1 /*
   2  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   3  *
   4  * This code is free software; you can redistribute it and/or modify it
   5  * under the terms of the GNU General Public License version 2 only, as
   6  * published by the Free Software Foundation.  Oracle designates this
   7  * particular file as subject to the "Classpath" exception as provided
   8  * by Oracle in the LICENSE file that accompanied this code.
   9  *
  10  * This code is distributed in the hope that it will be useful, but WITHOUT
  11  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  12  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  13  * version 2 for more details (a copy is included in the LICENSE file that
  14  * accompanied this code).
  15  *
  16  * You should have received a copy of the GNU General Public License version
  17  * 2 along with this work; if not, write to the Free Software Foundation,
  18  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  19  *
  20  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  21  * or visit www.oracle.com if you need additional information or have any
  22  * questions.
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  24 
  25 /*
  26  * This file is available under and governed by the GNU General Public
  27  * License version 2 only, as published by the Free Software Foundation.
  28  * However, the following notice accompanied the original version of this
  29  * file:
  30  *
  31  * Written by Doug Lea with assistance from members of JCP JSR-166
  32  * Expert Group and released to the public domain, as explained at
  33  * http://creativecommons.org/publicdomain/zero/1.0/
  34  */
  35 
  36 package java.util.concurrent;
  37 
  38 import java.lang.Thread.UncaughtExceptionHandler;
  39 import java.security.AccessControlContext;
  40 import java.security.Permissions;
  41 import java.security.ProtectionDomain;
  42 import java.util.ArrayList;
  43 import java.util.Arrays;
  44 import java.util.Collection;
  45 import java.util.Collections;
  46 import java.util.List;
  47 import java.util.concurrent.locks.ReentrantLock;
  48 import java.util.concurrent.locks.LockSupport;
  49 
  50 /**
  51  * An {@link ExecutorService} for running {@link ForkJoinTask}s.
  52  * A {@code ForkJoinPool} provides the entry point for submissions
  53  * from non-{@code ForkJoinTask} clients, as well as management and
  54  * monitoring operations.
  55  *
  56  * <p>A {@code ForkJoinPool} differs from other kinds of {@link
  57  * ExecutorService} mainly by virtue of employing
  58  * <em>work-stealing</em>: all threads in the pool attempt to find and
  59  * execute tasks submitted to the pool and/or created by other active
  60  * tasks (eventually blocking waiting for work if none exist). This
  61  * enables efficient processing when most tasks spawn other subtasks
  62  * (as do most {@code ForkJoinTask}s), as well as when many small
  63  * tasks are submitted to the pool from external clients.  Especially
  64  * when setting <em>asyncMode</em> to true in constructors, {@code
  65  * ForkJoinPool}s may also be appropriate for use with event-style
  66  * tasks that are never joined.
  67  *
  68  * <p>A static {@link #commonPool()} is available and appropriate for
  69  * most applications. The common pool is used by any ForkJoinTask that
  70  * is not explicitly submitted to a specified pool. Using the common
  71  * pool normally reduces resource usage (its threads are slowly
  72  * reclaimed during periods of non-use, and reinstated upon subsequent
  73  * use).
  74  *
  75  * <p>For applications that require separate or custom pools, a {@code
  76  * ForkJoinPool} may be constructed with a given target parallelism
  77  * level; by default, equal to the number of available processors.
  78  * The pool attempts to maintain enough active (or available) threads
  79  * by dynamically adding, suspending, or resuming internal worker
  80  * threads, even if some tasks are stalled waiting to join others.
  81  * However, no such adjustments are guaranteed in the face of blocked
  82  * I/O or other unmanaged synchronization. The nested {@link
  83  * ManagedBlocker} interface enables extension of the kinds of
  84  * synchronization accommodated.
  85  *
  86  * <p>In addition to execution and lifecycle control methods, this
  87  * class provides status check methods (for example
  88  * {@link #getStealCount}) that are intended to aid in developing,
  89  * tuning, and monitoring fork/join applications. Also, method
  90  * {@link #toString} returns indications of pool state in a
  91  * convenient form for informal monitoring.
  92  *
  93  * <p>As is the case with other ExecutorServices, there are three
  94  * main task execution methods summarized in the following table.
  95  * These are designed to be used primarily by clients not already
  96  * engaged in fork/join computations in the current pool.  The main
  97  * forms of these methods accept instances of {@code ForkJoinTask},
  98  * but overloaded forms also allow mixed execution of plain {@code
  99  * Runnable}- or {@code Callable}- based activities as well.  However,
 100  * tasks that are already executing in a pool should normally instead
 101  * use the within-computation forms listed in the table unless using
 102  * async event-style tasks that are not usually joined, in which case
 103  * there is little difference among choice of methods.
 104  *
 105  * <table BORDER CELLPADDING=3 CELLSPACING=1>
 106  * <caption>Summary of task execution methods</caption>
 107  *  <tr>
 108  *    <td></td>
 109  *    <td ALIGN=CENTER> <b>Call from non-fork/join clients</b></td>
 110  *    <td ALIGN=CENTER> <b>Call from within fork/join computations</b></td>
 111  *  </tr>
 112  *  <tr>
 113  *    <td> <b>Arrange async execution</b></td>
 114  *    <td> {@link #execute(ForkJoinTask)}</td>
 115  *    <td> {@link ForkJoinTask#fork}</td>
 116  *  </tr>
 117  *  <tr>
 118  *    <td> <b>Await and obtain result</b></td>
 119  *    <td> {@link #invoke(ForkJoinTask)}</td>
 120  *    <td> {@link ForkJoinTask#invoke}</td>
 121  *  </tr>
 122  *  <tr>
 123  *    <td> <b>Arrange exec and obtain Future</b></td>
 124  *    <td> {@link #submit(ForkJoinTask)}</td>
 125  *    <td> {@link ForkJoinTask#fork} (ForkJoinTasks <em>are</em> Futures)</td>
 126  *  </tr>
 127  * </table>
 128  *
 129  * <p>The common pool is by default constructed with default
 130  * parameters, but these may be controlled by setting three
 131  * {@linkplain System#getProperty system properties}:
 132  * <ul>
 133  * <li>{@code java.util.concurrent.ForkJoinPool.common.parallelism}
 134  * - the parallelism level, a non-negative integer
 135  * <li>{@code java.util.concurrent.ForkJoinPool.common.threadFactory}
 136  * - the class name of a {@link ForkJoinWorkerThreadFactory}
 137  * <li>{@code java.util.concurrent.ForkJoinPool.common.exceptionHandler}
 138  * - the class name of a {@link UncaughtExceptionHandler}
 139  * <li>{@code java.util.concurrent.ForkJoinPool.common.maximumSpares}
 140  * - the maximum number of allowed extra threads to maintain target
 141  * parallelism (default 256).
 142  * </ul>
 143  * If a {@link SecurityManager} is present and no factory is
 144  * specified, then the default pool uses a factory supplying
 145  * threads that have no {@link Permissions} enabled.
 146  * The system class loader is used to load these classes.
 147  * Upon any error in establishing these settings, default parameters
 148  * are used. It is possible to disable or limit the use of threads in
 149  * the common pool by setting the parallelism property to zero, and/or
 150  * using a factory that may return {@code null}. However doing so may
 151  * cause unjoined tasks to never be executed.
 152  *
 153  * <p><b>Implementation notes</b>: This implementation restricts the
 154  * maximum number of running threads to 32767. Attempts to create
 155  * pools with greater than the maximum number result in
 156  * {@code IllegalArgumentException}.
 157  *
 158  * <p>This implementation rejects submitted tasks (that is, by throwing
 159  * {@link RejectedExecutionException}) only when the pool is shut down
 160  * or internal resources have been exhausted.
 161  *
 162  * @since 1.7
 163  * @author Doug Lea
 164  */
 165 @jdk.internal.vm.annotation.Contended
 166 public class ForkJoinPool extends AbstractExecutorService {
 167 
 168     /*
 169      * Implementation Overview
 170      *
 171      * This class and its nested classes provide the main
 172      * functionality and control for a set of worker threads:
 173      * Submissions from non-FJ threads enter into submission queues.
 174      * Workers take these tasks and typically split them into subtasks
 175      * that may be stolen by other workers.  Preference rules give
 176      * first priority to processing tasks from their own queues (LIFO
 177      * or FIFO, depending on mode), then to randomized FIFO steals of
 178      * tasks in other queues.  This framework began as vehicle for
 179      * supporting tree-structured parallelism using work-stealing.
 180      * Over time, its scalability advantages led to extensions and
 181      * changes to better support more diverse usage contexts.  Because
 182      * most internal methods and nested classes are interrelated,
 183      * their main rationale and descriptions are presented here;
 184      * individual methods and nested classes contain only brief
 185      * comments about details.
 186      *
 187      * WorkQueues
 188      * ==========
 189      *
 190      * Most operations occur within work-stealing queues (in nested
 191      * class WorkQueue).  These are special forms of Deques that
 192      * support only three of the four possible end-operations -- push,
 193      * pop, and poll (aka steal), under the further constraints that
 194      * push and pop are called only from the owning thread (or, as
 195      * extended here, under a lock), while poll may be called from
 196      * other threads.  (If you are unfamiliar with them, you probably
 197      * want to read Herlihy and Shavit's book "The Art of
 198      * Multiprocessor programming", chapter 16 describing these in
 199      * more detail before proceeding.)  The main work-stealing queue
 200      * design is roughly similar to those in the papers "Dynamic
 201      * Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005
 202      * (http://research.sun.com/scalable/pubs/index.html) and
 203      * "Idempotent work stealing" by Michael, Saraswat, and Vechev,
 204      * PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186).
 205      * The main differences ultimately stem from GC requirements that
 206      * we null out taken slots as soon as we can, to maintain as small
 207      * a footprint as possible even in programs generating huge
 208      * numbers of tasks. To accomplish this, we shift the CAS
 209      * arbitrating pop vs poll (steal) from being on the indices
 210      * ("base" and "top") to the slots themselves.
 211      *
 212      * Adding tasks then takes the form of a classic array push(task)
 213      * in a circular buffer:
 214      *    q.array[q.top++ % length] = task;
 215      *
 216      * (The actual code needs to null-check and size-check the array,
 217      * uses masking, not mod, for indexing a power-of-two-sized array,
 218      * properly fences accesses, and possibly signals waiting workers
 219      * to start scanning -- see below.)  Both a successful pop and
 220      * poll mainly entail a CAS of a slot from non-null to null.
 221      *
 222      * The pop operation (always performed by owner) is:
 223      *   if ((the task at top slot is not null) and
 224      *        (CAS slot to null))
 225      *           decrement top and return task;
 226      *
 227      * And the poll operation (usually by a stealer) is
 228      *    if ((the task at base slot is not null) and
 229      *        (CAS slot to null))
 230      *           increment base and return task;
 231      *
 232      * There are several variants of each of these; for example most
 233      * versions of poll pre-screen the CAS by rechecking that the base
 234      * has not changed since reading the slot, and most methods only
 235      * attempt the CAS if base appears not to be equal to top.
 236      *
 237      * Memory ordering.  See "Correct and Efficient Work-Stealing for
 238      * Weak Memory Models" by Le, Pop, Cohen, and Nardelli, PPoPP 2013
 239      * (http://www.di.ens.fr/~zappa/readings/ppopp13.pdf) for an
 240      * analysis of memory ordering requirements in work-stealing
 241      * algorithms similar to (but different than) the one used here.
 242      * Extracting tasks in array slots via (fully fenced) CAS provides
 243      * primary synchronization. The base and top indices imprecisely
 244      * guide where to extract from. We do not always require strict
 245      * orderings of array and index updates, so sometimes let them be
 246      * subject to compiler and processor reorderings. However, the
 247      * volatile "base" index also serves as a basis for memory
 248      * ordering: Slot accesses are preceded by a read of base,
 249      * ensuring happens-before ordering with respect to stealers (so
 250      * the slots themselves can be read via plain array reads.)  The
 251      * only other memory orderings relied on are maintained in the
 252      * course of signalling and activation (see below).  A check that
 253      * base == top indicates (momentary) emptiness, but otherwise may
 254      * err on the side of possibly making the queue appear nonempty
 255      * when a push, pop, or poll have not fully committed, or making
 256      * it appear empty when an update of top has not yet been visibly
 257      * written.  (Method isEmpty() checks the case of a partially
 258      * completed removal of the last element.)  Because of this, the
 259      * poll operation, considered individually, is not wait-free. One
 260      * thief cannot successfully continue until another in-progress
 261      * one (or, if previously empty, a push) visibly completes.
 262      * However, in the aggregate, we ensure at least probabilistic
 263      * non-blockingness.  If an attempted steal fails, a scanning
 264      * thief chooses a different random victim target to try next. So,
 265      * in order for one thief to progress, it suffices for any
 266      * in-progress poll or new push on any empty queue to
 267      * complete. (This is why we normally use method pollAt and its
 268      * variants that try once at the apparent base index, else
 269      * consider alternative actions, rather than method poll, which
 270      * retries.)
 271      *
 272      * This approach also enables support of a user mode in which
 273      * local task processing is in FIFO, not LIFO order, simply by
 274      * using poll rather than pop.  This can be useful in
 275      * message-passing frameworks in which tasks are never joined.
 276      *
 277      * WorkQueues are also used in a similar way for tasks submitted
 278      * to the pool. We cannot mix these tasks in the same queues used
 279      * by workers. Instead, we randomly associate submission queues
 280      * with submitting threads, using a form of hashing.  The
 281      * ThreadLocalRandom probe value serves as a hash code for
 282      * choosing existing queues, and may be randomly repositioned upon
 283      * contention with other submitters.  In essence, submitters act
 284      * like workers except that they are restricted to executing local
 285      * tasks that they submitted (or in the case of CountedCompleters,
 286      * others with the same root task).  Insertion of tasks in shared
 287      * mode requires a lock but we use only a simple spinlock (using
 288      * field qlock), because submitters encountering a busy queue move
 289      * on to try or create other queues -- they block only when
 290      * creating and registering new queues. Because it is used only as
 291      * a spinlock, unlocking requires only a "releasing" store (using
 292      * putIntRelease).  The qlock is also used during termination
 293      * detection, in which case it is forced to a negative
 294      * non-lockable value.
 295      *
 296      * Management
 297      * ==========
 298      *
 299      * The main throughput advantages of work-stealing stem from
 300      * decentralized control -- workers mostly take tasks from
 301      * themselves or each other, at rates that can exceed a billion
 302      * per second.  The pool itself creates, activates (enables
 303      * scanning for and running tasks), deactivates, blocks, and
 304      * terminates threads, all with minimal central information.
 305      * There are only a few properties that we can globally track or
 306      * maintain, so we pack them into a small number of variables,
 307      * often maintaining atomicity without blocking or locking.
 308      * Nearly all essentially atomic control state is held in two
 309      * volatile variables that are by far most often read (not
 310      * written) as status and consistency checks. (Also, field
 311      * "config" holds unchanging configuration state.)
 312      *
 313      * Field "ctl" contains 64 bits holding information needed to
 314      * atomically decide to add, inactivate, enqueue (on an event
 315      * queue), dequeue, and/or re-activate workers.  To enable this
 316      * packing, we restrict maximum parallelism to (1<<15)-1 (which is
 317      * far in excess of normal operating range) to allow ids, counts,
 318      * and their negations (used for thresholding) to fit into 16bit
 319      * subfields.
 320      *
 321      * Field "runState" holds lifetime status, atomically and
 322      * monotonically setting STARTED, SHUTDOWN, STOP, and finally
 323      * TERMINATED bits.
 324      *
 325      * Field "auxState" is a ReentrantLock subclass that also
 326      * opportunistically holds some other bookkeeping fields accessed
 327      * only when locked.  It is mainly used to lock (infrequent)
 328      * updates to workQueues.  The auxState instance is itself lazily
 329      * constructed (see tryInitialize), requiring a double-check-style
 330      * bootstrapping use of field runState, and locking a private
 331      * static.
 332      *
 333      * Field "workQueues" holds references to WorkQueues.  It is
 334      * updated (only during worker creation and termination) under the
 335      * lock, but is otherwise concurrently readable, and accessed
 336      * directly. We also ensure that reads of the array reference
 337      * itself never become too stale (for example, re-reading before
 338      * each scan). To simplify index-based operations, the array size
 339      * is always a power of two, and all readers must tolerate null
 340      * slots. Worker queues are at odd indices. Shared (submission)
 341      * queues are at even indices, up to a maximum of 64 slots, to
 342      * limit growth even if array needs to expand to add more
 343      * workers. Grouping them together in this way simplifies and
 344      * speeds up task scanning.
 345      *
 346      * All worker thread creation is on-demand, triggered by task
 347      * submissions, replacement of terminated workers, and/or
 348      * compensation for blocked workers. However, all other support
 349      * code is set up to work with other policies.  To ensure that we
 350      * do not hold on to worker references that would prevent GC, all
 351      * accesses to workQueues are via indices into the workQueues
 352      * array (which is one source of some of the messy code
 353      * constructions here). In essence, the workQueues array serves as
 354      * a weak reference mechanism. Thus for example the stack top
 355      * subfield of ctl stores indices, not references.
 356      *
 357      * Queuing Idle Workers. Unlike HPC work-stealing frameworks, we
 358      * cannot let workers spin indefinitely scanning for tasks when
 359      * none can be found immediately, and we cannot start/resume
 360      * workers unless there appear to be tasks available.  On the
 361      * other hand, we must quickly prod them into action when new
 362      * tasks are submitted or generated. In many usages, ramp-up time
 363      * to activate workers is the main limiting factor in overall
 364      * performance, which is compounded at program start-up by JIT
 365      * compilation and allocation. So we streamline this as much as
 366      * possible.
 367      *
 368      * The "ctl" field atomically maintains active and total worker
 369      * counts as well as a queue to place waiting threads so they can
 370      * be located for signalling. Active counts also play the role of
 371      * quiescence indicators, so are decremented when workers believe
 372      * that there are no more tasks to execute. The "queue" is
 373      * actually a form of Treiber stack.  A stack is ideal for
 374      * activating threads in most-recently used order. This improves
 375      * performance and locality, outweighing the disadvantages of
 376      * being prone to contention and inability to release a worker
 377      * unless it is topmost on stack.  We block/unblock workers after
 378      * pushing on the idle worker stack (represented by the lower
 379      * 32bit subfield of ctl) when they cannot find work.  The top
 380      * stack state holds the value of the "scanState" field of the
 381      * worker: its index and status, plus a version counter that, in
 382      * addition to the count subfields (also serving as version
 383      * stamps) provide protection against Treiber stack ABA effects.
 384      *
 385      * Creating workers. To create a worker, we pre-increment total
 386      * count (serving as a reservation), and attempt to construct a
 387      * ForkJoinWorkerThread via its factory. Upon construction, the
 388      * new thread invokes registerWorker, where it constructs a
 389      * WorkQueue and is assigned an index in the workQueues array
 390      * (expanding the array if necessary). The thread is then started.
 391      * Upon any exception across these steps, or null return from
 392      * factory, deregisterWorker adjusts counts and records
 393      * accordingly.  If a null return, the pool continues running with
 394      * fewer than the target number workers. If exceptional, the
 395      * exception is propagated, generally to some external caller.
 396      * Worker index assignment avoids the bias in scanning that would
 397      * occur if entries were sequentially packed starting at the front
 398      * of the workQueues array. We treat the array as a simple
 399      * power-of-two hash table, expanding as needed. The seedIndex
 400      * increment ensures no collisions until a resize is needed or a
 401      * worker is deregistered and replaced, and thereafter keeps
 402      * probability of collision low. We cannot use
 403      * ThreadLocalRandom.getProbe() for similar purposes here because
 404      * the thread has not started yet, but do so for creating
 405      * submission queues for existing external threads (see
 406      * externalPush).
 407      *
 408      * WorkQueue field scanState is used by both workers and the pool
 409      * to manage and track whether a worker is UNSIGNALLED (possibly
 410      * blocked waiting for a signal).  When a worker is inactivated,
 411      * its scanState field is set, and is prevented from executing
 412      * tasks, even though it must scan once for them to avoid queuing
 413      * races. Note that scanState updates lag queue CAS releases so
 414      * usage requires care. When queued, the lower 16 bits of
 415      * scanState must hold its pool index. So we place the index there
 416      * upon initialization (see registerWorker) and otherwise keep it
 417      * there or restore it when necessary.
 418      *
 419      * The ctl field also serves as the basis for memory
 420      * synchronization surrounding activation. This uses a more
 421      * efficient version of a Dekker-like rule that task producers and
 422      * consumers sync with each other by both writing/CASing ctl (even
 423      * if to its current value).  This would be extremely costly. So
 424      * we relax it in several ways: (1) Producers only signal when
 425      * their queue is empty. Other workers propagate this signal (in
 426      * method scan) when they find tasks. (2) Workers only enqueue
 427      * after scanning (see below) and not finding any tasks.  (3)
 428      * Rather than CASing ctl to its current value in the common case
 429      * where no action is required, we reduce write contention by
 430      * equivalently prefacing signalWork when called by an external
 431      * task producer using a memory access with full-volatile
 432      * semantics or a "fullFence". (4) For internal task producers we
 433      * rely on the fact that even if no other workers awaken, the
 434      * producer itself will eventually see the task and execute it.
 435      *
 436      * Almost always, too many signals are issued. A task producer
 437      * cannot in general tell if some existing worker is in the midst
 438      * of finishing one task (or already scanning) and ready to take
 439      * another without being signalled. So the producer might instead
 440      * activate a different worker that does not find any work, and
 441      * then inactivates. This scarcely matters in steady-state
 442      * computations involving all workers, but can create contention
 443      * and bookkeeping bottlenecks during ramp-up, ramp-down, and small
 444      * computations involving only a few workers.
 445      *
 446      * Scanning. Method scan() performs top-level scanning for tasks.
 447      * Each scan traverses (and tries to poll from) each queue in
 448      * pseudorandom permutation order by randomly selecting an origin
 449      * index and a step value.  (The pseudorandom generator need not
 450      * have high-quality statistical properties in the long term, but
 451      * just within computations; We use 64bit and 32bit Marsaglia
 452      * XorShifts, which are cheap and suffice here.)  Scanning also
 453      * employs contention reduction: When scanning workers fail a CAS
 454      * polling for work, they soon restart with a different
 455      * pseudorandom scan order (thus likely retrying at different
 456      * intervals). This improves throughput when many threads are
 457      * trying to take tasks from few queues.  Scans do not otherwise
 458      * explicitly take into account core affinities, loads, cache
 459      * localities, etc, However, they do exploit temporal locality
 460      * (which usually approximates these) by preferring to re-poll (up
 461      * to POLL_LIMIT times) from the same queue after a successful
 462      * poll before trying others.  Restricted forms of scanning occur
 463      * in methods helpComplete and findNonEmptyStealQueue, and take
 464      * similar but simpler forms.
 465      *
 466      * Deactivation and waiting. Queuing encounters several intrinsic
 467      * races; most notably that an inactivating scanning worker can
 468      * miss seeing a task produced during a scan.  So when a worker
 469      * cannot find a task to steal, it inactivates and enqueues, and
 470      * then rescans to ensure that it didn't miss one, reactivating
 471      * upon seeing one with probability approximately proportional to
 472      * probability of a miss.  (In most cases, the worker will be
 473      * signalled before self-signalling, avoiding cascades of multiple
 474      * signals for the same task).
 475      *
 476      * Workers block (in method awaitWork) using park/unpark;
 477      * advertising the need for signallers to unpark by setting their
 478      * "parker" fields.
 479      *
 480      * Trimming workers. To release resources after periods of lack of
 481      * use, a worker starting to wait when the pool is quiescent will
 482      * time out and terminate (see awaitWork) if the pool has remained
 483      * quiescent for period given by IDLE_TIMEOUT_MS, increasing the
 484      * period as the number of threads decreases, eventually removing
 485      * all workers.
 486      *
 487      * Shutdown and Termination. A call to shutdownNow invokes
 488      * tryTerminate to atomically set a runState bit. The calling
 489      * thread, as well as every other worker thereafter terminating,
 490      * helps terminate others by setting their (qlock) status,
 491      * cancelling their unprocessed tasks, and waking them up, doing
 492      * so repeatedly until stable. Calls to non-abrupt shutdown()
 493      * preface this by checking whether termination should commence.
 494      * This relies primarily on the active count bits of "ctl"
 495      * maintaining consensus -- tryTerminate is called from awaitWork
 496      * whenever quiescent. However, external submitters do not take
 497      * part in this consensus.  So, tryTerminate sweeps through queues
 498      * (until stable) to ensure lack of in-flight submissions and
 499      * workers about to process them before triggering the "STOP"
 500      * phase of termination. (Note: there is an intrinsic conflict if
 501      * helpQuiescePool is called when shutdown is enabled. Both wait
 502      * for quiescence, but tryTerminate is biased to not trigger until
 503      * helpQuiescePool completes.)
 504      *
 505      * Joining Tasks
 506      * =============
 507      *
 508      * Any of several actions may be taken when one worker is waiting
 509      * to join a task stolen (or always held) by another.  Because we
 510      * are multiplexing many tasks on to a pool of workers, we can't
 511      * just let them block (as in Thread.join).  We also cannot just
 512      * reassign the joiner's run-time stack with another and replace
 513      * it later, which would be a form of "continuation", that even if
 514      * possible is not necessarily a good idea since we may need both
 515      * an unblocked task and its continuation to progress.  Instead we
 516      * combine two tactics:
 517      *
 518      *   Helping: Arranging for the joiner to execute some task that it
 519      *      would be running if the steal had not occurred.
 520      *
 521      *   Compensating: Unless there are already enough live threads,
 522      *      method tryCompensate() may create or re-activate a spare
 523      *      thread to compensate for blocked joiners until they unblock.
 524      *
 525      * A third form (implemented in tryRemoveAndExec) amounts to
 526      * helping a hypothetical compensator: If we can readily tell that
 527      * a possible action of a compensator is to steal and execute the
 528      * task being joined, the joining thread can do so directly,
 529      * without the need for a compensation thread (although at the
 530      * expense of larger run-time stacks, but the tradeoff is
 531      * typically worthwhile).
 532      *
 533      * The ManagedBlocker extension API can't use helping so relies
 534      * only on compensation in method awaitBlocker.
 535      *
 536      * The algorithm in helpStealer entails a form of "linear
 537      * helping".  Each worker records (in field currentSteal) the most
 538      * recent task it stole from some other worker (or a submission).
 539      * It also records (in field currentJoin) the task it is currently
 540      * actively joining. Method helpStealer uses these markers to try
 541      * to find a worker to help (i.e., steal back a task from and
 542      * execute it) that could hasten completion of the actively joined
 543      * task.  Thus, the joiner executes a task that would be on its
 544      * own local deque had the to-be-joined task not been stolen. This
 545      * is a conservative variant of the approach described in Wagner &
 546      * Calder "Leapfrogging: a portable technique for implementing
 547      * efficient futures" SIGPLAN Notices, 1993
 548      * (http://portal.acm.org/citation.cfm?id=155354). It differs in
 549      * that: (1) We only maintain dependency links across workers upon
 550      * steals, rather than use per-task bookkeeping.  This sometimes
 551      * requires a linear scan of workQueues array to locate stealers,
 552      * but often doesn't because stealers leave hints (that may become
 553      * stale/wrong) of where to locate them.  It is only a hint
 554      * because a worker might have had multiple steals and the hint
 555      * records only one of them (usually the most current).  Hinting
 556      * isolates cost to when it is needed, rather than adding to
 557      * per-task overhead.  (2) It is "shallow", ignoring nesting and
 558      * potentially cyclic mutual steals.  (3) It is intentionally
 559      * racy: field currentJoin is updated only while actively joining,
 560      * which means that we miss links in the chain during long-lived
 561      * tasks, GC stalls etc (which is OK since blocking in such cases
 562      * is usually a good idea).  (4) We bound the number of attempts
 563      * to find work using checksums and fall back to suspending the
 564      * worker and if necessary replacing it with another.
 565      *
 566      * Helping actions for CountedCompleters do not require tracking
 567      * currentJoins: Method helpComplete takes and executes any task
 568      * with the same root as the task being waited on (preferring
 569      * local pops to non-local polls). However, this still entails
 570      * some traversal of completer chains, so is less efficient than
 571      * using CountedCompleters without explicit joins.
 572      *
 573      * Compensation does not aim to keep exactly the target
 574      * parallelism number of unblocked threads running at any given
 575      * time. Some previous versions of this class employed immediate
 576      * compensations for any blocked join. However, in practice, the
 577      * vast majority of blockages are transient byproducts of GC and
 578      * other JVM or OS activities that are made worse by replacement.
 579      * Currently, compensation is attempted only after validating that
 580      * all purportedly active threads are processing tasks by checking
 581      * field WorkQueue.scanState, which eliminates most false
 582      * positives.  Also, compensation is bypassed (tolerating fewer
 583      * threads) in the most common case in which it is rarely
 584      * beneficial: when a worker with an empty queue (thus no
 585      * continuation tasks) blocks on a join and there still remain
 586      * enough threads to ensure liveness.
 587      *
 588      * Spare threads are removed as soon as they notice that the
 589      * target parallelism level has been exceeded, in method
 590      * tryDropSpare. (Method scan arranges returns for rechecks upon
 591      * each probe via the "bound" parameter.)
 592      *
 593      * The compensation mechanism may be bounded.  Bounds for the
 594      * commonPool (see COMMON_MAX_SPARES) better enable JVMs to cope
 595      * with programming errors and abuse before running out of
 596      * resources to do so. In other cases, users may supply factories
 597      * that limit thread construction. The effects of bounding in this
 598      * pool (like all others) is imprecise.  Total worker counts are
 599      * decremented when threads deregister, not when they exit and
 600      * resources are reclaimed by the JVM and OS. So the number of
 601      * simultaneously live threads may transiently exceed bounds.
 602      *
 603      * Common Pool
 604      * ===========
 605      *
 606      * The static common pool always exists after static
 607      * initialization.  Since it (or any other created pool) need
 608      * never be used, we minimize initial construction overhead and
 609      * footprint to the setup of about a dozen fields, with no nested
 610      * allocation. Most bootstrapping occurs within method
 611      * externalSubmit during the first submission to the pool.
 612      *
 613      * When external threads submit to the common pool, they can
 614      * perform subtask processing (see externalHelpComplete and
 615      * related methods) upon joins.  This caller-helps policy makes it
 616      * sensible to set common pool parallelism level to one (or more)
 617      * less than the total number of available cores, or even zero for
 618      * pure caller-runs.  We do not need to record whether external
 619      * submissions are to the common pool -- if not, external help
 620      * methods return quickly. These submitters would otherwise be
 621      * blocked waiting for completion, so the extra effort (with
 622      * liberally sprinkled task status checks) in inapplicable cases
 623      * amounts to an odd form of limited spin-wait before blocking in
 624      * ForkJoinTask.join.
 625      *
 626      * As a more appropriate default in managed environments, unless
 627      * overridden by system properties, we use workers of subclass
 628      * InnocuousForkJoinWorkerThread when there is a SecurityManager
 629      * present. These workers have no permissions set, do not belong
 630      * to any user-defined ThreadGroup, and erase all ThreadLocals
 631      * after executing any top-level task (see WorkQueue.runTask).
 632      * The associated mechanics (mainly in ForkJoinWorkerThread) may
 633      * be JVM-dependent and must access particular Thread class fields
 634      * to achieve this effect.
 635      *
 636      * Style notes
 637      * ===========
 638      *
 639      * Memory ordering relies mainly on Unsafe intrinsics that carry
 640      * the further responsibility of explicitly performing null- and
 641      * bounds- checks otherwise carried out implicitly by JVMs.  This
 642      * can be awkward and ugly, but also reflects the need to control
 643      * outcomes across the unusual cases that arise in very racy code
 644      * with very few invariants. So these explicit checks would exist
 645      * in some form anyway.  All fields are read into locals before
 646      * use, and null-checked if they are references.  This is usually
 647      * done in a "C"-like style of listing declarations at the heads
 648      * of methods or blocks, and using inline assignments on first
 649      * encounter.  Array bounds-checks are usually performed by
 650      * masking with array.length-1, which relies on the invariant that
 651      * these arrays are created with positive lengths, which is itself
 652      * paranoically checked. Nearly all explicit checks lead to
 653      * bypass/return, not exception throws, because they may
 654      * legitimately arise due to cancellation/revocation during
 655      * shutdown.
 656      *
 657      * There is a lot of representation-level coupling among classes
 658      * ForkJoinPool, ForkJoinWorkerThread, and ForkJoinTask.  The
 659      * fields of WorkQueue maintain data structures managed by
 660      * ForkJoinPool, so are directly accessed.  There is little point
 661      * trying to reduce this, since any associated future changes in
 662      * representations will need to be accompanied by algorithmic
 663      * changes anyway. Several methods intrinsically sprawl because
 664      * they must accumulate sets of consistent reads of fields held in
 665      * local variables.  There are also other coding oddities
 666      * (including several unnecessary-looking hoisted null checks)
 667      * that help some methods perform reasonably even when interpreted
 668      * (not compiled).
 669      *
 670      * The order of declarations in this file is (with a few exceptions):
 671      * (1) Static utility functions
 672      * (2) Nested (static) classes
 673      * (3) Static fields
 674      * (4) Fields, along with constants used when unpacking some of them
 675      * (5) Internal control methods
 676      * (6) Callbacks and other support for ForkJoinTask methods
 677      * (7) Exported methods
 678      * (8) Static block initializing statics in minimally dependent order
 679      */
 680 
 681     // Static utilities
 682 
 683     /**
 684      * If there is a security manager, makes sure caller has
 685      * permission to modify threads.
 686      */
 687     private static void checkPermission() {
 688         SecurityManager security = System.getSecurityManager();
 689         if (security != null)
 690             security.checkPermission(modifyThreadPermission);
 691     }
 692 
 693     // Nested classes
 694 
 695     /**
 696      * Factory for creating new {@link ForkJoinWorkerThread}s.
 697      * A {@code ForkJoinWorkerThreadFactory} must be defined and used
 698      * for {@code ForkJoinWorkerThread} subclasses that extend base
 699      * functionality or initialize threads with different contexts.
 700      */
 701     public static interface ForkJoinWorkerThreadFactory {
 702         /**
 703          * Returns a new worker thread operating in the given pool.
 704          *
 705          * @param pool the pool this thread works in
 706          * @return the new worker thread, or {@code null} if the request
 707          *         to create a thread is rejected
 708          * @throws NullPointerException if the pool is null
 709          */
 710         public ForkJoinWorkerThread newThread(ForkJoinPool pool);
 711     }
 712 
 713     /**
 714      * Default ForkJoinWorkerThreadFactory implementation; creates a
 715      * new ForkJoinWorkerThread.
 716      */
 717     private static final class DefaultForkJoinWorkerThreadFactory
 718         implements ForkJoinWorkerThreadFactory {
 719         public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
 720             return new ForkJoinWorkerThread(pool);
 721         }
 722     }
 723 
 724     /**
 725      * Class for artificial tasks that are used to replace the target
 726      * of local joins if they are removed from an interior queue slot
 727      * in WorkQueue.tryRemoveAndExec. We don't need the proxy to
 728      * actually do anything beyond having a unique identity.
 729      */
 730     private static final class EmptyTask extends ForkJoinTask<Void> {
 731         private static final long serialVersionUID = -7721805057305804111L;
 732         EmptyTask() { status = ForkJoinTask.NORMAL; } // force done
 733         public final Void getRawResult() { return null; }
 734         public final void setRawResult(Void x) {}
 735         public final boolean exec() { return true; }
 736     }
 737 
 738     /**
 739      * Additional fields and lock created upon initialization.
 740      */
 741     private static final class AuxState extends ReentrantLock {
 742         private static final long serialVersionUID = -6001602636862214147L;
 743         volatile long stealCount;     // cumulative steal count
 744         long indexSeed;               // index bits for registerWorker
 745         AuxState() {}
 746     }
 747 
 748     // Constants shared across ForkJoinPool and WorkQueue
 749 
 750     // Bounds
 751     static final int SMASK        = 0xffff;        // short bits == max index
 752     static final int MAX_CAP      = 0x7fff;        // max #workers - 1
 753     static final int EVENMASK     = 0xfffe;        // even short bits
 754     static final int SQMASK       = 0x007e;        // max 64 (even) slots
 755 
 756     // Masks and units for WorkQueue.scanState and ctl sp subfield
 757     static final int UNSIGNALLED  = 1 << 31;       // must be negative
 758     static final int SS_SEQ       = 1 << 16;       // version count
 759 
 760     // Mode bits for ForkJoinPool.config and WorkQueue.config
 761     static final int MODE_MASK    = 0xffff << 16;  // top half of int
 762     static final int SPARE_WORKER = 1 << 17;       // set if tc > 0 on creation
 763     static final int UNREGISTERED = 1 << 18;       // to skip some of deregister
 764     static final int FIFO_QUEUE   = 1 << 31;       // must be negative
 765     static final int LIFO_QUEUE   = 0;             // for clarity
 766     static final int IS_OWNED     = 1;             // low bit 0 if shared
 767 
 768     /**
 769      * The maximum number of task executions from the same queue
 770      * before checking other queues, bounding unfairness and impact of
 771      * infinite user task recursion.  Must be a power of two minus 1.
 772      */
 773     static final int POLL_LIMIT = (1 << 10) - 1;
 774 
 775     /**
 776      * Queues supporting work-stealing as well as external task
 777      * submission. See above for descriptions and algorithms.
 778      * Performance on most platforms is very sensitive to placement of
 779      * instances of both WorkQueues and their arrays -- we absolutely
 780      * do not want multiple WorkQueue instances or multiple queue
 781      * arrays sharing cache lines. The @Contended annotation alerts
 782      * JVMs to try to keep instances apart.
 783      */
 784     @jdk.internal.vm.annotation.Contended
 785     static final class WorkQueue {
 786 
 787         /**
 788          * Capacity of work-stealing queue array upon initialization.
 789          * Must be a power of two; at least 4, but should be larger to
 790          * reduce or eliminate cacheline sharing among queues.
 791          * Currently, it is much larger, as a partial workaround for
 792          * the fact that JVMs often place arrays in locations that
 793          * share GC bookkeeping (especially cardmarks) such that
 794          * per-write accesses encounter serious memory contention.
 795          */
 796         static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
 797 
 798         /**
 799          * Maximum size for queue arrays. Must be a power of two less
 800          * than or equal to 1 << (31 - width of array entry) to ensure
 801          * lack of wraparound of index calculations, but defined to a
 802          * value a bit less than this to help users trap runaway
 803          * programs before saturating systems.
 804          */
 805         static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M
 806 
 807         // Instance fields
 808 
 809         volatile int scanState;    // versioned, negative if inactive
 810         int stackPred;             // pool stack (ctl) predecessor
 811         int nsteals;               // number of steals
 812         int hint;                  // randomization and stealer index hint
 813         int config;                // pool index and mode
 814         volatile int qlock;        // 1: locked, < 0: terminate; else 0
 815         volatile int base;         // index of next slot for poll
 816         int top;                   // index of next slot for push
 817         ForkJoinTask<?>[] array;   // the elements (initially unallocated)
 818         final ForkJoinPool pool;   // the containing pool (may be null)
 819         final ForkJoinWorkerThread owner; // owning thread or null if shared
 820         volatile Thread parker;    // == owner during call to park; else null
 821         volatile ForkJoinTask<?> currentJoin; // task being joined in awaitJoin
 822 
 823         @jdk.internal.vm.annotation.Contended("group2") // segregate
 824         volatile ForkJoinTask<?> currentSteal; // nonnull when running some task
 825 
 826         WorkQueue(ForkJoinPool pool, ForkJoinWorkerThread owner) {
 827             this.pool = pool;
 828             this.owner = owner;
 829             // Place indices in the center of array (that is not yet allocated)
 830             base = top = INITIAL_QUEUE_CAPACITY >>> 1;
 831         }
 832 
 833         /**
 834          * Returns an exportable index (used by ForkJoinWorkerThread).
 835          */
 836         final int getPoolIndex() {
 837             return (config & 0xffff) >>> 1; // ignore odd/even tag bit
 838         }
 839 
 840         /**
 841          * Returns the approximate number of tasks in the queue.
 842          */
 843         final int queueSize() {
 844             int n = base - top;       // read base first
 845             return (n >= 0) ? 0 : -n; // ignore transient negative
 846         }
 847 
 848         /**
 849          * Provides a more accurate estimate of whether this queue has
 850          * any tasks than does queueSize, by checking whether a
 851          * near-empty queue has at least one unclaimed task.
 852          */
 853         final boolean isEmpty() {
 854             ForkJoinTask<?>[] a; int n, al, s;
 855             return ((n = base - (s = top)) >= 0 || // possibly one task
 856                     (n == -1 && ((a = array) == null ||
 857                                  (al = a.length) == 0 ||
 858                                  a[(al - 1) & (s - 1)] == null)));
 859         }
 860 
 861         /**
 862          * Pushes a task. Call only by owner in unshared queues.
 863          *
 864          * @param task the task. Caller must ensure non-null.
 865          * @throws RejectedExecutionException if array cannot be resized
 866          */
 867         final void push(ForkJoinTask<?> task) {
 868             U.storeFence();              // ensure safe publication
 869             int s = top, al, d; ForkJoinTask<?>[] a;
 870             if ((a = array) != null && (al = a.length) > 0) {
 871                 a[(al - 1) & s] = task;  // relaxed writes OK
 872                 top = s + 1;
 873                 ForkJoinPool p = pool;
 874                 if ((d = base - s) == 0 && p != null) {
 875                     U.fullFence();
 876                     p.signalWork();
 877                 }
 878                 else if (al + d == 1)
 879                     growArray();
 880             }
 881         }
 882 
 883         /**
 884          * Initializes or doubles the capacity of array. Call either
 885          * by owner or with lock held -- it is OK for base, but not
 886          * top, to move while resizings are in progress.
 887          */
 888         final ForkJoinTask<?>[] growArray() {
 889             ForkJoinTask<?>[] oldA = array;
 890             int size = oldA != null ? oldA.length << 1 : INITIAL_QUEUE_CAPACITY;
 891             if (size < INITIAL_QUEUE_CAPACITY || size > MAXIMUM_QUEUE_CAPACITY)
 892                 throw new RejectedExecutionException("Queue capacity exceeded");
 893             int oldMask, t, b;
 894             ForkJoinTask<?>[] a = array = new ForkJoinTask<?>[size];
 895             if (oldA != null && (oldMask = oldA.length - 1) > 0 &&
 896                 (t = top) - (b = base) > 0) {
 897                 int mask = size - 1;
 898                 do { // emulate poll from old array, push to new array
 899                     int index = b & oldMask;
 900                     long offset = ((long)index << ASHIFT) + ABASE;
 901                     ForkJoinTask<?> x = (ForkJoinTask<?>)
 902                         U.getObjectVolatile(oldA, offset);
 903                     if (x != null &&
 904                         U.compareAndSwapObject(oldA, offset, x, null))
 905                         a[b & mask] = x;
 906                 } while (++b != t);
 907                 U.storeFence();
 908             }
 909             return a;
 910         }
 911 
 912         /**
 913          * Takes next task, if one exists, in LIFO order.  Call only
 914          * by owner in unshared queues.
 915          */
 916         final ForkJoinTask<?> pop() {
 917             int b = base, s = top, al, i; ForkJoinTask<?>[] a;
 918             if ((a = array) != null && b != s && (al = a.length) > 0) {
 919                 int index = (al - 1) & --s;
 920                 long offset = ((long)index << ASHIFT) + ABASE;
 921                 ForkJoinTask<?> t = (ForkJoinTask<?>)
 922                     U.getObject(a, offset);
 923                 if (t != null &&
 924                     U.compareAndSwapObject(a, offset, t, null)) {
 925                     top = s;
 926                     return t;
 927                 }
 928             }
 929             return null;
 930         }
 931 
 932         /**
 933          * Takes a task in FIFO order if b is base of queue and a task
 934          * can be claimed without contention. Specialized versions
 935          * appear in ForkJoinPool methods scan and helpStealer.
 936          */
 937         final ForkJoinTask<?> pollAt(int b) {
 938             ForkJoinTask<?>[] a; int al;
 939             if ((a = array) != null && (al = a.length) > 0) {
 940                 int index = (al - 1) & b;
 941                 long offset = ((long)index << ASHIFT) + ABASE;
 942                 ForkJoinTask<?> t = (ForkJoinTask<?>)
 943                     U.getObjectVolatile(a, offset);
 944                 if (t != null && b++ == base &&
 945                     U.compareAndSwapObject(a, offset, t, null)) {
 946                     base = b;
 947                     return t;
 948                 }
 949             }
 950             return null;
 951         }
 952 
 953         /**
 954          * Takes next task, if one exists, in FIFO order.
 955          */
 956         final ForkJoinTask<?> poll() {
 957             for (;;) {
 958                 int b = base, s = top, d, al; ForkJoinTask<?>[] a;
 959                 if ((a = array) != null && (d = b - s) < 0 &&
 960                     (al = a.length) > 0) {
 961                     int index = (al - 1) & b;
 962                     long offset = ((long)index << ASHIFT) + ABASE;
 963                     ForkJoinTask<?> t = (ForkJoinTask<?>)
 964                         U.getObjectVolatile(a, offset);
 965                     if (b++ == base) {
 966                         if (t != null) {
 967                             if (U.compareAndSwapObject(a, offset, t, null)) {
 968                                 base = b;
 969                                 return t;
 970                             }
 971                         }
 972                         else if (d == -1)
 973                             break; // now empty
 974                     }
 975                 }
 976                 else
 977                     break;
 978             }
 979             return null;
 980         }
 981 
 982         /**
 983          * Takes next task, if one exists, in order specified by mode.
 984          */
 985         final ForkJoinTask<?> nextLocalTask() {
 986             return (config < 0) ? poll() : pop();
 987         }
 988 
 989         /**
 990          * Returns next task, if one exists, in order specified by mode.
 991          */
 992         final ForkJoinTask<?> peek() {
 993             int al; ForkJoinTask<?>[] a;
 994             return ((a = array) != null && (al = a.length) > 0) ?
 995                 a[(al - 1) & (config < 0 ? base : top - 1)] : null;
 996         }
 997 
 998         /**
 999          * Pops the given task only if it is at the current top.
1000          */
1001         final boolean tryUnpush(ForkJoinTask<?> task) {
1002             int b = base, s = top, al; ForkJoinTask<?>[] a;
1003             if ((a = array) != null && b != s && (al = a.length) > 0) {
1004                 int index = (al - 1) & --s;
1005                 long offset = ((long)index << ASHIFT) + ABASE;
1006                 if (U.compareAndSwapObject(a, offset, task, null)) {
1007                     top = s;
1008                     return true;
1009                 }
1010             }
1011             return false;
1012         }
1013 
1014         /**
1015          * Shared version of push. Fails if already locked.
1016          *
1017          * @return status: > 0 locked, 0 possibly was empty, < 0 was nonempty
1018          */
1019         final int sharedPush(ForkJoinTask<?> task) {
1020             int stat;
1021             if (U.compareAndSwapInt(this, QLOCK, 0, 1)) {
1022                 int b = base, s = top, al, d; ForkJoinTask<?>[] a;
1023                 if ((a = array) != null && (al = a.length) > 0 &&
1024                     al - 1 + (d = b - s) > 0) {
1025                     a[(al - 1) & s] = task;
1026                     top = s + 1;                 // relaxed writes OK here
1027                     qlock = 0;
1028                     stat = (d < 0 && b == base) ? d : 0;
1029                 }
1030                 else {
1031                     growAndSharedPush(task);
1032                     stat = 0;
1033                 }
1034             }
1035             else
1036                 stat = 1;
1037             return stat;
1038         }
1039 
1040         /**
1041          * Helper for sharedPush; called only when locked and resize
1042          * needed.
1043          */
1044         private void growAndSharedPush(ForkJoinTask<?> task) {
1045             try {
1046                 growArray();
1047                 int s = top, al; ForkJoinTask<?>[] a;
1048                 if ((a = array) != null && (al = a.length) > 0) {
1049                     a[(al - 1) & s] = task;
1050                     top = s + 1;
1051                 }
1052             } finally {
1053                 qlock = 0;
1054             }
1055         }
1056 
1057         /**
1058          * Shared version of tryUnpush.
1059          */
1060         final boolean trySharedUnpush(ForkJoinTask<?> task) {
1061             boolean popped = false;
1062             int s = top - 1, al; ForkJoinTask<?>[] a;
1063             if ((a = array) != null && (al = a.length) > 0) {
1064                 int index = (al - 1) & s;
1065                 long offset = ((long)index << ASHIFT) + ABASE;
1066                 ForkJoinTask<?> t = (ForkJoinTask<?>) U.getObject(a, offset);
1067                 if (t == task &&
1068                     U.compareAndSwapInt(this, QLOCK, 0, 1)) {
1069                     if (top == s + 1 && array == a &&
1070                         U.compareAndSwapObject(a, offset, task, null)) {
1071                         popped = true;
1072                         top = s;
1073                     }
1074                     U.putIntRelease(this, QLOCK, 0);
1075                 }
1076             }
1077             return popped;
1078         }
1079 
1080         /**
1081          * Removes and cancels all known tasks, ignoring any exceptions.
1082          */
1083         final void cancelAll() {
1084             ForkJoinTask<?> t;
1085             if ((t = currentJoin) != null) {
1086                 currentJoin = null;
1087                 ForkJoinTask.cancelIgnoringExceptions(t);
1088             }
1089             if ((t = currentSteal) != null) {
1090                 currentSteal = null;
1091                 ForkJoinTask.cancelIgnoringExceptions(t);
1092             }
1093             while ((t = poll()) != null)
1094                 ForkJoinTask.cancelIgnoringExceptions(t);
1095         }
1096 
1097         // Specialized execution methods
1098 
1099         /**
1100          * Pops and executes up to POLL_LIMIT tasks or until empty.
1101          */
1102         final void localPopAndExec() {
1103             for (int nexec = 0;;) {
1104                 int b = base, s = top, al; ForkJoinTask<?>[] a;
1105                 if ((a = array) != null && b != s && (al = a.length) > 0) {
1106                     int index = (al - 1) & --s;
1107                     long offset = ((long)index << ASHIFT) + ABASE;
1108                     ForkJoinTask<?> t = (ForkJoinTask<?>)
1109                         U.getAndSetObject(a, offset, null);
1110                     if (t != null) {
1111                         top = s;
1112                         (currentSteal = t).doExec();
1113                         if (++nexec > POLL_LIMIT)
1114                             break;
1115                     }
1116                     else
1117                         break;
1118                 }
1119                 else
1120                     break;
1121             }
1122         }
1123 
1124         /**
1125          * Polls and executes up to POLL_LIMIT tasks or until empty.
1126          */
1127         final void localPollAndExec() {
1128             for (int nexec = 0;;) {
1129                 int b = base, s = top, al; ForkJoinTask<?>[] a;
1130                 if ((a = array) != null && b != s && (al = a.length) > 0) {
1131                     int index = (al - 1) & b++;
1132                     long offset = ((long)index << ASHIFT) + ABASE;
1133                     ForkJoinTask<?> t = (ForkJoinTask<?>)
1134                         U.getAndSetObject(a, offset, null);
1135                     if (t != null) {
1136                         base = b;
1137                         t.doExec();
1138                         if (++nexec > POLL_LIMIT)
1139                             break;
1140                     }
1141                 }
1142                 else
1143                     break;
1144             }
1145         }
1146 
1147         /**
1148          * Executes the given task and (some) remaining local tasks.
1149          */
1150         final void runTask(ForkJoinTask<?> task) {
1151             if (task != null) {
1152                 task.doExec();
1153                 if (config < 0)
1154                     localPollAndExec();
1155                 else
1156                     localPopAndExec();
1157                 int ns = ++nsteals;
1158                 ForkJoinWorkerThread thread = owner;
1159                 currentSteal = null;
1160                 if (ns < 0)           // collect on overflow
1161                     transferStealCount(pool);
1162                 if (thread != null)
1163                     thread.afterTopLevelExec();
1164             }
1165         }
1166 
1167         /**
1168          * Adds steal count to pool steal count if it exists, and resets.
1169          */
1170         final void transferStealCount(ForkJoinPool p) {
1171             AuxState aux;
1172             if (p != null && (aux = p.auxState) != null) {
1173                 long s = nsteals;
1174                 nsteals = 0;            // if negative, correct for overflow
1175                 if (s < 0) s = Integer.MAX_VALUE;
1176                 aux.lock();
1177                 try {
1178                     aux.stealCount += s;
1179                 } finally {
1180                     aux.unlock();
1181                 }
1182             }
1183         }
1184 
1185         /**
1186          * If present, removes from queue and executes the given task,
1187          * or any other cancelled task. Used only by awaitJoin.
1188          *
1189          * @return true if queue empty and task not known to be done
1190          */
1191         final boolean tryRemoveAndExec(ForkJoinTask<?> task) {
1192             if (task != null && task.status >= 0) {
1193                 int b, s, d, al; ForkJoinTask<?>[] a;
1194                 while ((d = (b = base) - (s = top)) < 0 &&
1195                        (a = array) != null && (al = a.length) > 0) {
1196                     for (;;) {      // traverse from s to b
1197                         int index = --s & (al - 1);
1198                         long offset = (index << ASHIFT) + ABASE;
1199                         ForkJoinTask<?> t = (ForkJoinTask<?>)
1200                             U.getObjectVolatile(a, offset);
1201                         if (t == null)
1202                             break;                   // restart
1203                         else if (t == task) {
1204                             boolean removed = false;
1205                             if (s + 1 == top) {      // pop
1206                                 if (U.compareAndSwapObject(a, offset, t, null)) {
1207                                     top = s;
1208                                     removed = true;
1209                                 }
1210                             }
1211                             else if (base == b)      // replace with proxy
1212                                 removed = U.compareAndSwapObject(a, offset, t,
1213                                                                  new EmptyTask());
1214                             if (removed) {
1215                                 ForkJoinTask<?> ps = currentSteal;
1216                                 (currentSteal = task).doExec();
1217                                 currentSteal = ps;
1218                             }
1219                             break;
1220                         }
1221                         else if (t.status < 0 && s + 1 == top) {
1222                             if (U.compareAndSwapObject(a, offset, t, null)) {
1223                                 top = s;
1224                             }
1225                             break;                  // was cancelled
1226                         }
1227                         else if (++d == 0) {
1228                             if (base != b)          // rescan
1229                                 break;
1230                             return false;
1231                         }
1232                     }
1233                     if (task.status < 0)
1234                         return false;
1235                 }
1236             }
1237             return true;
1238         }
1239 
1240         /**
1241          * Pops task if in the same CC computation as the given task,
1242          * in either shared or owned mode. Used only by helpComplete.
1243          */
1244         final CountedCompleter<?> popCC(CountedCompleter<?> task, int mode) {
1245             int b = base, s = top, al; ForkJoinTask<?>[] a;
1246             if ((a = array) != null && b != s && (al = a.length) > 0) {
1247                 int index = (al - 1) & (s - 1);
1248                 long offset = ((long)index << ASHIFT) + ABASE;
1249                 ForkJoinTask<?> o = (ForkJoinTask<?>)
1250                     U.getObjectVolatile(a, offset);
1251                 if (o instanceof CountedCompleter) {
1252                     CountedCompleter<?> t = (CountedCompleter<?>)o;
1253                     for (CountedCompleter<?> r = t;;) {
1254                         if (r == task) {
1255                             if ((mode & IS_OWNED) == 0) {
1256                                 boolean popped = false;
1257                                 if (U.compareAndSwapInt(this, QLOCK, 0, 1)) {
1258                                     if (top == s && array == a &&
1259                                         U.compareAndSwapObject(a, offset,
1260                                                                t, null)) {
1261                                         popped = true;
1262                                         top = s - 1;
1263                                     }
1264                                     U.putIntRelease(this, QLOCK, 0);
1265                                     if (popped)
1266                                         return t;
1267                                 }
1268                             }
1269                             else if (U.compareAndSwapObject(a, offset,
1270                                                             t, null)) {
1271                                 top = s - 1;
1272                                 return t;
1273                             }
1274                             break;
1275                         }
1276                         else if ((r = r.completer) == null) // try parent
1277                             break;
1278                     }
1279                 }
1280             }
1281             return null;
1282         }
1283 
1284         /**
1285          * Steals and runs a task in the same CC computation as the
1286          * given task if one exists and can be taken without
1287          * contention. Otherwise returns a checksum/control value for
1288          * use by method helpComplete.
1289          *
1290          * @return 1 if successful, 2 if retryable (lost to another
1291          * stealer), -1 if non-empty but no matching task found, else
1292          * the base index, forced negative.
1293          */
1294         final int pollAndExecCC(CountedCompleter<?> task) {
1295             ForkJoinTask<?>[] a;
1296             int b = base, s = top, al, h;
1297             if ((a = array) != null && b != s && (al = a.length) > 0) {
1298                 int index = (al - 1) & b;
1299                 long offset = ((long)index << ASHIFT) + ABASE;
1300                 ForkJoinTask<?> o = (ForkJoinTask<?>)
1301                     U.getObjectVolatile(a, offset);
1302                 if (o == null)
1303                     h = 2;                      // retryable
1304                 else if (!(o instanceof CountedCompleter))
1305                     h = -1;                     // unmatchable
1306                 else {
1307                     CountedCompleter<?> t = (CountedCompleter<?>)o;
1308                     for (CountedCompleter<?> r = t;;) {
1309                         if (r == task) {
1310                             if (b++ == base &&
1311                                 U.compareAndSwapObject(a, offset, t, null)) {
1312                                 base = b;
1313                                 t.doExec();
1314                                 h = 1;          // success
1315                             }
1316                             else
1317                                 h = 2;          // lost CAS
1318                             break;
1319                         }
1320                         else if ((r = r.completer) == null) {
1321                             h = -1;             // unmatched
1322                             break;
1323                         }
1324                     }
1325                 }
1326             }
1327             else
1328                 h = b | Integer.MIN_VALUE;      // to sense movement on re-poll
1329             return h;
1330         }
1331 
1332         /**
1333          * Returns true if owned and not known to be blocked.
1334          */
1335         final boolean isApparentlyUnblocked() {
1336             Thread wt; Thread.State s;
1337             return (scanState >= 0 &&
1338                     (wt = owner) != null &&
1339                     (s = wt.getState()) != Thread.State.BLOCKED &&
1340                     s != Thread.State.WAITING &&
1341                     s != Thread.State.TIMED_WAITING);
1342         }
1343 
1344         // Unsafe mechanics. Note that some are (and must be) the same as in FJP
1345         private static final jdk.internal.misc.Unsafe U = jdk.internal.misc.Unsafe.getUnsafe();
1346         private static final long QLOCK;
1347         private static final int ABASE;
1348         private static final int ASHIFT;
1349         static {
1350             try {
1351                 QLOCK = U.objectFieldOffset
1352                     (WorkQueue.class.getDeclaredField("qlock"));
1353                 ABASE = U.arrayBaseOffset(ForkJoinTask[].class);
1354                 int scale = U.arrayIndexScale(ForkJoinTask[].class);
1355                 if ((scale & (scale - 1)) != 0)
1356                     throw new Error("array index scale not a power of two");
1357                 ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
1358             } catch (ReflectiveOperationException e) {
1359                 throw new Error(e);
1360             }
1361         }
1362     }
1363 
1364     // static fields (initialized in static initializer below)
1365 
1366     /**
1367      * Creates a new ForkJoinWorkerThread. This factory is used unless
1368      * overridden in ForkJoinPool constructors.
1369      */
1370     public static final ForkJoinWorkerThreadFactory
1371         defaultForkJoinWorkerThreadFactory;
1372 
1373     /**
1374      * Permission required for callers of methods that may start or
1375      * kill threads.  Also used as a static lock in tryInitialize.
1376      */
1377     static final RuntimePermission modifyThreadPermission;
1378 
1379     /**
1380      * Common (static) pool. Non-null for public use unless a static
1381      * construction exception, but internal usages null-check on use
1382      * to paranoically avoid potential initialization circularities
1383      * as well as to simplify generated code.
1384      */
1385     static final ForkJoinPool common;
1386 
1387     /**
1388      * Common pool parallelism. To allow simpler use and management
1389      * when common pool threads are disabled, we allow the underlying
1390      * common.parallelism field to be zero, but in that case still report
1391      * parallelism as 1 to reflect resulting caller-runs mechanics.
1392      */
1393     static final int COMMON_PARALLELISM;
1394 
1395     /**
1396      * Limit on spare thread construction in tryCompensate.
1397      */
1398     private static final int COMMON_MAX_SPARES;
1399 
1400     /**
1401      * Sequence number for creating workerNamePrefix.
1402      */
1403     private static int poolNumberSequence;
1404 
1405     /**
1406      * Returns the next sequence number. We don't expect this to
1407      * ever contend, so use simple builtin sync.
1408      */
1409     private static final synchronized int nextPoolId() {
1410         return ++poolNumberSequence;
1411     }
1412 
1413     // static configuration constants
1414 
1415     /**
1416      * Initial timeout value (in milliseconds) for the thread
1417      * triggering quiescence to park waiting for new work. On timeout,
1418      * the thread will instead try to shrink the number of workers.
1419      * The value should be large enough to avoid overly aggressive
1420      * shrinkage during most transient stalls (long GCs etc).
1421      */
1422     private static final long IDLE_TIMEOUT_MS = 2000L; // 2sec
1423 
1424     /**
1425      * Tolerance for idle timeouts, to cope with timer undershoots.
1426      */
1427     private static final long TIMEOUT_SLOP_MS =   20L; // 20ms
1428 
1429     /**
1430      * The default value for COMMON_MAX_SPARES.  Overridable using the
1431      * "java.util.concurrent.ForkJoinPool.common.maximumSpares" system
1432      * property.  The default value is far in excess of normal
1433      * requirements, but also far short of MAX_CAP and typical OS
1434      * thread limits, so allows JVMs to catch misuse/abuse before
1435      * running out of resources needed to do so.
1436      */
1437     private static final int DEFAULT_COMMON_MAX_SPARES = 256;
1438 
1439     /**
1440      * Increment for seed generators. See class ThreadLocal for
1441      * explanation.
1442      */
1443     private static final int SEED_INCREMENT = 0x9e3779b9;
1444 
1445     /*
1446      * Bits and masks for field ctl, packed with 4 16 bit subfields:
1447      * AC: Number of active running workers minus target parallelism
1448      * TC: Number of total workers minus target parallelism
1449      * SS: version count and status of top waiting thread
1450      * ID: poolIndex of top of Treiber stack of waiters
1451      *
1452      * When convenient, we can extract the lower 32 stack top bits
1453      * (including version bits) as sp=(int)ctl.  The offsets of counts
1454      * by the target parallelism and the positionings of fields makes
1455      * it possible to perform the most common checks via sign tests of
1456      * fields: When ac is negative, there are not enough active
1457      * workers, when tc is negative, there are not enough total
1458      * workers.  When sp is non-zero, there are waiting workers.  To
1459      * deal with possibly negative fields, we use casts in and out of
1460      * "short" and/or signed shifts to maintain signedness.
1461      *
1462      * Because it occupies uppermost bits, we can add one active count
1463      * using getAndAddLong of AC_UNIT, rather than CAS, when returning
1464      * from a blocked join.  Other updates entail multiple subfields
1465      * and masking, requiring CAS.
1466      */
1467 
1468     // Lower and upper word masks
1469     private static final long SP_MASK    = 0xffffffffL;
1470     private static final long UC_MASK    = ~SP_MASK;
1471 
1472     // Active counts
1473     private static final int  AC_SHIFT   = 48;
1474     private static final long AC_UNIT    = 0x0001L << AC_SHIFT;
1475     private static final long AC_MASK    = 0xffffL << AC_SHIFT;
1476 
1477     // Total counts
1478     private static final int  TC_SHIFT   = 32;
1479     private static final long TC_UNIT    = 0x0001L << TC_SHIFT;
1480     private static final long TC_MASK    = 0xffffL << TC_SHIFT;
1481     private static final long ADD_WORKER = 0x0001L << (TC_SHIFT + 15); // sign
1482 
1483     // runState bits: SHUTDOWN must be negative, others arbitrary powers of two
1484     private static final int  STARTED    = 1;
1485     private static final int  STOP       = 1 << 1;
1486     private static final int  TERMINATED = 1 << 2;
1487     private static final int  SHUTDOWN   = 1 << 31;
1488 
1489     // Instance fields
1490     volatile long ctl;                   // main pool control
1491     volatile int runState;
1492     final int config;                    // parallelism, mode
1493     AuxState auxState;                   // lock, steal counts
1494     volatile WorkQueue[] workQueues;     // main registry
1495     final String workerNamePrefix;       // to create worker name string
1496     final ForkJoinWorkerThreadFactory factory;
1497     final UncaughtExceptionHandler ueh;  // per-worker UEH
1498 
1499     /**
1500      * Instantiates fields upon first submission, or upon shutdown if
1501      * no submissions. If checkTermination true, also responds to
1502      * termination by external calls submitting tasks.
1503      */
1504     private void tryInitialize(boolean checkTermination) {
1505         if (runState == 0) { // bootstrap by locking static field
1506             int p = config & SMASK;
1507             int n = (p > 1) ? p - 1 : 1; // ensure at least 2 slots
1508             n |= n >>> 1;    // create workQueues array with size a power of two
1509             n |= n >>> 2;
1510             n |= n >>> 4;
1511             n |= n >>> 8;
1512             n |= n >>> 16;
1513             n = ((n + 1) << 1) & SMASK;
1514             AuxState aux = new AuxState();
1515             WorkQueue[] ws = new WorkQueue[n];
1516             synchronized (modifyThreadPermission) { // double-check
1517                 if (runState == 0) {
1518                     workQueues = ws;
1519                     auxState = aux;
1520                     runState = STARTED;
1521                 }
1522             }
1523         }
1524         if (checkTermination && runState < 0) {
1525             tryTerminate(false, false); // help terminate
1526             throw new RejectedExecutionException();
1527         }
1528     }
1529 
1530     // Creating, registering and deregistering workers
1531 
1532     /**
1533      * Tries to construct and start one worker. Assumes that total
1534      * count has already been incremented as a reservation.  Invokes
1535      * deregisterWorker on any failure.
1536      *
1537      * @param isSpare true if this is a spare thread
1538      * @return true if successful
1539      */
1540     private boolean createWorker(boolean isSpare) {
1541         ForkJoinWorkerThreadFactory fac = factory;
1542         Throwable ex = null;
1543         ForkJoinWorkerThread wt = null;
1544         WorkQueue q;
1545         try {
1546             if (fac != null && (wt = fac.newThread(this)) != null) {
1547                 if (isSpare && (q = wt.workQueue) != null)
1548                     q.config |= SPARE_WORKER;
1549                 wt.start();
1550                 return true;
1551             }
1552         } catch (Throwable rex) {
1553             ex = rex;
1554         }
1555         deregisterWorker(wt, ex);
1556         return false;
1557     }
1558 
1559     /**
1560      * Tries to add one worker, incrementing ctl counts before doing
1561      * so, relying on createWorker to back out on failure.
1562      *
1563      * @param c incoming ctl value, with total count negative and no
1564      * idle workers.  On CAS failure, c is refreshed and retried if
1565      * this holds (otherwise, a new worker is not needed).
1566      */
1567     private void tryAddWorker(long c) {
1568         do {
1569             long nc = ((AC_MASK & (c + AC_UNIT)) |
1570                        (TC_MASK & (c + TC_UNIT)));
1571             if (ctl == c && U.compareAndSwapLong(this, CTL, c, nc)) {
1572                 createWorker(false);
1573                 break;
1574             }
1575         } while (((c = ctl) & ADD_WORKER) != 0L && (int)c == 0);
1576     }
1577 
1578     /**
1579      * Callback from ForkJoinWorkerThread constructor to establish and
1580      * record its WorkQueue.
1581      *
1582      * @param wt the worker thread
1583      * @return the worker's queue
1584      */
1585     final WorkQueue registerWorker(ForkJoinWorkerThread wt) {
1586         UncaughtExceptionHandler handler;
1587         AuxState aux;
1588         wt.setDaemon(true);                           // configure thread
1589         if ((handler = ueh) != null)
1590             wt.setUncaughtExceptionHandler(handler);
1591         WorkQueue w = new WorkQueue(this, wt);
1592         int i = 0;                                    // assign a pool index
1593         int mode = config & MODE_MASK;
1594         if ((aux = auxState) != null) {
1595             aux.lock();
1596             try {
1597                 int s = (int)(aux.indexSeed += SEED_INCREMENT), n, m;
1598                 WorkQueue[] ws = workQueues;
1599                 if (ws != null && (n = ws.length) > 0) {
1600                     i = (m = n - 1) & ((s << 1) | 1); // odd-numbered indices
1601                     if (ws[i] != null) {              // collision
1602                         int probes = 0;               // step by approx half n
1603                         int step = (n <= 4) ? 2 : ((n >>> 1) & EVENMASK) + 2;
1604                         while (ws[i = (i + step) & m] != null) {
1605                             if (++probes >= n) {
1606                                 workQueues = ws = Arrays.copyOf(ws, n <<= 1);
1607                                 m = n - 1;
1608                                 probes = 0;
1609                             }
1610                         }
1611                     }
1612                     w.hint = s;                       // use as random seed
1613                     w.config = i | mode;
1614                     w.scanState = i | (s & 0x7fff0000); // random seq bits
1615                     ws[i] = w;
1616                 }
1617             } finally {
1618                 aux.unlock();
1619             }
1620         }
1621         wt.setName(workerNamePrefix.concat(Integer.toString(i >>> 1)));
1622         return w;
1623     }
1624 
1625     /**
1626      * Final callback from terminating worker, as well as upon failure
1627      * to construct or start a worker.  Removes record of worker from
1628      * array, and adjusts counts. If pool is shutting down, tries to
1629      * complete termination.
1630      *
1631      * @param wt the worker thread, or null if construction failed
1632      * @param ex the exception causing failure, or null if none
1633      */
1634     final void deregisterWorker(ForkJoinWorkerThread wt, Throwable ex) {
1635         WorkQueue w = null;
1636         if (wt != null && (w = wt.workQueue) != null) {
1637             AuxState aux; WorkQueue[] ws;          // remove index from array
1638             int idx = w.config & SMASK;
1639             int ns = w.nsteals;
1640             if ((aux = auxState) != null) {
1641                 aux.lock();
1642                 try {
1643                     if ((ws = workQueues) != null && ws.length > idx &&
1644                         ws[idx] == w)
1645                         ws[idx] = null;
1646                     aux.stealCount += ns;
1647                 } finally {
1648                     aux.unlock();
1649                 }
1650             }
1651         }
1652         if (w == null || (w.config & UNREGISTERED) == 0) { // else pre-adjusted
1653             long c;                                   // decrement counts
1654             do {} while (!U.compareAndSwapLong
1655                          (this, CTL, c = ctl, ((AC_MASK & (c - AC_UNIT)) |
1656                                                (TC_MASK & (c - TC_UNIT)) |
1657                                                (SP_MASK & c))));
1658         }
1659         if (w != null) {
1660             w.currentSteal = null;
1661             w.qlock = -1;                             // ensure set
1662             w.cancelAll();                            // cancel remaining tasks
1663         }
1664         while (tryTerminate(false, false) >= 0) {     // possibly replace
1665             WorkQueue[] ws; int wl, sp; long c;
1666             if (w == null || w.array == null ||
1667                 (ws = workQueues) == null || (wl = ws.length) <= 0)
1668                 break;
1669             else if ((sp = (int)(c = ctl)) != 0) {    // wake up replacement
1670                 if (tryRelease(c, ws[(wl - 1) & sp], AC_UNIT))
1671                     break;
1672             }
1673             else if (ex != null && (c & ADD_WORKER) != 0L) {
1674                 tryAddWorker(c);                      // create replacement
1675                 break;
1676             }
1677             else                                      // don't need replacement
1678                 break;
1679         }
1680         if (ex == null)                               // help clean on way out
1681             ForkJoinTask.helpExpungeStaleExceptions();
1682         else                                          // rethrow
1683             ForkJoinTask.rethrow(ex);
1684     }
1685 
1686     // Signalling
1687 
1688     /**
1689      * Tries to create or activate a worker if too few are active.
1690      */
1691     final void signalWork() {
1692         for (;;) {
1693             long c; int sp, i; WorkQueue v; WorkQueue[] ws;
1694             if ((c = ctl) >= 0L)                      // enough workers
1695                 break;
1696             else if ((sp = (int)c) == 0) {            // no idle workers
1697                 if ((c & ADD_WORKER) != 0L)           // too few workers
1698                     tryAddWorker(c);
1699                 break;
1700             }
1701             else if ((ws = workQueues) == null)
1702                 break;                                // unstarted/terminated
1703             else if (ws.length <= (i = sp & SMASK))
1704                 break;                                // terminated
1705             else if ((v = ws[i]) == null)
1706                 break;                                // terminating
1707             else {
1708                 int ns = sp & ~UNSIGNALLED;
1709                 int vs = v.scanState;
1710                 long nc = (v.stackPred & SP_MASK) | (UC_MASK & (c + AC_UNIT));
1711                 if (sp == vs && U.compareAndSwapLong(this, CTL, c, nc)) {
1712                     v.scanState = ns;
1713                     LockSupport.unpark(v.parker);
1714                     break;
1715                 }
1716             }
1717         }
1718     }
1719 
1720     /**
1721      * Signals and releases worker v if it is top of idle worker
1722      * stack.  This performs a one-shot version of signalWork only if
1723      * there is (apparently) at least one idle worker.
1724      *
1725      * @param c incoming ctl value
1726      * @param v if non-null, a worker
1727      * @param inc the increment to active count (zero when compensating)
1728      * @return true if successful
1729      */
1730     private boolean tryRelease(long c, WorkQueue v, long inc) {
1731         int sp = (int)c, ns = sp & ~UNSIGNALLED;
1732         if (v != null) {
1733             int vs = v.scanState;
1734             long nc = (v.stackPred & SP_MASK) | (UC_MASK & (c + inc));
1735             if (sp == vs && U.compareAndSwapLong(this, CTL, c, nc)) {
1736                 v.scanState = ns;
1737                 LockSupport.unpark(v.parker);
1738                 return true;
1739             }
1740         }
1741         return false;
1742     }
1743 
1744     /**
1745      * With approx probability of a missed signal, tries (once) to
1746      * reactivate worker w (or some other worker), failing if stale or
1747      * known to be already active.
1748      *
1749      * @param w the worker
1750      * @param ws the workQueue array to use
1751      * @param r random seed
1752      */
1753     private void tryReactivate(WorkQueue w, WorkQueue[] ws, int r) {
1754         long c; int sp, wl; WorkQueue v;
1755         if ((sp = (int)(c = ctl)) != 0 && w != null &&
1756             ws != null && (wl = ws.length) > 0 &&
1757             ((sp ^ r) & SS_SEQ) == 0 &&
1758             (v = ws[(wl - 1) & sp]) != null) {
1759             long nc = (v.stackPred & SP_MASK) | (UC_MASK & (c + AC_UNIT));
1760             int ns = sp & ~UNSIGNALLED;
1761             if (w.scanState < 0 &&
1762                 v.scanState == sp &&
1763                 U.compareAndSwapLong(this, CTL, c, nc)) {
1764                 v.scanState = ns;
1765                 LockSupport.unpark(v.parker);
1766             }
1767         }
1768     }
1769 
1770     /**
1771      * If worker w exists and is active, enqueues and sets status to inactive.
1772      *
1773      * @param w the worker
1774      * @param ss current (non-negative) scanState
1775      */
1776     private void inactivate(WorkQueue w, int ss) {
1777         int ns = (ss + SS_SEQ) | UNSIGNALLED;
1778         long lc = ns & SP_MASK, nc, c;
1779         if (w != null) {
1780             w.scanState = ns;
1781             do {
1782                 nc = lc | (UC_MASK & ((c = ctl) - AC_UNIT));
1783                 w.stackPred = (int)c;
1784             } while (!U.compareAndSwapLong(this, CTL, c, nc));
1785         }
1786     }
1787 
1788     /**
1789      * Possibly blocks worker w waiting for signal, or returns
1790      * negative status if the worker should terminate. May return
1791      * without status change if multiple stale unparks and/or
1792      * interrupts occur.
1793      *
1794      * @param w the calling worker
1795      * @return negative if w should terminate
1796      */
1797     private int awaitWork(WorkQueue w) {
1798         int stat = 0;
1799         if (w != null && w.scanState < 0) {
1800             long c = ctl;
1801             if ((int)(c >> AC_SHIFT) + (config & SMASK) <= 0)
1802                 stat = timedAwaitWork(w, c);     // possibly quiescent
1803             else if ((runState & STOP) != 0)
1804                 stat = w.qlock = -1;             // pool terminating
1805             else if (w.scanState < 0) {
1806                 w.parker = Thread.currentThread();
1807                 if (w.scanState < 0)             // recheck after write
1808                     LockSupport.park(this);
1809                 w.parker = null;
1810                 if ((runState & STOP) != 0)
1811                     stat = w.qlock = -1;         // recheck
1812                 else if (w.scanState < 0)
1813                     Thread.interrupted();        // clear status
1814             }
1815         }
1816         return stat;
1817     }
1818 
1819     /**
1820      * Possibly triggers shutdown and tries (once) to block worker
1821      * when pool is (or may be) quiescent. Waits up to a duration
1822      * determined by number of workers.  On timeout, if ctl has not
1823      * changed, terminates the worker, which will in turn wake up
1824      * another worker to possibly repeat this process.
1825      *
1826      * @param w the calling worker
1827      * @return negative if w should terminate
1828      */
1829     private int timedAwaitWork(WorkQueue w, long c) {
1830         int stat = 0;
1831         int scale = 1 - (short)(c >>> TC_SHIFT);
1832         long deadline = (((scale <= 0) ? 1 : scale) * IDLE_TIMEOUT_MS +
1833                          System.currentTimeMillis());
1834         if ((runState >= 0 || (stat = tryTerminate(false, false)) > 0) &&
1835             w != null && w.scanState < 0) {
1836             int ss; AuxState aux;
1837             w.parker = Thread.currentThread();
1838             if (w.scanState < 0)
1839                 LockSupport.parkUntil(this, deadline);
1840             w.parker = null;
1841             if ((runState & STOP) != 0)
1842                 stat = w.qlock = -1;         // pool terminating
1843             else if ((ss = w.scanState) < 0 && !Thread.interrupted() &&
1844                      (int)c == ss && (aux = auxState) != null && ctl == c &&
1845                      deadline - System.currentTimeMillis() <= TIMEOUT_SLOP_MS) {
1846                 aux.lock();
1847                 try {                        // pre-deregister
1848                     WorkQueue[] ws;
1849                     int cfg = w.config, idx = cfg & SMASK;
1850                     long nc = ((UC_MASK & (c - TC_UNIT)) |
1851                                (SP_MASK & w.stackPred));
1852                     if ((runState & STOP) == 0 &&
1853                         (ws = workQueues) != null &&
1854                         idx < ws.length && idx >= 0 && ws[idx] == w &&
1855                         U.compareAndSwapLong(this, CTL, c, nc)) {
1856                         ws[idx] = null;
1857                         w.config = cfg | UNREGISTERED;
1858                         stat = w.qlock = -1;
1859                     }
1860                 } finally {
1861                     aux.unlock();
1862                 }
1863             }
1864         }
1865         return stat;
1866     }
1867 
1868     /**
1869      * If the given worker is a spare with no queued tasks, and there
1870      * are enough existing workers, drops it from ctl counts and sets
1871      * its state to terminated.
1872      *
1873      * @param w the calling worker -- must be a spare
1874      * @return true if dropped (in which case it must not process more tasks)
1875      */
1876     private boolean tryDropSpare(WorkQueue w) {
1877         if (w != null && w.isEmpty()) {           // no local tasks
1878             long c; int sp, wl; WorkQueue[] ws; WorkQueue v;
1879             while ((short)((c = ctl) >> TC_SHIFT) > 0 &&
1880                    ((sp = (int)c) != 0 || (int)(c >> AC_SHIFT) > 0) &&
1881                    (ws = workQueues) != null && (wl = ws.length) > 0) {
1882                 boolean dropped, canDrop;
1883                 if (sp == 0) {                    // no queued workers
1884                     long nc = ((AC_MASK & (c - AC_UNIT)) |
1885                                (TC_MASK & (c - TC_UNIT)) | (SP_MASK & c));
1886                     dropped = U.compareAndSwapLong(this, CTL, c, nc);
1887                 }
1888                 else if (
1889                     (v = ws[(wl - 1) & sp]) == null || v.scanState != sp)
1890                     dropped = false;              // stale; retry
1891                 else {
1892                     long nc = v.stackPred & SP_MASK;
1893                     if (w == v || w.scanState >= 0) {
1894                         canDrop = true;           // w unqueued or topmost
1895                         nc |= ((AC_MASK & c) |    // ensure replacement
1896                                (TC_MASK & (c - TC_UNIT)));
1897                     }
1898                     else {                        // w may be queued
1899                         canDrop = false;          // help uncover
1900                         nc |= ((AC_MASK & (c + AC_UNIT)) |
1901                                (TC_MASK & c));
1902                     }
1903                     if (U.compareAndSwapLong(this, CTL, c, nc)) {
1904                         v.scanState = sp & ~UNSIGNALLED;
1905                         LockSupport.unpark(v.parker);
1906                         dropped = canDrop;
1907                     }
1908                     else
1909                         dropped = false;
1910                 }
1911                 if (dropped) {                    // pre-deregister
1912                     int cfg = w.config, idx = cfg & SMASK;
1913                     if (idx >= 0 && idx < ws.length && ws[idx] == w)
1914                         ws[idx] = null;
1915                     w.config = cfg | UNREGISTERED;
1916                     w.qlock = -1;
1917                     return true;
1918                 }
1919             }
1920         }
1921         return false;
1922     }
1923 
1924     /**
1925      * Top-level runloop for workers, called by ForkJoinWorkerThread.run.
1926      */
1927     final void runWorker(WorkQueue w) {
1928         w.growArray();                                  // allocate queue
1929         int bound = (w.config & SPARE_WORKER) != 0 ? 0 : POLL_LIMIT;
1930         long seed = w.hint * 0xdaba0b6eb09322e3L;       // initial random seed
1931         if ((runState & STOP) == 0) {
1932             for (long r = (seed == 0L) ? 1L : seed;;) { // ensure nonzero
1933                 if (bound == 0 && tryDropSpare(w))
1934                     break;
1935                 // high bits of prev seed for step; current low bits for idx
1936                 int step = (int)(r >>> 48) | 1;
1937                 r ^= r >>> 12; r ^= r << 25; r ^= r >>> 27; // xorshift
1938                 if (scan(w, bound, step, (int)r) < 0 && awaitWork(w) < 0)
1939                     break;
1940             }
1941         }
1942     }
1943 
1944     // Scanning for tasks
1945 
1946     /**
1947      * Repeatedly scans for and tries to steal and execute (via
1948      * workQueue.runTask) a queued task. Each scan traverses queues in
1949      * pseudorandom permutation. Upon finding a non-empty queue, makes
1950      * at most the given bound attempts to re-poll (fewer if
1951      * contended) on the same queue before returning (impossible
1952      * scanState value) 0 to restart scan. Else returns after at least
1953      * 1 and at most 32 full scans.
1954      *
1955      * @param w the worker (via its WorkQueue)
1956      * @param bound repoll bound as bitmask (0 if spare)
1957      * @param step (circular) index increment per iteration (must be odd)
1958      * @param r a random seed for origin index
1959      * @return negative if should await signal
1960      */
1961     private int scan(WorkQueue w, int bound, int step, int r) {
1962         int stat = 0, wl; WorkQueue[] ws;
1963         if ((ws = workQueues) != null && w != null && (wl = ws.length) > 0) {
1964             for (int m = wl - 1,
1965                      origin = m & r, idx = origin,
1966                      npolls = 0,
1967                      ss = w.scanState;;) {         // negative if inactive
1968                 WorkQueue q; ForkJoinTask<?>[] a; int b, al;
1969                 if ((q = ws[idx]) != null && (b = q.base) - q.top < 0 &&
1970                     (a = q.array) != null && (al = a.length) > 0) {
1971                     int index = (al - 1) & b;
1972                     long offset = ((long)index << ASHIFT) + ABASE;
1973                     ForkJoinTask<?> t = (ForkJoinTask<?>)
1974                         U.getObjectVolatile(a, offset);
1975                     if (t == null)
1976                         break;                     // empty or busy
1977                     else if (b++ != q.base)
1978                         break;                     // busy
1979                     else if (ss < 0) {
1980                         tryReactivate(w, ws, r);
1981                         break;                     // retry upon rescan
1982                     }
1983                     else if (!U.compareAndSwapObject(a, offset, t, null))
1984                         break;                     // contended
1985                     else {
1986                         q.base = b;
1987                         w.currentSteal = t;
1988                         if (b != q.top)            // propagate signal
1989                             signalWork();
1990                         w.runTask(t);
1991                         if (++npolls > bound)
1992                             break;
1993                     }
1994                 }
1995                 else if (npolls != 0)              // rescan
1996                     break;
1997                 else if ((idx = (idx + step) & m) == origin) {
1998                     if (ss < 0) {                  // await signal
1999                         stat = ss;
2000                         break;
2001                     }
2002                     else if (r >= 0) {
2003                         inactivate(w, ss);
2004                         break;
2005                     }
2006                     else
2007                         r <<= 1;                   // at most 31 rescans
2008                 }
2009             }
2010         }
2011         return stat;
2012     }
2013 
2014     // Joining tasks
2015 
2016     /**
2017      * Tries to steal and run tasks within the target's computation.
2018      * Uses a variant of the top-level algorithm, restricted to tasks
2019      * with the given task as ancestor: It prefers taking and running
2020      * eligible tasks popped from the worker's own queue (via
2021      * popCC). Otherwise it scans others, randomly moving on
2022      * contention or execution, deciding to give up based on a
2023      * checksum (via return codes from pollAndExecCC). The maxTasks
2024      * argument supports external usages; internal calls use zero,
2025      * allowing unbounded steps (external calls trap non-positive
2026      * values).
2027      *
2028      * @param w caller
2029      * @param maxTasks if non-zero, the maximum number of other tasks to run
2030      * @return task status on exit
2031      */
2032     final int helpComplete(WorkQueue w, CountedCompleter<?> task,
2033                            int maxTasks) {
2034         WorkQueue[] ws; int s = 0, wl;
2035         if ((ws = workQueues) != null && (wl = ws.length) > 1 &&
2036             task != null && w != null) {
2037             for (int m = wl - 1,
2038                      mode = w.config,
2039                      r = ~mode,                  // scanning seed
2040                      origin = r & m, k = origin, // first queue to scan
2041                      step = 3,                   // first scan step
2042                      h = 1,                      // 1:ran, >1:contended, <0:hash
2043                      oldSum = 0, checkSum = 0;;) {
2044                 CountedCompleter<?> p; WorkQueue q; int i;
2045                 if ((s = task.status) < 0)
2046                     break;
2047                 if (h == 1 && (p = w.popCC(task, mode)) != null) {
2048                     p.doExec();                  // run local task
2049                     if (maxTasks != 0 && --maxTasks == 0)
2050                         break;
2051                     origin = k;                  // reset
2052                     oldSum = checkSum = 0;
2053                 }
2054                 else {                           // poll other worker queues
2055                     if ((i = k | 1) < 0 || i > m || (q = ws[i]) == null)
2056                         h = 0;
2057                     else if ((h = q.pollAndExecCC(task)) < 0)
2058                         checkSum += h;
2059                     if (h > 0) {
2060                         if (h == 1 && maxTasks != 0 && --maxTasks == 0)
2061                             break;
2062                         step = (r >>> 16) | 3;
2063                         r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift
2064                         k = origin = r & m;      // move and restart
2065                         oldSum = checkSum = 0;
2066                     }
2067                     else if ((k = (k + step) & m) == origin) {
2068                         if (oldSum == (oldSum = checkSum))
2069                             break;
2070                         checkSum = 0;
2071                     }
2072                 }
2073             }
2074         }
2075         return s;
2076     }
2077 
2078     /**
2079      * Tries to locate and execute tasks for a stealer of the given
2080      * task, or in turn one of its stealers. Traces currentSteal ->
2081      * currentJoin links looking for a thread working on a descendant
2082      * of the given task and with a non-empty queue to steal back and
2083      * execute tasks from. The first call to this method upon a
2084      * waiting join will often entail scanning/search, (which is OK
2085      * because the joiner has nothing better to do), but this method
2086      * leaves hints in workers to speed up subsequent calls.
2087      *
2088      * @param w caller
2089      * @param task the task to join
2090      */
2091     private void helpStealer(WorkQueue w, ForkJoinTask<?> task) {
2092         if (task != null && w != null) {
2093             ForkJoinTask<?> ps = w.currentSteal;
2094             WorkQueue[] ws; int wl, oldSum = 0;
2095             outer: while (w.tryRemoveAndExec(task) && task.status >= 0 &&
2096                           (ws = workQueues) != null && (wl = ws.length) > 0) {
2097                 ForkJoinTask<?> subtask;
2098                 int m = wl - 1, checkSum = 0;          // for stability check
2099                 WorkQueue j = w, v;                    // v is subtask stealer
2100                 descent: for (subtask = task; subtask.status >= 0; ) {
2101                     for (int h = j.hint | 1, k = 0, i;;) {
2102                         if ((v = ws[i = (h + (k << 1)) & m]) != null) {
2103                             if (v.currentSteal == subtask) {
2104                                 j.hint = i;
2105                                 break;
2106                             }
2107                             checkSum += v.base;
2108                         }
2109                         if (++k > m)                   // can't find stealer
2110                             break outer;
2111                     }
2112 
2113                     for (;;) {                         // help v or descend
2114                         ForkJoinTask<?>[] a; int b, al;
2115                         if (subtask.status < 0)        // too late to help
2116                             break descent;
2117                         checkSum += (b = v.base);
2118                         ForkJoinTask<?> next = v.currentJoin;
2119                         ForkJoinTask<?> t = null;
2120                         if ((a = v.array) != null && (al = a.length) > 0) {
2121                             int index = (al - 1) & b;
2122                             long offset = ((long)index << ASHIFT) + ABASE;
2123                             t = (ForkJoinTask<?>)
2124                                 U.getObjectVolatile(a, offset);
2125                             if (t != null && b++ == v.base) {
2126                                 if (j.currentJoin != subtask ||
2127                                     v.currentSteal != subtask ||
2128                                     subtask.status < 0)
2129                                     break descent;     // stale
2130                                 if (U.compareAndSwapObject(a, offset, t, null)) {
2131                                     v.base = b;
2132                                     w.currentSteal = t;
2133                                     for (int top = w.top;;) {
2134                                         t.doExec();    // help
2135                                         w.currentSteal = ps;
2136                                         if (task.status < 0)
2137                                             break outer;
2138                                         if (w.top == top)
2139                                             break;     // run local tasks
2140                                         if ((t = w.pop()) == null)
2141                                             break descent;
2142                                         w.currentSteal = t;
2143                                     }
2144                                 }
2145                             }
2146                         }
2147                         if (t == null && b == v.base && b - v.top >= 0) {
2148                             if ((subtask = next) == null) {  // try to descend
2149                                 if (next == v.currentJoin &&
2150                                     oldSum == (oldSum = checkSum))
2151                                     break outer;
2152                                 break descent;
2153                             }
2154                             j = v;
2155                             break;
2156                         }
2157                     }
2158                 }
2159             }
2160         }
2161     }
2162 
2163     /**
2164      * Tries to decrement active count (sometimes implicitly) and
2165      * possibly release or create a compensating worker in preparation
2166      * for blocking. Returns false (retryable by caller), on
2167      * contention, detected staleness, instability, or termination.
2168      *
2169      * @param w caller
2170      */
2171     private boolean tryCompensate(WorkQueue w) {
2172         boolean canBlock; int wl;
2173         long c = ctl;
2174         WorkQueue[] ws = workQueues;
2175         int pc = config & SMASK;
2176         int ac = pc + (int)(c >> AC_SHIFT);
2177         int tc = pc + (short)(c >> TC_SHIFT);
2178         if (w == null || w.qlock < 0 || pc == 0 ||  // terminating or disabled
2179             ws == null || (wl = ws.length) <= 0)
2180             canBlock = false;
2181         else {
2182             int m = wl - 1, sp;
2183             boolean busy = true;                    // validate ac
2184             for (int i = 0; i <= m; ++i) {
2185                 int k; WorkQueue v;
2186                 if ((k = (i << 1) | 1) <= m && k >= 0 && (v = ws[k]) != null &&
2187                     v.scanState >= 0 && v.currentSteal == null) {
2188                     busy = false;
2189                     break;
2190                 }
2191             }
2192             if (!busy || ctl != c)
2193                 canBlock = false;                   // unstable or stale
2194             else if ((sp = (int)c) != 0)            // release idle worker
2195                 canBlock = tryRelease(c, ws[m & sp], 0L);
2196             else if (tc >= pc && ac > 1 && w.isEmpty()) {
2197                 long nc = ((AC_MASK & (c - AC_UNIT)) |
2198                            (~AC_MASK & c));         // uncompensated
2199                 canBlock = U.compareAndSwapLong(this, CTL, c, nc);
2200             }
2201             else if (tc >= MAX_CAP ||
2202                      (this == common && tc >= pc + COMMON_MAX_SPARES))
2203                 throw new RejectedExecutionException(
2204                     "Thread limit exceeded replacing blocked worker");
2205             else {                                  // similar to tryAddWorker
2206                 boolean isSpare = (tc >= pc);
2207                 long nc = (AC_MASK & c) | (TC_MASK & (c + TC_UNIT));
2208                 canBlock = (U.compareAndSwapLong(this, CTL, c, nc) &&
2209                             createWorker(isSpare)); // throws on exception
2210             }
2211         }
2212         return canBlock;
2213     }
2214 
2215     /**
2216      * Helps and/or blocks until the given task is done or timeout.
2217      *
2218      * @param w caller
2219      * @param task the task
2220      * @param deadline for timed waits, if nonzero
2221      * @return task status on exit
2222      */
2223     final int awaitJoin(WorkQueue w, ForkJoinTask<?> task, long deadline) {
2224         int s = 0;
2225         if (w != null) {
2226             ForkJoinTask<?> prevJoin = w.currentJoin;
2227             if (task != null && (s = task.status) >= 0) {
2228                 w.currentJoin = task;
2229                 CountedCompleter<?> cc = (task instanceof CountedCompleter) ?
2230                     (CountedCompleter<?>)task : null;
2231                 for (;;) {
2232                     if (cc != null)
2233                         helpComplete(w, cc, 0);
2234                     else
2235                         helpStealer(w, task);
2236                     if ((s = task.status) < 0)
2237                         break;
2238                     long ms, ns;
2239                     if (deadline == 0L)
2240                         ms = 0L;
2241                     else if ((ns = deadline - System.nanoTime()) <= 0L)
2242                         break;
2243                     else if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) <= 0L)
2244                         ms = 1L;
2245                     if (tryCompensate(w)) {
2246                         task.internalWait(ms);
2247                         U.getAndAddLong(this, CTL, AC_UNIT);
2248                     }
2249                     if ((s = task.status) < 0)
2250                         break;
2251                 }
2252                 w.currentJoin = prevJoin;
2253             }
2254         }
2255         return s;
2256     }
2257 
2258     // Specialized scanning
2259 
2260     /**
2261      * Returns a (probably) non-empty steal queue, if one is found
2262      * during a scan, else null.  This method must be retried by
2263      * caller if, by the time it tries to use the queue, it is empty.
2264      */
2265     private WorkQueue findNonEmptyStealQueue() {
2266         WorkQueue[] ws; int wl;  // one-shot version of scan loop
2267         int r = ThreadLocalRandom.nextSecondarySeed();
2268         if ((ws = workQueues) != null && (wl = ws.length) > 0) {
2269             int m = wl - 1, origin = r & m;
2270             for (int k = origin, oldSum = 0, checkSum = 0;;) {
2271                 WorkQueue q; int b;
2272                 if ((q = ws[k]) != null) {
2273                     if ((b = q.base) - q.top < 0)
2274                         return q;
2275                     checkSum += b;
2276                 }
2277                 if ((k = (k + 1) & m) == origin) {
2278                     if (oldSum == (oldSum = checkSum))
2279                         break;
2280                     checkSum = 0;
2281                 }
2282             }
2283         }
2284         return null;
2285     }
2286 
2287     /**
2288      * Runs tasks until {@code isQuiescent()}. We piggyback on
2289      * active count ctl maintenance, but rather than blocking
2290      * when tasks cannot be found, we rescan until all others cannot
2291      * find tasks either.
2292      */
2293     final void helpQuiescePool(WorkQueue w) {
2294         ForkJoinTask<?> ps = w.currentSteal; // save context
2295         int wc = w.config;
2296         for (boolean active = true;;) {
2297             long c; WorkQueue q; ForkJoinTask<?> t;
2298             if (wc >= 0 && (t = w.pop()) != null) { // run locals if LIFO
2299                 (w.currentSteal = t).doExec();
2300                 w.currentSteal = ps;
2301             }
2302             else if ((q = findNonEmptyStealQueue()) != null) {
2303                 if (!active) {      // re-establish active count
2304                     active = true;
2305                     U.getAndAddLong(this, CTL, AC_UNIT);
2306                 }
2307                 if ((t = q.pollAt(q.base)) != null) {
2308                     (w.currentSteal = t).doExec();
2309                     w.currentSteal = ps;
2310                     if (++w.nsteals < 0)
2311                         w.transferStealCount(this);
2312                 }
2313             }
2314             else if (active) {      // decrement active count without queuing
2315                 long nc = (AC_MASK & ((c = ctl) - AC_UNIT)) | (~AC_MASK & c);
2316                 if (U.compareAndSwapLong(this, CTL, c, nc))
2317                     active = false;
2318             }
2319             else if ((int)((c = ctl) >> AC_SHIFT) + (config & SMASK) <= 0 &&
2320                      U.compareAndSwapLong(this, CTL, c, c + AC_UNIT))
2321                 break;
2322         }
2323     }
2324 
2325     /**
2326      * Gets and removes a local or stolen task for the given worker.
2327      *
2328      * @return a task, if available
2329      */
2330     final ForkJoinTask<?> nextTaskFor(WorkQueue w) {
2331         for (ForkJoinTask<?> t;;) {
2332             WorkQueue q;
2333             if ((t = w.nextLocalTask()) != null)
2334                 return t;
2335             if ((q = findNonEmptyStealQueue()) == null)
2336                 return null;
2337             if ((t = q.pollAt(q.base)) != null)
2338                 return t;
2339         }
2340     }
2341 
2342     /**
2343      * Returns a cheap heuristic guide for task partitioning when
2344      * programmers, frameworks, tools, or languages have little or no
2345      * idea about task granularity.  In essence, by offering this
2346      * method, we ask users only about tradeoffs in overhead vs
2347      * expected throughput and its variance, rather than how finely to
2348      * partition tasks.
2349      *
2350      * In a steady state strict (tree-structured) computation, each
2351      * thread makes available for stealing enough tasks for other
2352      * threads to remain active. Inductively, if all threads play by
2353      * the same rules, each thread should make available only a
2354      * constant number of tasks.
2355      *
2356      * The minimum useful constant is just 1. But using a value of 1
2357      * would require immediate replenishment upon each steal to
2358      * maintain enough tasks, which is infeasible.  Further,
2359      * partitionings/granularities of offered tasks should minimize
2360      * steal rates, which in general means that threads nearer the top
2361      * of computation tree should generate more than those nearer the
2362      * bottom. In perfect steady state, each thread is at
2363      * approximately the same level of computation tree. However,
2364      * producing extra tasks amortizes the uncertainty of progress and
2365      * diffusion assumptions.
2366      *
2367      * So, users will want to use values larger (but not much larger)
2368      * than 1 to both smooth over transient shortages and hedge
2369      * against uneven progress; as traded off against the cost of
2370      * extra task overhead. We leave the user to pick a threshold
2371      * value to compare with the results of this call to guide
2372      * decisions, but recommend values such as 3.
2373      *
2374      * When all threads are active, it is on average OK to estimate
2375      * surplus strictly locally. In steady-state, if one thread is
2376      * maintaining say 2 surplus tasks, then so are others. So we can
2377      * just use estimated queue length.  However, this strategy alone
2378      * leads to serious mis-estimates in some non-steady-state
2379      * conditions (ramp-up, ramp-down, other stalls). We can detect
2380      * many of these by further considering the number of "idle"
2381      * threads, that are known to have zero queued tasks, so
2382      * compensate by a factor of (#idle/#active) threads.
2383      */
2384     static int getSurplusQueuedTaskCount() {
2385         Thread t; ForkJoinWorkerThread wt; ForkJoinPool pool; WorkQueue q;
2386         if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
2387             int p = (pool = (wt = (ForkJoinWorkerThread)t).pool).config & SMASK;
2388             int n = (q = wt.workQueue).top - q.base;
2389             int a = (int)(pool.ctl >> AC_SHIFT) + p;
2390             return n - (a > (p >>>= 1) ? 0 :
2391                         a > (p >>>= 1) ? 1 :
2392                         a > (p >>>= 1) ? 2 :
2393                         a > (p >>>= 1) ? 4 :
2394                         8);
2395         }
2396         return 0;
2397     }
2398 
2399     //  Termination
2400 
2401     /**
2402      * Possibly initiates and/or completes termination.
2403      *
2404      * @param now if true, unconditionally terminate, else only
2405      * if no work and no active workers
2406      * @param enable if true, terminate when next possible
2407      * @return -1: terminating/terminated, 0: retry if internal caller, else 1
2408      */
2409     private int tryTerminate(boolean now, boolean enable) {
2410         int rs; // 3 phases: try to set SHUTDOWN, then STOP, then TERMINATED
2411 
2412         while ((rs = runState) >= 0) {
2413             if (!enable || this == common)        // cannot shutdown
2414                 return 1;
2415             else if (rs == 0)
2416                 tryInitialize(false);             // ensure initialized
2417             else
2418                 U.compareAndSwapInt(this, RUNSTATE, rs, rs | SHUTDOWN);
2419         }
2420 
2421         if ((rs & STOP) == 0) {                   // try to initiate termination
2422             if (!now) {                           // check quiescence
2423                 for (long oldSum = 0L;;) {        // repeat until stable
2424                     WorkQueue[] ws; WorkQueue w; int b;
2425                     long checkSum = ctl;
2426                     if ((int)(checkSum >> AC_SHIFT) + (config & SMASK) > 0)
2427                         return 0;                 // still active workers
2428                     if ((ws = workQueues) != null) {
2429                         for (int i = 0; i < ws.length; ++i) {
2430                             if ((w = ws[i]) != null) {
2431                                 checkSum += (b = w.base);
2432                                 if (w.currentSteal != null || b != w.top)
2433                                     return 0;     // retry if internal caller
2434                             }
2435                         }
2436                     }
2437                     if (oldSum == (oldSum = checkSum))
2438                         break;
2439                 }
2440             }
2441             do {} while (!U.compareAndSwapInt(this, RUNSTATE,
2442                                               rs = runState, rs | STOP));
2443         }
2444 
2445         for (long oldSum = 0L;;) {                // repeat until stable
2446             WorkQueue[] ws; WorkQueue w; ForkJoinWorkerThread wt;
2447             long checkSum = ctl;
2448             if ((ws = workQueues) != null) {      // help terminate others
2449                 for (int i = 0; i < ws.length; ++i) {
2450                     if ((w = ws[i]) != null) {
2451                         w.cancelAll();            // clear queues
2452                         checkSum += w.base;
2453                         if (w.qlock >= 0) {
2454                             w.qlock = -1;         // racy set OK
2455                             if ((wt = w.owner) != null) {
2456                                 try {             // unblock join or park
2457                                     wt.interrupt();
2458                                 } catch (Throwable ignore) {
2459                                 }
2460                             }
2461                         }
2462                     }
2463                 }
2464             }
2465             if (oldSum == (oldSum = checkSum))
2466                 break;
2467         }
2468 
2469         if ((short)(ctl >>> TC_SHIFT) + (config & SMASK) <= 0) {
2470             runState = (STARTED | SHUTDOWN | STOP | TERMINATED); // final write
2471             synchronized (this) {
2472                 notifyAll();                      // for awaitTermination
2473             }
2474         }
2475 
2476         return -1;
2477     }
2478 
2479     // External operations
2480 
2481     /**
2482      * Constructs and tries to install a new external queue,
2483      * failing if the workQueues array already has a queue at
2484      * the given index.
2485      *
2486      * @param index the index of the new queue
2487      */
2488     private void tryCreateExternalQueue(int index) {
2489         AuxState aux;
2490         if ((aux = auxState) != null && index >= 0) {
2491             WorkQueue q = new WorkQueue(this, null);
2492             q.config = index;
2493             q.scanState = ~UNSIGNALLED;
2494             q.qlock = 1;                   // lock queue
2495             boolean installed = false;
2496             aux.lock();
2497             try {                          // lock pool to install
2498                 WorkQueue[] ws;
2499                 if ((ws = workQueues) != null && index < ws.length &&
2500                     ws[index] == null) {
2501                     ws[index] = q;         // else throw away
2502                     installed = true;
2503                 }
2504             } finally {
2505                 aux.unlock();
2506             }
2507             if (installed) {
2508                 try {
2509                     q.growArray();
2510                 } finally {
2511                     q.qlock = 0;
2512                 }
2513             }
2514         }
2515     }
2516 
2517     /**
2518      * Adds the given task to a submission queue at submitter's
2519      * current queue. Also performs secondary initialization upon the
2520      * first submission of the first task to the pool, and detects
2521      * first submission by an external thread and creates a new shared
2522      * queue if the one at index if empty or contended.
2523      *
2524      * @param task the task. Caller must ensure non-null.
2525      */
2526     final void externalPush(ForkJoinTask<?> task) {
2527         int r;                            // initialize caller's probe
2528         if ((r = ThreadLocalRandom.getProbe()) == 0) {
2529             ThreadLocalRandom.localInit();
2530             r = ThreadLocalRandom.getProbe();
2531         }
2532         for (;;) {
2533             WorkQueue q; int wl, k, stat;
2534             int rs = runState;
2535             WorkQueue[] ws = workQueues;
2536             if (rs <= 0 || ws == null || (wl = ws.length) <= 0)
2537                 tryInitialize(true);
2538             else if ((q = ws[k = (wl - 1) & r & SQMASK]) == null)
2539                 tryCreateExternalQueue(k);
2540             else if ((stat = q.sharedPush(task)) < 0)
2541                 break;
2542             else if (stat == 0) {
2543                 signalWork();
2544                 break;
2545             }
2546             else                          // move if busy
2547                 r = ThreadLocalRandom.advanceProbe(r);
2548         }
2549     }
2550 
2551     /**
2552      * Pushes a possibly-external submission.
2553      */
2554     private <T> ForkJoinTask<T> externalSubmit(ForkJoinTask<T> task) {
2555         Thread t; ForkJoinWorkerThread w; WorkQueue q;
2556         if (task == null)
2557             throw new NullPointerException();
2558         if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) &&
2559             (w = (ForkJoinWorkerThread)t).pool == this &&
2560             (q = w.workQueue) != null)
2561             q.push(task);
2562         else
2563             externalPush(task);
2564         return task;
2565     }
2566 
2567     /**
2568      * Returns common pool queue for an external thread.
2569      */
2570     static WorkQueue commonSubmitterQueue() {
2571         ForkJoinPool p = common;
2572         int r = ThreadLocalRandom.getProbe();
2573         WorkQueue[] ws; int wl;
2574         return (p != null && (ws = p.workQueues) != null &&
2575                 (wl = ws.length) > 0) ?
2576             ws[(wl - 1) & r & SQMASK] : null;
2577     }
2578 
2579     /**
2580      * Performs tryUnpush for an external submitter.
2581      */
2582     final boolean tryExternalUnpush(ForkJoinTask<?> task) {
2583         int r = ThreadLocalRandom.getProbe();
2584         WorkQueue[] ws; WorkQueue w; int wl;
2585         return ((ws = workQueues) != null &&
2586                 (wl = ws.length) > 0 &&
2587                 (w = ws[(wl - 1) & r & SQMASK]) != null &&
2588                 w.trySharedUnpush(task));
2589     }
2590 
2591     /**
2592      * Performs helpComplete for an external submitter.
2593      */
2594     final int externalHelpComplete(CountedCompleter<?> task, int maxTasks) {
2595         WorkQueue[] ws; int wl;
2596         int r = ThreadLocalRandom.getProbe();
2597         return ((ws = workQueues) != null && (wl = ws.length) > 0) ?
2598             helpComplete(ws[(wl - 1) & r & SQMASK], task, maxTasks) : 0;
2599     }
2600 
2601     // Exported methods
2602 
2603     // Constructors
2604 
2605     /**
2606      * Creates a {@code ForkJoinPool} with parallelism equal to {@link
2607      * java.lang.Runtime#availableProcessors}, using the {@linkplain
2608      * #defaultForkJoinWorkerThreadFactory default thread factory},
2609      * no UncaughtExceptionHandler, and non-async LIFO processing mode.
2610      *
2611      * @throws SecurityException if a security manager exists and
2612      *         the caller is not permitted to modify threads
2613      *         because it does not hold {@link
2614      *         java.lang.RuntimePermission}{@code ("modifyThread")}
2615      */
2616     public ForkJoinPool() {
2617         this(Math.min(MAX_CAP, Runtime.getRuntime().availableProcessors()),
2618              defaultForkJoinWorkerThreadFactory, null, false);
2619     }
2620 
2621     /**
2622      * Creates a {@code ForkJoinPool} with the indicated parallelism
2623      * level, the {@linkplain
2624      * #defaultForkJoinWorkerThreadFactory default thread factory},
2625      * no UncaughtExceptionHandler, and non-async LIFO processing mode.
2626      *
2627      * @param parallelism the parallelism level
2628      * @throws IllegalArgumentException if parallelism less than or
2629      *         equal to zero, or greater than implementation limit
2630      * @throws SecurityException if a security manager exists and
2631      *         the caller is not permitted to modify threads
2632      *         because it does not hold {@link
2633      *         java.lang.RuntimePermission}{@code ("modifyThread")}
2634      */
2635     public ForkJoinPool(int parallelism) {
2636         this(parallelism, defaultForkJoinWorkerThreadFactory, null, false);
2637     }
2638 
2639     /**
2640      * Creates a {@code ForkJoinPool} with the given parameters.
2641      *
2642      * @param parallelism the parallelism level. For default value,
2643      * use {@link java.lang.Runtime#availableProcessors}.
2644      * @param factory the factory for creating new threads. For default value,
2645      * use {@link #defaultForkJoinWorkerThreadFactory}.
2646      * @param handler the handler for internal worker threads that
2647      * terminate due to unrecoverable errors encountered while executing
2648      * tasks. For default value, use {@code null}.
2649      * @param asyncMode if true,
2650      * establishes local first-in-first-out scheduling mode for forked
2651      * tasks that are never joined. This mode may be more appropriate
2652      * than default locally stack-based mode in applications in which
2653      * worker threads only process event-style asynchronous tasks.
2654      * For default value, use {@code false}.
2655      * @throws IllegalArgumentException if parallelism less than or
2656      *         equal to zero, or greater than implementation limit
2657      * @throws NullPointerException if the factory is null
2658      * @throws SecurityException if a security manager exists and
2659      *         the caller is not permitted to modify threads
2660      *         because it does not hold {@link
2661      *         java.lang.RuntimePermission}{@code ("modifyThread")}
2662      */
2663     public ForkJoinPool(int parallelism,
2664                         ForkJoinWorkerThreadFactory factory,
2665                         UncaughtExceptionHandler handler,
2666                         boolean asyncMode) {
2667         this(checkParallelism(parallelism),
2668              checkFactory(factory),
2669              handler,
2670              asyncMode ? FIFO_QUEUE : LIFO_QUEUE,
2671              "ForkJoinPool-" + nextPoolId() + "-worker-");
2672         checkPermission();
2673     }
2674 
2675     private static int checkParallelism(int parallelism) {
2676         if (parallelism <= 0 || parallelism > MAX_CAP)
2677             throw new IllegalArgumentException();
2678         return parallelism;
2679     }
2680 
2681     private static ForkJoinWorkerThreadFactory checkFactory
2682         (ForkJoinWorkerThreadFactory factory) {
2683         if (factory == null)
2684             throw new NullPointerException();
2685         return factory;
2686     }
2687 
2688     /**
2689      * Creates a {@code ForkJoinPool} with the given parameters, without
2690      * any security checks or parameter validation.  Invoked directly by
2691      * makeCommonPool.
2692      */
2693     private ForkJoinPool(int parallelism,
2694                          ForkJoinWorkerThreadFactory factory,
2695                          UncaughtExceptionHandler handler,
2696                          int mode,
2697                          String workerNamePrefix) {
2698         this.workerNamePrefix = workerNamePrefix;
2699         this.factory = factory;
2700         this.ueh = handler;
2701         this.config = (parallelism & SMASK) | mode;
2702         long np = (long)(-parallelism); // offset ctl counts
2703         this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
2704     }
2705 
2706     /**
2707      * Returns the common pool instance. This pool is statically
2708      * constructed; its run state is unaffected by attempts to {@link
2709      * #shutdown} or {@link #shutdownNow}. However this pool and any
2710      * ongoing processing are automatically terminated upon program
2711      * {@link System#exit}.  Any program that relies on asynchronous
2712      * task processing to complete before program termination should
2713      * invoke {@code commonPool().}{@link #awaitQuiescence awaitQuiescence},
2714      * before exit.
2715      *
2716      * @return the common pool instance
2717      * @since 1.8
2718      */
2719     public static ForkJoinPool commonPool() {
2720         // assert common != null : "static init error";
2721         return common;
2722     }
2723 
2724     // Execution methods
2725 
2726     /**
2727      * Performs the given task, returning its result upon completion.
2728      * If the computation encounters an unchecked Exception or Error,
2729      * it is rethrown as the outcome of this invocation.  Rethrown
2730      * exceptions behave in the same way as regular exceptions, but,
2731      * when possible, contain stack traces (as displayed for example
2732      * using {@code ex.printStackTrace()}) of both the current thread
2733      * as well as the thread actually encountering the exception;
2734      * minimally only the latter.
2735      *
2736      * @param task the task
2737      * @param <T> the type of the task's result
2738      * @return the task's result
2739      * @throws NullPointerException if the task is null
2740      * @throws RejectedExecutionException if the task cannot be
2741      *         scheduled for execution
2742      */
2743     public <T> T invoke(ForkJoinTask<T> task) {
2744         if (task == null)
2745             throw new NullPointerException();
2746         externalSubmit(task);
2747         return task.join();
2748     }
2749 
2750     /**
2751      * Arranges for (asynchronous) execution of the given task.
2752      *
2753      * @param task the task
2754      * @throws NullPointerException if the task is null
2755      * @throws RejectedExecutionException if the task cannot be
2756      *         scheduled for execution
2757      */
2758     public void execute(ForkJoinTask<?> task) {
2759         externalSubmit(task);
2760     }
2761 
2762     // AbstractExecutorService methods
2763 
2764     /**
2765      * @throws NullPointerException if the task is null
2766      * @throws RejectedExecutionException if the task cannot be
2767      *         scheduled for execution
2768      */
2769     public void execute(Runnable task) {
2770         if (task == null)
2771             throw new NullPointerException();
2772         ForkJoinTask<?> job;
2773         if (task instanceof ForkJoinTask<?>) // avoid re-wrap
2774             job = (ForkJoinTask<?>) task;
2775         else
2776             job = new ForkJoinTask.RunnableExecuteAction(task);
2777         externalSubmit(job);
2778     }
2779 
2780     /**
2781      * Submits a ForkJoinTask for execution.
2782      *
2783      * @param task the task to submit
2784      * @param <T> the type of the task's result
2785      * @return the task
2786      * @throws NullPointerException if the task is null
2787      * @throws RejectedExecutionException if the task cannot be
2788      *         scheduled for execution
2789      */
2790     public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
2791         return externalSubmit(task);
2792     }
2793 
2794     /**
2795      * @throws NullPointerException if the task is null
2796      * @throws RejectedExecutionException if the task cannot be
2797      *         scheduled for execution
2798      */
2799     public <T> ForkJoinTask<T> submit(Callable<T> task) {
2800         return externalSubmit(new ForkJoinTask.AdaptedCallable<T>(task));
2801     }
2802 
2803     /**
2804      * @throws NullPointerException if the task is null
2805      * @throws RejectedExecutionException if the task cannot be
2806      *         scheduled for execution
2807      */
2808     public <T> ForkJoinTask<T> submit(Runnable task, T result) {
2809         return externalSubmit(new ForkJoinTask.AdaptedRunnable<T>(task, result));
2810     }
2811 
2812     /**
2813      * @throws NullPointerException if the task is null
2814      * @throws RejectedExecutionException if the task cannot be
2815      *         scheduled for execution
2816      */
2817     public ForkJoinTask<?> submit(Runnable task) {
2818         if (task == null)
2819             throw new NullPointerException();
2820         ForkJoinTask<?> job;
2821         if (task instanceof ForkJoinTask<?>) // avoid re-wrap
2822             job = (ForkJoinTask<?>) task;
2823         else
2824             job = new ForkJoinTask.AdaptedRunnableAction(task);
2825         return externalSubmit(job);
2826     }
2827 
2828     /**
2829      * @throws NullPointerException       {@inheritDoc}
2830      * @throws RejectedExecutionException {@inheritDoc}
2831      */
2832     public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
2833         // In previous versions of this class, this method constructed
2834         // a task to run ForkJoinTask.invokeAll, but now external
2835         // invocation of multiple tasks is at least as efficient.
2836         ArrayList<Future<T>> futures = new ArrayList<>(tasks.size());
2837 
2838         try {
2839             for (Callable<T> t : tasks) {
2840                 ForkJoinTask<T> f = new ForkJoinTask.AdaptedCallable<T>(t);
2841                 futures.add(f);
2842                 externalSubmit(f);
2843             }
2844             for (int i = 0, size = futures.size(); i < size; i++)
2845                 ((ForkJoinTask<?>)futures.get(i)).quietlyJoin();
2846             return futures;
2847         } catch (Throwable t) {
2848             for (int i = 0, size = futures.size(); i < size; i++)
2849                 futures.get(i).cancel(false);
2850             throw t;
2851         }
2852     }
2853 
2854     /**
2855      * Returns the factory used for constructing new workers.
2856      *
2857      * @return the factory used for constructing new workers
2858      */
2859     public ForkJoinWorkerThreadFactory getFactory() {
2860         return factory;
2861     }
2862 
2863     /**
2864      * Returns the handler for internal worker threads that terminate
2865      * due to unrecoverable errors encountered while executing tasks.
2866      *
2867      * @return the handler, or {@code null} if none
2868      */
2869     public UncaughtExceptionHandler getUncaughtExceptionHandler() {
2870         return ueh;
2871     }
2872 
2873     /**
2874      * Returns the targeted parallelism level of this pool.
2875      *
2876      * @return the targeted parallelism level of this pool
2877      */
2878     public int getParallelism() {
2879         int par;
2880         return ((par = config & SMASK) > 0) ? par : 1;
2881     }
2882 
2883     /**
2884      * Returns the targeted parallelism level of the common pool.
2885      *
2886      * @return the targeted parallelism level of the common pool
2887      * @since 1.8
2888      */
2889     public static int getCommonPoolParallelism() {
2890         return COMMON_PARALLELISM;
2891     }
2892 
2893     /**
2894      * Returns the number of worker threads that have started but not
2895      * yet terminated.  The result returned by this method may differ
2896      * from {@link #getParallelism} when threads are created to
2897      * maintain parallelism when others are cooperatively blocked.
2898      *
2899      * @return the number of worker threads
2900      */
2901     public int getPoolSize() {
2902         return (config & SMASK) + (short)(ctl >>> TC_SHIFT);
2903     }
2904 
2905     /**
2906      * Returns {@code true} if this pool uses local first-in-first-out
2907      * scheduling mode for forked tasks that are never joined.
2908      *
2909      * @return {@code true} if this pool uses async mode
2910      */
2911     public boolean getAsyncMode() {
2912         return (config & FIFO_QUEUE) != 0;
2913     }
2914 
2915     /**
2916      * Returns an estimate of the number of worker threads that are
2917      * not blocked waiting to join tasks or for other managed
2918      * synchronization. This method may overestimate the
2919      * number of running threads.
2920      *
2921      * @return the number of worker threads
2922      */
2923     public int getRunningThreadCount() {
2924         int rc = 0;
2925         WorkQueue[] ws; WorkQueue w;
2926         if ((ws = workQueues) != null) {
2927             for (int i = 1; i < ws.length; i += 2) {
2928                 if ((w = ws[i]) != null && w.isApparentlyUnblocked())
2929                     ++rc;
2930             }
2931         }
2932         return rc;
2933     }
2934 
2935     /**
2936      * Returns an estimate of the number of threads that are currently
2937      * stealing or executing tasks. This method may overestimate the
2938      * number of active threads.
2939      *
2940      * @return the number of active threads
2941      */
2942     public int getActiveThreadCount() {
2943         int r = (config & SMASK) + (int)(ctl >> AC_SHIFT);
2944         return (r <= 0) ? 0 : r; // suppress momentarily negative values
2945     }
2946 
2947     /**
2948      * Returns {@code true} if all worker threads are currently idle.
2949      * An idle worker is one that cannot obtain a task to execute
2950      * because none are available to steal from other threads, and
2951      * there are no pending submissions to the pool. This method is
2952      * conservative; it might not return {@code true} immediately upon
2953      * idleness of all threads, but will eventually become true if
2954      * threads remain inactive.
2955      *
2956      * @return {@code true} if all threads are currently idle
2957      */
2958     public boolean isQuiescent() {
2959         return (config & SMASK) + (int)(ctl >> AC_SHIFT) <= 0;
2960     }
2961 
2962     /**
2963      * Returns an estimate of the total number of tasks stolen from
2964      * one thread's work queue by another. The reported value
2965      * underestimates the actual total number of steals when the pool
2966      * is not quiescent. This value may be useful for monitoring and
2967      * tuning fork/join programs: in general, steal counts should be
2968      * high enough to keep threads busy, but low enough to avoid
2969      * overhead and contention across threads.
2970      *
2971      * @return the number of steals
2972      */
2973     public long getStealCount() {
2974         AuxState sc = auxState;
2975         long count = (sc == null) ? 0L : sc.stealCount;
2976         WorkQueue[] ws; WorkQueue w;
2977         if ((ws = workQueues) != null) {
2978             for (int i = 1; i < ws.length; i += 2) {
2979                 if ((w = ws[i]) != null)
2980                     count += w.nsteals;
2981             }
2982         }
2983         return count;
2984     }
2985 
2986     /**
2987      * Returns an estimate of the total number of tasks currently held
2988      * in queues by worker threads (but not including tasks submitted
2989      * to the pool that have not begun executing). This value is only
2990      * an approximation, obtained by iterating across all threads in
2991      * the pool. This method may be useful for tuning task
2992      * granularities.
2993      *
2994      * @return the number of queued tasks
2995      */
2996     public long getQueuedTaskCount() {
2997         long count = 0;
2998         WorkQueue[] ws; WorkQueue w;
2999         if ((ws = workQueues) != null) {
3000             for (int i = 1; i < ws.length; i += 2) {
3001                 if ((w = ws[i]) != null)
3002                     count += w.queueSize();
3003             }
3004         }
3005         return count;
3006     }
3007 
3008     /**
3009      * Returns an estimate of the number of tasks submitted to this
3010      * pool that have not yet begun executing.  This method may take
3011      * time proportional to the number of submissions.
3012      *
3013      * @return the number of queued submissions
3014      */
3015     public int getQueuedSubmissionCount() {
3016         int count = 0;
3017         WorkQueue[] ws; WorkQueue w;
3018         if ((ws = workQueues) != null) {
3019             for (int i = 0; i < ws.length; i += 2) {
3020                 if ((w = ws[i]) != null)
3021                     count += w.queueSize();
3022             }
3023         }
3024         return count;
3025     }
3026 
3027     /**
3028      * Returns {@code true} if there are any tasks submitted to this
3029      * pool that have not yet begun executing.
3030      *
3031      * @return {@code true} if there are any queued submissions
3032      */
3033     public boolean hasQueuedSubmissions() {
3034         WorkQueue[] ws; WorkQueue w;
3035         if ((ws = workQueues) != null) {
3036             for (int i = 0; i < ws.length; i += 2) {
3037                 if ((w = ws[i]) != null && !w.isEmpty())
3038                     return true;
3039             }
3040         }
3041         return false;
3042     }
3043 
3044     /**
3045      * Removes and returns the next unexecuted submission if one is
3046      * available.  This method may be useful in extensions to this
3047      * class that re-assign work in systems with multiple pools.
3048      *
3049      * @return the next submission, or {@code null} if none
3050      */
3051     protected ForkJoinTask<?> pollSubmission() {
3052         WorkQueue[] ws; int wl; WorkQueue w; ForkJoinTask<?> t;
3053         int r = ThreadLocalRandom.nextSecondarySeed();
3054         if ((ws = workQueues) != null && (wl = ws.length) > 0) {
3055             for (int m = wl - 1, i = 0; i < wl; ++i) {
3056                 if ((w = ws[(i << 1) & m]) != null && (t = w.poll()) != null)
3057                     return t;
3058             }
3059         }
3060         return null;
3061     }
3062 
3063     /**
3064      * Removes all available unexecuted submitted and forked tasks
3065      * from scheduling queues and adds them to the given collection,
3066      * without altering their execution status. These may include
3067      * artificially generated or wrapped tasks. This method is
3068      * designed to be invoked only when the pool is known to be
3069      * quiescent. Invocations at other times may not remove all
3070      * tasks. A failure encountered while attempting to add elements
3071      * to collection {@code c} may result in elements being in
3072      * neither, either or both collections when the associated
3073      * exception is thrown.  The behavior of this operation is
3074      * undefined if the specified collection is modified while the
3075      * operation is in progress.
3076      *
3077      * @param c the collection to transfer elements into
3078      * @return the number of elements transferred
3079      */
3080     protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
3081         int count = 0;
3082         WorkQueue[] ws; WorkQueue w; ForkJoinTask<?> t;
3083         if ((ws = workQueues) != null) {
3084             for (int i = 0; i < ws.length; ++i) {
3085                 if ((w = ws[i]) != null) {
3086                     while ((t = w.poll()) != null) {
3087                         c.add(t);
3088                         ++count;
3089                     }
3090                 }
3091             }
3092         }
3093         return count;
3094     }
3095 
3096     /**
3097      * Returns a string identifying this pool, as well as its state,
3098      * including indications of run state, parallelism level, and
3099      * worker and task counts.
3100      *
3101      * @return a string identifying this pool, as well as its state
3102      */
3103     public String toString() {
3104         // Use a single pass through workQueues to collect counts
3105         long qt = 0L, qs = 0L; int rc = 0;
3106         AuxState sc = auxState;
3107         long st = (sc == null) ? 0L : sc.stealCount;
3108         long c = ctl;
3109         WorkQueue[] ws; WorkQueue w;
3110         if ((ws = workQueues) != null) {
3111             for (int i = 0; i < ws.length; ++i) {
3112                 if ((w = ws[i]) != null) {
3113                     int size = w.queueSize();
3114                     if ((i & 1) == 0)
3115                         qs += size;
3116                     else {
3117                         qt += size;
3118                         st += w.nsteals;
3119                         if (w.isApparentlyUnblocked())
3120                             ++rc;
3121                     }
3122                 }
3123             }
3124         }
3125         int pc = (config & SMASK);
3126         int tc = pc + (short)(c >>> TC_SHIFT);
3127         int ac = pc + (int)(c >> AC_SHIFT);
3128         if (ac < 0) // ignore transient negative
3129             ac = 0;
3130         int rs = runState;
3131         String level = ((rs & TERMINATED) != 0 ? "Terminated" :
3132                         (rs & STOP)       != 0 ? "Terminating" :
3133                         (rs & SHUTDOWN)   != 0 ? "Shutting down" :
3134                         "Running");
3135         return super.toString() +
3136             "[" + level +
3137             ", parallelism = " + pc +
3138             ", size = " + tc +
3139             ", active = " + ac +
3140             ", running = " + rc +
3141             ", steals = " + st +
3142             ", tasks = " + qt +
3143             ", submissions = " + qs +
3144             "]";
3145     }
3146 
3147     /**
3148      * Possibly initiates an orderly shutdown in which previously
3149      * submitted tasks are executed, but no new tasks will be
3150      * accepted. Invocation has no effect on execution state if this
3151      * is the {@link #commonPool()}, and no additional effect if
3152      * already shut down.  Tasks that are in the process of being
3153      * submitted concurrently during the course of this method may or
3154      * may not be rejected.
3155      *
3156      * @throws SecurityException if a security manager exists and
3157      *         the caller is not permitted to modify threads
3158      *         because it does not hold {@link
3159      *         java.lang.RuntimePermission}{@code ("modifyThread")}
3160      */
3161     public void shutdown() {
3162         checkPermission();
3163         tryTerminate(false, true);
3164     }
3165 
3166     /**
3167      * Possibly attempts to cancel and/or stop all tasks, and reject
3168      * all subsequently submitted tasks.  Invocation has no effect on
3169      * execution state if this is the {@link #commonPool()}, and no
3170      * additional effect if already shut down. Otherwise, tasks that
3171      * are in the process of being submitted or executed concurrently
3172      * during the course of this method may or may not be
3173      * rejected. This method cancels both existing and unexecuted
3174      * tasks, in order to permit termination in the presence of task
3175      * dependencies. So the method always returns an empty list
3176      * (unlike the case for some other Executors).
3177      *
3178      * @return an empty list
3179      * @throws SecurityException if a security manager exists and
3180      *         the caller is not permitted to modify threads
3181      *         because it does not hold {@link
3182      *         java.lang.RuntimePermission}{@code ("modifyThread")}
3183      */
3184     public List<Runnable> shutdownNow() {
3185         checkPermission();
3186         tryTerminate(true, true);
3187         return Collections.emptyList();
3188     }
3189 
3190     /**
3191      * Returns {@code true} if all tasks have completed following shut down.
3192      *
3193      * @return {@code true} if all tasks have completed following shut down
3194      */
3195     public boolean isTerminated() {
3196         return (runState & TERMINATED) != 0;
3197     }
3198 
3199     /**
3200      * Returns {@code true} if the process of termination has
3201      * commenced but not yet completed.  This method may be useful for
3202      * debugging. A return of {@code true} reported a sufficient
3203      * period after shutdown may indicate that submitted tasks have
3204      * ignored or suppressed interruption, or are waiting for I/O,
3205      * causing this executor not to properly terminate. (See the
3206      * advisory notes for class {@link ForkJoinTask} stating that
3207      * tasks should not normally entail blocking operations.  But if
3208      * they do, they must abort them on interrupt.)
3209      *
3210      * @return {@code true} if terminating but not yet terminated
3211      */
3212     public boolean isTerminating() {
3213         int rs = runState;
3214         return (rs & STOP) != 0 && (rs & TERMINATED) == 0;
3215     }
3216 
3217     /**
3218      * Returns {@code true} if this pool has been shut down.
3219      *
3220      * @return {@code true} if this pool has been shut down
3221      */
3222     public boolean isShutdown() {
3223         return (runState & SHUTDOWN) != 0;
3224     }
3225 
3226     /**
3227      * Blocks until all tasks have completed execution after a
3228      * shutdown request, or the timeout occurs, or the current thread
3229      * is interrupted, whichever happens first. Because the {@link
3230      * #commonPool()} never terminates until program shutdown, when
3231      * applied to the common pool, this method is equivalent to {@link
3232      * #awaitQuiescence(long, TimeUnit)} but always returns {@code false}.
3233      *
3234      * @param timeout the maximum time to wait
3235      * @param unit the time unit of the timeout argument
3236      * @return {@code true} if this executor terminated and
3237      *         {@code false} if the timeout elapsed before termination
3238      * @throws InterruptedException if interrupted while waiting
3239      */
3240     public boolean awaitTermination(long timeout, TimeUnit unit)
3241         throws InterruptedException {
3242         if (Thread.interrupted())
3243             throw new InterruptedException();
3244         if (this == common) {
3245             awaitQuiescence(timeout, unit);
3246             return false;
3247         }
3248         long nanos = unit.toNanos(timeout);
3249         if (isTerminated())
3250             return true;
3251         if (nanos <= 0L)
3252             return false;
3253         long deadline = System.nanoTime() + nanos;
3254         synchronized (this) {
3255             for (;;) {
3256                 if (isTerminated())
3257                     return true;
3258                 if (nanos <= 0L)
3259                     return false;
3260                 long millis = TimeUnit.NANOSECONDS.toMillis(nanos);
3261                 wait(millis > 0L ? millis : 1L);
3262                 nanos = deadline - System.nanoTime();
3263             }
3264         }
3265     }
3266 
3267     /**
3268      * If called by a ForkJoinTask operating in this pool, equivalent
3269      * in effect to {@link ForkJoinTask#helpQuiesce}. Otherwise,
3270      * waits and/or attempts to assist performing tasks until this
3271      * pool {@link #isQuiescent} or the indicated timeout elapses.
3272      *
3273      * @param timeout the maximum time to wait
3274      * @param unit the time unit of the timeout argument
3275      * @return {@code true} if quiescent; {@code false} if the
3276      * timeout elapsed.
3277      */
3278     public boolean awaitQuiescence(long timeout, TimeUnit unit) {
3279         long nanos = unit.toNanos(timeout);
3280         ForkJoinWorkerThread wt;
3281         Thread thread = Thread.currentThread();
3282         if ((thread instanceof ForkJoinWorkerThread) &&
3283             (wt = (ForkJoinWorkerThread)thread).pool == this) {
3284             helpQuiescePool(wt.workQueue);
3285             return true;
3286         }
3287         long startTime = System.nanoTime();
3288         WorkQueue[] ws;
3289         int r = 0, wl;
3290         boolean found = true;
3291         while (!isQuiescent() && (ws = workQueues) != null &&
3292                (wl = ws.length) > 0) {
3293             if (!found) {
3294                 if ((System.nanoTime() - startTime) > nanos)
3295                     return false;
3296                 Thread.yield(); // cannot block
3297             }
3298             found = false;
3299             for (int m = wl - 1, j = (m + 1) << 2; j >= 0; --j) {
3300                 ForkJoinTask<?> t; WorkQueue q; int b, k;
3301                 if ((k = r++ & m) <= m && k >= 0 && (q = ws[k]) != null &&
3302                     (b = q.base) - q.top < 0) {
3303                     found = true;
3304                     if ((t = q.pollAt(b)) != null)
3305                         t.doExec();
3306                     break;
3307                 }
3308             }
3309         }
3310         return true;
3311     }
3312 
3313     /**
3314      * Waits and/or attempts to assist performing tasks indefinitely
3315      * until the {@link #commonPool()} {@link #isQuiescent}.
3316      */
3317     static void quiesceCommonPool() {
3318         common.awaitQuiescence(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
3319     }
3320 
3321     /**
3322      * Interface for extending managed parallelism for tasks running
3323      * in {@link ForkJoinPool}s.
3324      *
3325      * <p>A {@code ManagedBlocker} provides two methods.  Method
3326      * {@link #isReleasable} must return {@code true} if blocking is
3327      * not necessary. Method {@link #block} blocks the current thread
3328      * if necessary (perhaps internally invoking {@code isReleasable}
3329      * before actually blocking). These actions are performed by any
3330      * thread invoking {@link ForkJoinPool#managedBlock(ManagedBlocker)}.
3331      * The unusual methods in this API accommodate synchronizers that
3332      * may, but don't usually, block for long periods. Similarly, they
3333      * allow more efficient internal handling of cases in which
3334      * additional workers may be, but usually are not, needed to
3335      * ensure sufficient parallelism.  Toward this end,
3336      * implementations of method {@code isReleasable} must be amenable
3337      * to repeated invocation.
3338      *
3339      * <p>For example, here is a ManagedBlocker based on a
3340      * ReentrantLock:
3341      * <pre> {@code
3342      * class ManagedLocker implements ManagedBlocker {
3343      *   final ReentrantLock lock;
3344      *   boolean hasLock = false;
3345      *   ManagedLocker(ReentrantLock lock) { this.lock = lock; }
3346      *   public boolean block() {
3347      *     if (!hasLock)
3348      *       lock.lock();
3349      *     return true;
3350      *   }
3351      *   public boolean isReleasable() {
3352      *     return hasLock || (hasLock = lock.tryLock());
3353      *   }
3354      * }}</pre>
3355      *
3356      * <p>Here is a class that possibly blocks waiting for an
3357      * item on a given queue:
3358      * <pre> {@code
3359      * class QueueTaker<E> implements ManagedBlocker {
3360      *   final BlockingQueue<E> queue;
3361      *   volatile E item = null;
3362      *   QueueTaker(BlockingQueue<E> q) { this.queue = q; }
3363      *   public boolean block() throws InterruptedException {
3364      *     if (item == null)
3365      *       item = queue.take();
3366      *     return true;
3367      *   }
3368      *   public boolean isReleasable() {
3369      *     return item != null || (item = queue.poll()) != null;
3370      *   }
3371      *   public E getItem() { // call after pool.managedBlock completes
3372      *     return item;
3373      *   }
3374      * }}</pre>
3375      */
3376     public static interface ManagedBlocker {
3377         /**
3378          * Possibly blocks the current thread, for example waiting for
3379          * a lock or condition.
3380          *
3381          * @return {@code true} if no additional blocking is necessary
3382          * (i.e., if isReleasable would return true)
3383          * @throws InterruptedException if interrupted while waiting
3384          * (the method is not required to do so, but is allowed to)
3385          */
3386         boolean block() throws InterruptedException;
3387 
3388         /**
3389          * Returns {@code true} if blocking is unnecessary.
3390          * @return {@code true} if blocking is unnecessary
3391          */
3392         boolean isReleasable();
3393     }
3394 
3395     /**
3396      * Runs the given possibly blocking task.  When {@linkplain
3397      * ForkJoinTask#inForkJoinPool() running in a ForkJoinPool}, this
3398      * method possibly arranges for a spare thread to be activated if
3399      * necessary to ensure sufficient parallelism while the current
3400      * thread is blocked in {@link ManagedBlocker#block blocker.block()}.
3401      *
3402      * <p>This method repeatedly calls {@code blocker.isReleasable()} and
3403      * {@code blocker.block()} until either method returns {@code true}.
3404      * Every call to {@code blocker.block()} is preceded by a call to
3405      * {@code blocker.isReleasable()} that returned {@code false}.
3406      *
3407      * <p>If not running in a ForkJoinPool, this method is
3408      * behaviorally equivalent to
3409      * <pre> {@code
3410      * while (!blocker.isReleasable())
3411      *   if (blocker.block())
3412      *     break;}</pre>
3413      *
3414      * If running in a ForkJoinPool, the pool may first be expanded to
3415      * ensure sufficient parallelism available during the call to
3416      * {@code blocker.block()}.
3417      *
3418      * @param blocker the blocker task
3419      * @throws InterruptedException if {@code blocker.block()} did so
3420      */
3421     public static void managedBlock(ManagedBlocker blocker)
3422         throws InterruptedException {
3423         ForkJoinPool p;
3424         ForkJoinWorkerThread wt;
3425         Thread t = Thread.currentThread();
3426         if ((t instanceof ForkJoinWorkerThread) &&
3427             (p = (wt = (ForkJoinWorkerThread)t).pool) != null) {
3428             WorkQueue w = wt.workQueue;
3429             while (!blocker.isReleasable()) {
3430                 if (p.tryCompensate(w)) {
3431                     try {
3432                         do {} while (!blocker.isReleasable() &&
3433                                      !blocker.block());
3434                     } finally {
3435                         U.getAndAddLong(p, CTL, AC_UNIT);
3436                     }
3437                     break;
3438                 }
3439             }
3440         }
3441         else {
3442             do {} while (!blocker.isReleasable() &&
3443                          !blocker.block());
3444         }
3445     }
3446 
3447     // AbstractExecutorService overrides.  These rely on undocumented
3448     // fact that ForkJoinTask.adapt returns ForkJoinTasks that also
3449     // implement RunnableFuture.
3450 
3451     protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
3452         return new ForkJoinTask.AdaptedRunnable<T>(runnable, value);
3453     }
3454 
3455     protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
3456         return new ForkJoinTask.AdaptedCallable<T>(callable);
3457     }
3458 
3459     // Unsafe mechanics
3460     private static final jdk.internal.misc.Unsafe U = jdk.internal.misc.Unsafe.getUnsafe();
3461     private static final long CTL;
3462     private static final long RUNSTATE;
3463     private static final int ABASE;
3464     private static final int ASHIFT;
3465 
3466     static {
3467         try {
3468             CTL = U.objectFieldOffset
3469                 (ForkJoinPool.class.getDeclaredField("ctl"));
3470             RUNSTATE = U.objectFieldOffset
3471                 (ForkJoinPool.class.getDeclaredField("runState"));
3472             ABASE = U.arrayBaseOffset(ForkJoinTask[].class);
3473             int scale = U.arrayIndexScale(ForkJoinTask[].class);
3474             if ((scale & (scale - 1)) != 0)
3475                 throw new Error("array index scale not a power of two");
3476             ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
3477         } catch (ReflectiveOperationException e) {
3478             throw new Error(e);
3479         }
3480 
3481         // Reduce the risk of rare disastrous classloading in first call to
3482         // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
3483         Class<?> ensureLoaded = LockSupport.class;
3484 
3485         int commonMaxSpares = DEFAULT_COMMON_MAX_SPARES;
3486         try {
3487             String p = System.getProperty
3488                 ("java.util.concurrent.ForkJoinPool.common.maximumSpares");
3489             if (p != null)
3490                 commonMaxSpares = Integer.parseInt(p);
3491         } catch (Exception ignore) {}
3492         COMMON_MAX_SPARES = commonMaxSpares;
3493 
3494         defaultForkJoinWorkerThreadFactory =
3495             new DefaultForkJoinWorkerThreadFactory();
3496         modifyThreadPermission = new RuntimePermission("modifyThread");
3497 
3498         common = java.security.AccessController.doPrivileged
3499             (new java.security.PrivilegedAction<ForkJoinPool>() {
3500                 public ForkJoinPool run() { return makeCommonPool(); }});
3501 
3502         // report 1 even if threads disabled
3503         COMMON_PARALLELISM = Math.max(common.config & SMASK, 1);
3504     }
3505 
3506     /**
3507      * Creates and returns the common pool, respecting user settings
3508      * specified via system properties.
3509      */
3510     @SuppressWarnings("deprecation") // Class.newInstance
3511     static ForkJoinPool makeCommonPool() {
3512         int parallelism = -1;
3513         ForkJoinWorkerThreadFactory factory = null;
3514         UncaughtExceptionHandler handler = null;
3515         try {  // ignore exceptions in accessing/parsing properties
3516             String pp = System.getProperty
3517                 ("java.util.concurrent.ForkJoinPool.common.parallelism");
3518             String fp = System.getProperty
3519                 ("java.util.concurrent.ForkJoinPool.common.threadFactory");
3520             String hp = System.getProperty
3521                 ("java.util.concurrent.ForkJoinPool.common.exceptionHandler");
3522             if (pp != null)
3523                 parallelism = Integer.parseInt(pp);
3524             if (fp != null)
3525                 factory = ((ForkJoinWorkerThreadFactory)ClassLoader.
3526                            getSystemClassLoader().loadClass(fp).newInstance());
3527             if (hp != null)
3528                 handler = ((UncaughtExceptionHandler)ClassLoader.
3529                            getSystemClassLoader().loadClass(hp).newInstance());
3530         } catch (Exception ignore) {
3531         }
3532         if (factory == null) {
3533             if (System.getSecurityManager() == null)
3534                 factory = defaultForkJoinWorkerThreadFactory;
3535             else // use security-managed default
3536                 factory = new InnocuousForkJoinWorkerThreadFactory();
3537         }
3538         if (parallelism < 0 && // default 1 less than #cores
3539             (parallelism = Runtime.getRuntime().availableProcessors() - 1) <= 0)
3540             parallelism = 1;
3541         if (parallelism > MAX_CAP)
3542             parallelism = MAX_CAP;
3543         return new ForkJoinPool(parallelism, factory, handler, LIFO_QUEUE,
3544                                 "ForkJoinPool.commonPool-worker-");
3545     }
3546 
3547     /**
3548      * Factory for innocuous worker threads.
3549      */
3550     private static final class InnocuousForkJoinWorkerThreadFactory
3551         implements ForkJoinWorkerThreadFactory {
3552 
3553         /**
3554          * An ACC to restrict permissions for the factory itself.
3555          * The constructed workers have no permissions set.
3556          */
3557         private static final AccessControlContext innocuousAcc;
3558         static {
3559             Permissions innocuousPerms = new Permissions();
3560             innocuousPerms.add(modifyThreadPermission);
3561             innocuousPerms.add(new RuntimePermission(
3562                                    "enableContextClassLoaderOverride"));
3563             innocuousPerms.add(new RuntimePermission(
3564                                    "modifyThreadGroup"));
3565             innocuousAcc = new AccessControlContext(new ProtectionDomain[] {
3566                     new ProtectionDomain(null, innocuousPerms)
3567                 });
3568         }
3569 
3570         public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
3571             return java.security.AccessController.doPrivileged(
3572                 new java.security.PrivilegedAction<ForkJoinWorkerThread>() {
3573                     public ForkJoinWorkerThread run() {
3574                         return new ForkJoinWorkerThread.
3575                             InnocuousForkJoinWorkerThread(pool);
3576                     }}, innocuousAcc);
3577         }
3578     }
3579 
3580 }