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.
23 */
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 @sun.misc.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 * putOrderedInt). 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
707 * @throws NullPointerException if the pool is null
708 */
709 public ForkJoinWorkerThread newThread(ForkJoinPool pool);
710 }
711
712 /**
713 * Default ForkJoinWorkerThreadFactory implementation; creates a
714 * new ForkJoinWorkerThread.
715 */
716 private static final class DefaultForkJoinWorkerThreadFactory
717 implements ForkJoinWorkerThreadFactory {
718 public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
719 return new ForkJoinWorkerThread(pool);
720 }
721 }
722
723 /**
724 * Class for artificial tasks that are used to replace the target
725 * of local joins if they are removed from an interior queue slot
726 * in WorkQueue.tryRemoveAndExec. We don't need the proxy to
727 * actually do anything beyond having a unique identity.
728 */
729 private static final class EmptyTask extends ForkJoinTask<Void> {
730 private static final long serialVersionUID = -7721805057305804111L;
731 EmptyTask() { status = ForkJoinTask.NORMAL; } // force done
732 public final Void getRawResult() { return null; }
733 public final void setRawResult(Void x) {}
734 public final boolean exec() { return true; }
735 }
736
737 /**
738 * Additional fields and lock created upon initialization.
739 */
740 private static final class AuxState extends ReentrantLock {
741 private static final long serialVersionUID = -6001602636862214147L;
742 volatile long stealCount; // cumulative steal count
743 long indexSeed; // index bits for registerWorker
744 AuxState() {}
745 }
746
747 // Constants shared across ForkJoinPool and WorkQueue
748
749 // Bounds
750 static final int SMASK = 0xffff; // short bits == max index
751 static final int MAX_CAP = 0x7fff; // max #workers - 1
752 static final int EVENMASK = 0xfffe; // even short bits
753 static final int SQMASK = 0x007e; // max 64 (even) slots
754
755 // Masks and units for WorkQueue.scanState and ctl sp subfield
756 static final int UNSIGNALLED = 1 << 31; // must be negative
757 static final int SS_SEQ = 1 << 16; // version count
758
759 // Mode bits for ForkJoinPool.config and WorkQueue.config
760 static final int MODE_MASK = 0xffff << 16; // top half of int
761 static final int SPARE_WORKER = 1 << 17; // set if tc > 0 on creation
762 static final int UNREGISTERED = 1 << 18; // to skip some of deregister
763 static final int FIFO_QUEUE = 1 << 31; // must be negative
764 static final int LIFO_QUEUE = 0; // for clarity
765 static final int IS_OWNED = 1; // low bit 0 if shared
766
767 /**
768 * The maximum number of task executions from the same queue
769 * before checking other queues, bounding unfairness and impact of
770 * infinite user task recursion. Must be a power of two minus 1.
771 */
772 static final int POLL_LIMIT = (1 << 10) - 1;
773
774 /**
775 * Queues supporting work-stealing as well as external task
776 * submission. See above for descriptions and algorithms.
777 * Performance on most platforms is very sensitive to placement of
778 * instances of both WorkQueues and their arrays -- we absolutely
779 * do not want multiple WorkQueue instances or multiple queue
780 * arrays sharing cache lines. The @Contended annotation alerts
781 * JVMs to try to keep instances apart.
782 */
783 @sun.misc.Contended
784 static final class WorkQueue {
785
786 /**
787 * Capacity of work-stealing queue array upon initialization.
788 * Must be a power of two; at least 4, but should be larger to
789 * reduce or eliminate cacheline sharing among queues.
790 * Currently, it is much larger, as a partial workaround for
791 * the fact that JVMs often place arrays in locations that
792 * share GC bookkeeping (especially cardmarks) such that
793 * per-write accesses encounter serious memory contention.
794 */
795 static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
796
797 /**
798 * Maximum size for queue arrays. Must be a power of two less
799 * than or equal to 1 << (31 - width of array entry) to ensure
800 * lack of wraparound of index calculations, but defined to a
801 * value a bit less than this to help users trap runaway
802 * programs before saturating systems.
803 */
804 static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M
805
806 // Instance fields
807
808 volatile int scanState; // versioned, negative if inactive
809 int stackPred; // pool stack (ctl) predecessor
810 int nsteals; // number of steals
811 int hint; // randomization and stealer index hint
812 int config; // pool index and mode
813 volatile int qlock; // 1: locked, < 0: terminate; else 0
814 volatile int base; // index of next slot for poll
815 int top; // index of next slot for push
816 ForkJoinTask<?>[] array; // the elements (initially unallocated)
817 final ForkJoinPool pool; // the containing pool (may be null)
818 final ForkJoinWorkerThread owner; // owning thread or null if shared
819 volatile Thread parker; // == owner during call to park; else null
820 volatile ForkJoinTask<?> currentJoin; // task being joined in awaitJoin
821 @sun.misc.Contended("group2") // separate from other fields
822 volatile ForkJoinTask<?> currentSteal; // nonnull when running some task
823
824 WorkQueue(ForkJoinPool pool, ForkJoinWorkerThread owner) {
825 this.pool = pool;
826 this.owner = owner;
827 // Place indices in the center of array (that is not yet allocated)
828 base = top = INITIAL_QUEUE_CAPACITY >>> 1;
829 }
830
831 /**
832 * Returns an exportable index (used by ForkJoinWorkerThread).
833 */
834 final int getPoolIndex() {
835 return (config & 0xffff) >>> 1; // ignore odd/even tag bit
836 }
837
838 /**
839 * Returns the approximate number of tasks in the queue.
840 */
841 final int queueSize() {
842 int n = base - top; // read base first
843 return (n >= 0) ? 0 : -n; // ignore transient negative
844 }
845
846 /**
847 * Provides a more accurate estimate of whether this queue has
848 * any tasks than does queueSize, by checking whether a
849 * near-empty queue has at least one unclaimed task.
850 */
851 final boolean isEmpty() {
852 ForkJoinTask<?>[] a; int n, al, s;
853 return ((n = base - (s = top)) >= 0 || // possibly one task
854 (n == -1 && ((a = array) == null ||
855 (al = a.length) == 0 ||
856 a[(al - 1) & (s - 1)] == null)));
857 }
858
859 /**
860 * Pushes a task. Call only by owner in unshared queues.
861 *
862 * @param task the task. Caller must ensure non-null.
863 * @throws RejectedExecutionException if array cannot be resized
864 */
865 final void push(ForkJoinTask<?> task) {
866 U.storeFence(); // ensure safe publication
867 int s = top, al, d; ForkJoinTask<?>[] a;
868 if ((a = array) != null && (al = a.length) > 0) {
869 a[(al - 1) & s] = task; // relaxed writes OK
870 top = s + 1;
871 ForkJoinPool p = pool;
872 if ((d = base - s) == 0 && p != null) {
873 U.fullFence();
874 p.signalWork();
875 }
876 else if (al + d == 1)
877 growArray();
878 }
879 }
880
881 /**
882 * Initializes or doubles the capacity of array. Call either
883 * by owner or with lock held -- it is OK for base, but not
884 * top, to move while resizings are in progress.
885 */
886 final ForkJoinTask<?>[] growArray() {
887 ForkJoinTask<?>[] oldA = array;
888 int size = oldA != null ? oldA.length << 1 : INITIAL_QUEUE_CAPACITY;
889 if (size < INITIAL_QUEUE_CAPACITY || size > MAXIMUM_QUEUE_CAPACITY)
890 throw new RejectedExecutionException("Queue capacity exceeded");
891 int oldMask, t, b;
892 ForkJoinTask<?>[] a = array = new ForkJoinTask<?>[size];
893 if (oldA != null && (oldMask = oldA.length - 1) > 0 &&
894 (t = top) - (b = base) > 0) {
895 int mask = size - 1;
896 do { // emulate poll from old array, push to new array
897 int index = b & oldMask;
898 long offset = ((long)index << ASHIFT) + ABASE;
899 ForkJoinTask<?> x = (ForkJoinTask<?>)
900 U.getObjectVolatile(oldA, offset);
901 if (x != null &&
902 U.compareAndSwapObject(oldA, offset, x, null))
903 a[b & mask] = x;
904 } while (++b != t);
905 U.storeFence();
906 }
907 return a;
908 }
909
910 /**
911 * Takes next task, if one exists, in LIFO order. Call only
912 * by owner in unshared queues.
913 */
914 final ForkJoinTask<?> pop() {
915 int b = base, s = top, al, i; ForkJoinTask<?>[] a;
916 if ((a = array) != null && b != s && (al = a.length) > 0) {
917 int index = (al - 1) & --s;
918 long offset = ((long)index << ASHIFT) + ABASE;
919 ForkJoinTask<?> t = (ForkJoinTask<?>)
920 U.getObject(a, offset);
921 if (t != null &&
922 U.compareAndSwapObject(a, offset, t, null)) {
923 top = s;
924 return t;
925 }
926 }
927 return null;
928 }
929
930 /**
931 * Takes a task in FIFO order if b is base of queue and a task
932 * can be claimed without contention. Specialized versions
933 * appear in ForkJoinPool methods scan and helpStealer.
934 */
935 final ForkJoinTask<?> pollAt(int b) {
936 ForkJoinTask<?>[] a; int al;
937 if ((a = array) != null && (al = a.length) > 0) {
938 int index = (al - 1) & b;
939 long offset = ((long)index << ASHIFT) + ABASE;
940 ForkJoinTask<?> t = (ForkJoinTask<?>)
941 U.getObjectVolatile(a, offset);
942 if (t != null && b++ == base &&
943 U.compareAndSwapObject(a, offset, t, null)) {
944 base = b;
945 return t;
946 }
947 }
948 return null;
949 }
950
951 /**
952 * Takes next task, if one exists, in FIFO order.
953 */
954 final ForkJoinTask<?> poll() {
955 for (;;) {
956 int b = base, s = top, d, al; ForkJoinTask<?>[] a;
957 if ((a = array) != null && (d = b - s) < 0 &&
958 (al = a.length) > 0) {
959 int index = (al - 1) & b;
960 long offset = ((long)index << ASHIFT) + ABASE;
961 ForkJoinTask<?> t = (ForkJoinTask<?>)
962 U.getObjectVolatile(a, offset);
963 if (b++ == base) {
964 if (t != null) {
965 if (U.compareAndSwapObject(a, offset, t, null)) {
966 base = b;
967 return t;
968 }
969 }
970 else if (d == -1)
971 break; // now empty
972 }
973 }
974 else
975 break;
976 }
977 return null;
978 }
979
980 /**
981 * Takes next task, if one exists, in order specified by mode.
982 */
983 final ForkJoinTask<?> nextLocalTask() {
984 return (config < 0) ? poll() : pop();
985 }
986
987 /**
988 * Returns next task, if one exists, in order specified by mode.
989 */
990 final ForkJoinTask<?> peek() {
991 int al; ForkJoinTask<?>[] a;
992 return ((a = array) != null && (al = a.length) > 0) ?
993 a[(al - 1) & (config < 0 ? base : top - 1)] : null;
994 }
995
996 /**
997 * Pops the given task only if it is at the current top.
998 */
999 final boolean tryUnpush(ForkJoinTask<?> task) {
1000 int b = base, s = top, al; ForkJoinTask<?>[] a;
1001 if ((a = array) != null && b != s && (al = a.length) > 0) {
1002 int index = (al - 1) & --s;
1003 long offset = ((long)index << ASHIFT) + ABASE;
1004 if (U.compareAndSwapObject(a, offset, task, null)) {
1005 top = s;
1006 return true;
1007 }
1008 }
1009 return false;
1010 }
1011
1012 /**
1013 * Shared version of push. Fails if already locked.
1014 *
1015 * @return status: > 0 locked, 0 possibly was empty, < 0 was nonempty
1016 */
1017 final int sharedPush(ForkJoinTask<?> task) {
1018 int stat;
1019 if (U.compareAndSwapInt(this, QLOCK, 0, 1)) {
1020 int b = base, s = top, al, d; ForkJoinTask<?>[] a;
1021 if ((a = array) != null && (al = a.length) > 0 &&
1022 al - 1 + (d = b - s) > 0) {
1023 a[(al - 1) & s] = task;
1024 top = s + 1; // relaxed writes OK here
1025 qlock = 0;
1026 stat = (d < 0 && b == base) ? d : 0;
1027 }
1028 else {
1029 growAndSharedPush(task);
1030 stat = 0;
1031 }
1032 }
1033 else
1034 stat = 1;
1035 return stat;
1036 }
1037
1038 /**
1039 * Helper for sharedPush; called only when locked and resize
1040 * needed.
1041 */
1042 private void growAndSharedPush(ForkJoinTask<?> task) {
1043 try {
1044 growArray();
1045 int s = top, al; ForkJoinTask<?>[] a;
1046 if ((a = array) != null && (al = a.length) > 0) {
1047 a[(al - 1) & s] = task;
1048 top = s + 1;
1049 }
1050 } finally {
1051 qlock = 0;
1052 }
1053 }
1054
1055 /**
1056 * Shared version of pop.
1057 */
1058 final boolean trySharedUnpush(ForkJoinTask<?> task) {
1059 boolean popped = false;
1060 int s = top - 1, al; ForkJoinTask<?>[] a;
1061 if ((a = array) != null && (al = a.length) > 0) {
1062 int index = (al - 1) & s;
1063 long offset = ((long)index << ASHIFT) + ABASE;
1064 ForkJoinTask<?> t = (ForkJoinTask<?>) U.getObject(a, offset);
1065 if (t == task &&
1066 U.compareAndSwapInt(this, QLOCK, 0, 1)) {
1067 if (U.compareAndSwapObject(a, offset, task, null)) {
1068 popped = true;
1069 top = s;
1070 }
1071 U.putOrderedInt(this, QLOCK, 0);
1072 }
1073 }
1074 return popped;
1075 }
1076
1077 /**
1078 * Removes and cancels all known tasks, ignoring any exceptions.
1079 */
1080 final void cancelAll() {
1081 ForkJoinTask<?> t;
1082 if ((t = currentJoin) != null) {
1083 currentJoin = null;
1084 ForkJoinTask.cancelIgnoringExceptions(t);
1085 }
1086 if ((t = currentSteal) != null) {
1087 currentSteal = null;
1088 ForkJoinTask.cancelIgnoringExceptions(t);
1089 }
1090 while ((t = poll()) != null)
1091 ForkJoinTask.cancelIgnoringExceptions(t);
1092 }
1093
1094 // Specialized execution methods
1095
1096 /**
1097 * Pops and executes up to POLL_LIMIT tasks or until empty.
1098 */
1099 final void localPopAndExec() {
1100 for (int nexec = 0;;) {
1101 int b = base, s = top, al; ForkJoinTask<?>[] a;
1102 if ((a = array) != null && b != s && (al = a.length) > 0) {
1103 int index = (al - 1) & --s;
1104 long offset = ((long)index << ASHIFT) + ABASE;
1105 ForkJoinTask<?> t = (ForkJoinTask<?>)
1106 U.getAndSetObject(a, offset, null);
1107 if (t != null) {
1108 top = s;
1109 (currentSteal = t).doExec();
1110 if (++nexec > POLL_LIMIT)
1111 break;
1112 }
1113 else
1114 break;
1115 }
1116 else
1117 break;
1118 }
1119 }
1120
1121 /**
1122 * Polls and executes up to POLL_LIMIT tasks or until empty.
1123 */
1124 final void localPollAndExec() {
1125 for (int nexec = 0;;) {
1126 int b = base, s = top, al; ForkJoinTask<?>[] a;
1127 if ((a = array) != null && b != s && (al = a.length) > 0) {
1128 int index = (al - 1) & b++;
1129 long offset = ((long)index << ASHIFT) + ABASE;
1130 ForkJoinTask<?> t = (ForkJoinTask<?>)
1131 U.getAndSetObject(a, offset, null);
1132 if (t != null) {
1133 base = b;
1134 t.doExec();
1135 if (++nexec > POLL_LIMIT)
1136 break;
1137 }
1138 }
1139 else
1140 break;
1141 }
1142 }
1143
1144 /**
1145 * Executes the given task and (some) remaining local tasks.
1146 */
1147 final void runTask(ForkJoinTask<?> task) {
1148 if (task != null) {
1149 task.doExec();
1150 if (config < 0)
1151 localPollAndExec();
1152 else
1153 localPopAndExec();
1154 int ns = ++nsteals;
1155 ForkJoinWorkerThread thread = owner;
1156 currentSteal = null;
1157 if (ns < 0) // collect on overflow
1158 transferStealCount(pool);
1159 if (thread != null)
1160 thread.afterTopLevelExec();
1161 }
1162 }
1163
1164 /**
1165 * Adds steal count to pool steal count if it exists, and resets.
1166 */
1167 final void transferStealCount(ForkJoinPool p) {
1168 AuxState aux;
1169 if (p != null && (aux = p.auxState) != null) {
1170 long s = nsteals;
1171 nsteals = 0; // if negative, correct for overflow
1172 if (s < 0) s = Integer.MAX_VALUE;
1173 aux.lock();
1174 try {
1175 aux.stealCount += s;
1176 } finally {
1177 aux.unlock();
1178 }
1179 }
1180 }
1181
1182 /**
1183 * If present, removes from queue and executes the given task,
1184 * or any other cancelled task. Used only by awaitJoin.
1185 *
1186 * @return true if queue empty and task not known to be done
1187 */
1188 final boolean tryRemoveAndExec(ForkJoinTask<?> task) {
1189 if (task != null && task.status >= 0) {
1190 int b, s, d, al; ForkJoinTask<?>[] a;
1191 while ((d = (b = base) - (s = top)) < 0 &&
1192 (a = array) != null && (al = a.length) > 0) {
1193 for (;;) { // traverse from s to b
1194 int index = --s & (al - 1);
1195 long offset = (index << ASHIFT) + ABASE;
1196 ForkJoinTask<?> t = (ForkJoinTask<?>)
1197 U.getObjectVolatile(a, offset);
1198 if (t == null)
1199 break; // restart
1200 else if (t == task) {
1201 boolean removed = false;
1202 if (s + 1 == top) { // pop
1203 if (U.compareAndSwapObject(a, offset, t, null)) {
1204 top = s;
1205 removed = true;
1206 }
1207 }
1208 else if (base == b) // replace with proxy
1209 removed = U.compareAndSwapObject(a, offset, t,
1210 new EmptyTask());
1211 if (removed) {
1212 ForkJoinTask<?> ps = currentSteal;
1213 (currentSteal = task).doExec();
1214 currentSteal = ps;
1215 }
1216 break;
1217 }
1218 else if (t.status < 0 && s + 1 == top) {
1219 if (U.compareAndSwapObject(a, offset, t, null)) {
1220 top = s;
1221 }
1222 break; // was cancelled
1223 }
1224 else if (++d == 0) {
1225 if (base != b) // rescan
1226 break;
1227 return false;
1228 }
1229 }
1230 if (task.status < 0)
1231 return false;
1232 }
1233 }
1234 return true;
1235 }
1236
1237 /**
1238 * Pops task if in the same CC computation as the given task,
1239 * in either shared or owned mode. Used only by helpComplete.
1240 */
1241 final CountedCompleter<?> popCC(CountedCompleter<?> task, int mode) {
1242 int b = base, s = top, al; ForkJoinTask<?>[] a;
1243 if ((a = array) != null && b != s && (al = a.length) > 0) {
1244 int index = (al - 1) & (s - 1);
1245 long offset = ((long)index << ASHIFT) + ABASE;
1246 ForkJoinTask<?> o = (ForkJoinTask<?>)
1247 U.getObjectVolatile(a, offset);
1248 if (o instanceof CountedCompleter) {
1249 CountedCompleter<?> t = (CountedCompleter<?>)o;
1250 for (CountedCompleter<?> r = t;;) {
1251 if (r == task) {
1252 if ((mode & IS_OWNED) == 0) {
1253 boolean popped;
1254 if (U.compareAndSwapInt(this, QLOCK, 0, 1)) {
1255 if (popped =
1256 U.compareAndSwapObject(a, offset,
1257 t, null))
1258 top = s - 1;
1259 U.putOrderedInt(this, QLOCK, 0);
1260 if (popped)
1261 return t;
1262 }
1263 }
1264 else if (U.compareAndSwapObject(a, offset,
1265 t, null)) {
1266 top = s - 1;
1267 return t;
1268 }
1269 break;
1270 }
1271 else if ((r = r.completer) == null) // try parent
1272 break;
1273 }
1274 }
1275 }
1276 return null;
1277 }
1278
1279 /**
1280 * Steals and runs a task in the same CC computation as the
1281 * given task if one exists and can be taken without
1282 * contention. Otherwise returns a checksum/control value for
1283 * use by method helpComplete.
1284 *
1285 * @return 1 if successful, 2 if retryable (lost to another
1286 * stealer), -1 if non-empty but no matching task found, else
1287 * the base index, forced negative.
1288 */
1289 final int pollAndExecCC(CountedCompleter<?> task) {
1290 ForkJoinTask<?>[] a;
1291 int b = base, s = top, al, h;
1292 if ((a = array) != null && b != s && (al = a.length) > 0) {
1293 int index = (al - 1) & b;
1294 long offset = ((long)index << ASHIFT) + ABASE;
1295 ForkJoinTask<?> o = (ForkJoinTask<?>)
1296 U.getObjectVolatile(a, offset);
1297 if (o == null)
1298 h = 2; // retryable
1299 else if (!(o instanceof CountedCompleter))
1300 h = -1; // unmatchable
1301 else {
1302 CountedCompleter<?> t = (CountedCompleter<?>)o;
1303 for (CountedCompleter<?> r = t;;) {
1304 if (r == task) {
1305 if (b++ == base &&
1306 U.compareAndSwapObject(a, offset, t, null)) {
1307 base = b;
1308 t.doExec();
1309 h = 1; // success
1310 }
1311 else
1312 h = 2; // lost CAS
1313 break;
1314 }
1315 else if ((r = r.completer) == null) {
1316 h = -1; // unmatched
1317 break;
1318 }
1319 }
1320 }
1321 }
1322 else
1323 h = b | Integer.MIN_VALUE; // to sense movement on re-poll
1324 return h;
1325 }
1326
1327 /**
1328 * Returns true if owned and not known to be blocked.
1329 */
1330 final boolean isApparentlyUnblocked() {
1331 Thread wt; Thread.State s;
1332 return (scanState >= 0 &&
1333 (wt = owner) != null &&
1334 (s = wt.getState()) != Thread.State.BLOCKED &&
1335 s != Thread.State.WAITING &&
1336 s != Thread.State.TIMED_WAITING);
1337 }
1338
1339 // Unsafe mechanics. Note that some are (and must be) the same as in FJP
1340 private static final jdk.internal.misc.Unsafe U = jdk.internal.misc.Unsafe.getUnsafe();
1341 private static final long QLOCK;
1342 private static final int ABASE;
1343 private static final int ASHIFT;
1344 static {
1345 try {
1346 QLOCK = U.objectFieldOffset
1347 (WorkQueue.class.getDeclaredField("qlock"));
1348 ABASE = U.arrayBaseOffset(ForkJoinTask[].class);
1349 int scale = U.arrayIndexScale(ForkJoinTask[].class);
1350 if ((scale & (scale - 1)) != 0)
1351 throw new Error("array index scale not a power of two");
1352 ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
1353 } catch (ReflectiveOperationException e) {
1354 throw new Error(e);
1355 }
1356 }
1357 }
1358
1359 // static fields (initialized in static initializer below)
1360
1361 /**
1362 * Creates a new ForkJoinWorkerThread. This factory is used unless
1363 * overridden in ForkJoinPool constructors.
1364 */
1365 public static final ForkJoinWorkerThreadFactory
1366 defaultForkJoinWorkerThreadFactory;
1367
1368 /**
1369 * Permission required for callers of methods that may start or
1370 * kill threads. Also used as a static lock in tryInitialize.
1371 */
1372 static final RuntimePermission modifyThreadPermission;
1373
1374 /**
1375 * Common (static) pool. Non-null for public use unless a static
1376 * construction exception, but internal usages null-check on use
1377 * to paranoically avoid potential initialization circularities
1378 * as well as to simplify generated code.
1379 */
1380 static final ForkJoinPool common;
1381
1382 /**
1383 * Common pool parallelism. To allow simpler use and management
1384 * when common pool threads are disabled, we allow the underlying
1385 * common.parallelism field to be zero, but in that case still report
1386 * parallelism as 1 to reflect resulting caller-runs mechanics.
1387 */
1388 static final int COMMON_PARALLELISM;
1389
1390 /**
1391 * Limit on spare thread construction in tryCompensate.
1392 */
1393 private static final int COMMON_MAX_SPARES;
1394
1395 /**
1396 * Sequence number for creating workerNamePrefix.
1397 */
1398 private static int poolNumberSequence;
1399
1400 /**
1401 * Returns the next sequence number. We don't expect this to
1402 * ever contend, so use simple builtin sync.
1403 */
1404 private static final synchronized int nextPoolId() {
1405 return ++poolNumberSequence;
1406 }
1407
1408 // static configuration constants
1409
1410 /**
1411 * Initial timeout value (in milliseconds) for the thread
1412 * triggering quiescence to park waiting for new work. On timeout,
1413 * the thread will instead try to shrink the number of workers.
1414 * The value should be large enough to avoid overly aggressive
1415 * shrinkage during most transient stalls (long GCs etc).
1416 */
1417 private static final long IDLE_TIMEOUT_MS = 2000L; // 2sec
1418
1419 /**
1420 * Tolerance for idle timeouts, to cope with timer undershoots.
1421 */
1422 private static final long TIMEOUT_SLOP_MS = 20L; // 20ms
1423
1424 /**
1425 * The default value for COMMON_MAX_SPARES. Overridable using the
1426 * "java.util.concurrent.ForkJoinPool.common.maximumSpares" system
1427 * property. The default value is far in excess of normal
1428 * requirements, but also far short of MAX_CAP and typical OS
1429 * thread limits, so allows JVMs to catch misuse/abuse before
1430 * running out of resources needed to do so.
1431 */
1432 private static final int DEFAULT_COMMON_MAX_SPARES = 256;
1433
1434 /**
1435 * Increment for seed generators. See class ThreadLocal for
1436 * explanation.
1437 */
1438 private static final int SEED_INCREMENT = 0x9e3779b9;
1439
1440 /*
1441 * Bits and masks for field ctl, packed with 4 16 bit subfields:
1442 * AC: Number of active running workers minus target parallelism
1443 * TC: Number of total workers minus target parallelism
1444 * SS: version count and status of top waiting thread
1445 * ID: poolIndex of top of Treiber stack of waiters
1446 *
1447 * When convenient, we can extract the lower 32 stack top bits
1448 * (including version bits) as sp=(int)ctl. The offsets of counts
1449 * by the target parallelism and the positionings of fields makes
1450 * it possible to perform the most common checks via sign tests of
1451 * fields: When ac is negative, there are not enough active
1452 * workers, when tc is negative, there are not enough total
1453 * workers. When sp is non-zero, there are waiting workers. To
1454 * deal with possibly negative fields, we use casts in and out of
1455 * "short" and/or signed shifts to maintain signedness.
1456 *
1457 * Because it occupies uppermost bits, we can add one active count
1458 * using getAndAddLong of AC_UNIT, rather than CAS, when returning
1459 * from a blocked join. Other updates entail multiple subfields
1460 * and masking, requiring CAS.
1461 */
1462
1463 // Lower and upper word masks
1464 private static final long SP_MASK = 0xffffffffL;
1465 private static final long UC_MASK = ~SP_MASK;
1466
1467 // Active counts
1468 private static final int AC_SHIFT = 48;
1469 private static final long AC_UNIT = 0x0001L << AC_SHIFT;
1470 private static final long AC_MASK = 0xffffL << AC_SHIFT;
1471
1472 // Total counts
1473 private static final int TC_SHIFT = 32;
1474 private static final long TC_UNIT = 0x0001L << TC_SHIFT;
1475 private static final long TC_MASK = 0xffffL << TC_SHIFT;
1476 private static final long ADD_WORKER = 0x0001L << (TC_SHIFT + 15); // sign
1477
1478 // runState bits: SHUTDOWN must be negative, others arbitrary powers of two
1479 private static final int STARTED = 1;
1480 private static final int STOP = 1 << 1;
1481 private static final int TERMINATED = 1 << 2;
1482 private static final int SHUTDOWN = 1 << 31;
1483
1484 // Instance fields
1485 volatile long ctl; // main pool control
1486 volatile int runState;
1487 final int config; // parallelism, mode
1488 AuxState auxState; // lock, steal counts
1489 volatile WorkQueue[] workQueues; // main registry
1490 final String workerNamePrefix; // to create worker name string
1491 final ForkJoinWorkerThreadFactory factory;
1492 final UncaughtExceptionHandler ueh; // per-worker UEH
1493
1494 /**
1495 * Instantiates fields upon first submission, or upon shutdown if
1496 * no submissions. If checkTermination true, also responds to
1497 * termination by external calls submitting tasks.
1498 */
1499 private void tryInitialize(boolean checkTermination) {
1500 if (runState == 0) { // bootstrap by locking static field
1501 int p = config & SMASK;
1502 int n = (p > 1) ? p - 1 : 1; // ensure at least 2 slots
1503 n |= n >>> 1; // create workQueues array with size a power of two
1504 n |= n >>> 2;
1505 n |= n >>> 4;
1506 n |= n >>> 8;
1507 n |= n >>> 16;
1508 n = ((n + 1) << 1) & SMASK;
1509 AuxState aux = new AuxState();
1510 WorkQueue[] ws = new WorkQueue[n];
1511 synchronized (modifyThreadPermission) { // double-check
1512 if (runState == 0) {
1513 workQueues = ws;
1514 auxState = aux;
1515 runState = STARTED;
1516 }
1517 }
1518 }
1519 if (checkTermination && runState < 0) {
1520 tryTerminate(false, false); // help terminate
1521 throw new RejectedExecutionException();
1522 }
1523 }
1524
1525 // Creating, registering and deregistering workers
1526
1527 /**
1528 * Tries to construct and start one worker. Assumes that total
1529 * count has already been incremented as a reservation. Invokes
1530 * deregisterWorker on any failure.
1531 *
1532 * @param isSpare true if this is a spare thread
1533 * @return true if successful
1534 */
1535 private boolean createWorker(boolean isSpare) {
1536 ForkJoinWorkerThreadFactory fac = factory;
1537 Throwable ex = null;
1538 ForkJoinWorkerThread wt = null;
1539 WorkQueue q;
1540 try {
1541 if (fac != null && (wt = fac.newThread(this)) != null) {
1542 if (isSpare && (q = wt.workQueue) != null)
1543 q.config |= SPARE_WORKER;
1544 wt.start();
1545 return true;
1546 }
1547 } catch (Throwable rex) {
1548 ex = rex;
1549 }
1550 deregisterWorker(wt, ex);
1551 return false;
1552 }
1553
1554 /**
1555 * Tries to add one worker, incrementing ctl counts before doing
1556 * so, relying on createWorker to back out on failure.
1557 *
1558 * @param c incoming ctl value, with total count negative and no
1559 * idle workers. On CAS failure, c is refreshed and retried if
1560 * this holds (otherwise, a new worker is not needed).
1561 */
1562 private void tryAddWorker(long c) {
1563 do {
1564 long nc = ((AC_MASK & (c + AC_UNIT)) |
1565 (TC_MASK & (c + TC_UNIT)));
1566 if (ctl == c && U.compareAndSwapLong(this, CTL, c, nc)) {
1567 createWorker(false);
1568 break;
1569 }
1570 } while (((c = ctl) & ADD_WORKER) != 0L && (int)c == 0);
1571 }
1572
1573 /**
1574 * Callback from ForkJoinWorkerThread constructor to establish and
1575 * record its WorkQueue.
1576 *
1577 * @param wt the worker thread
1578 * @return the worker's queue
1579 */
1580 final WorkQueue registerWorker(ForkJoinWorkerThread wt) {
1581 UncaughtExceptionHandler handler;
1582 AuxState aux;
1583 wt.setDaemon(true); // configure thread
1584 if ((handler = ueh) != null)
1585 wt.setUncaughtExceptionHandler(handler);
1586 WorkQueue w = new WorkQueue(this, wt);
1587 int i = 0; // assign a pool index
1588 int mode = config & MODE_MASK;
1589 if ((aux = auxState) != null) {
1590 aux.lock();
1591 try {
1592 int s = (int)(aux.indexSeed += SEED_INCREMENT), n, m;
1593 WorkQueue[] ws = workQueues;
1594 if (ws != null && (n = ws.length) > 0) {
1595 i = (m = n - 1) & ((s << 1) | 1); // odd-numbered indices
1596 if (ws[i] != null) { // collision
1597 int probes = 0; // step by approx half n
1598 int step = (n <= 4) ? 2 : ((n >>> 1) & EVENMASK) + 2;
1599 while (ws[i = (i + step) & m] != null) {
1600 if (++probes >= n) {
1601 workQueues = ws = Arrays.copyOf(ws, n <<= 1);
1602 m = n - 1;
1603 probes = 0;
1604 }
1605 }
1606 }
1607 w.hint = s; // use as random seed
1608 w.config = i | mode;
1609 w.scanState = i | (s & 0x7fff0000); // random seq bits
1610 ws[i] = w;
1611 }
1612 } finally {
1613 aux.unlock();
1614 }
1615 }
1616 wt.setName(workerNamePrefix.concat(Integer.toString(i >>> 1)));
1617 return w;
1618 }
1619
1620 /**
1621 * Final callback from terminating worker, as well as upon failure
1622 * to construct or start a worker. Removes record of worker from
1623 * array, and adjusts counts. If pool is shutting down, tries to
1624 * complete termination.
1625 *
1626 * @param wt the worker thread, or null if construction failed
1627 * @param ex the exception causing failure, or null if none
1628 */
1629 final void deregisterWorker(ForkJoinWorkerThread wt, Throwable ex) {
1630 WorkQueue w = null;
1631 if (wt != null && (w = wt.workQueue) != null) {
1632 AuxState aux; WorkQueue[] ws; // remove index from array
1633 int idx = w.config & SMASK;
1634 int ns = w.nsteals;
1635 if ((aux = auxState) != null) {
1636 aux.lock();
1637 try {
1638 if ((ws = workQueues) != null && ws.length > idx &&
1639 ws[idx] == w)
1640 ws[idx] = null;
1641 aux.stealCount += ns;
1642 } finally {
1643 aux.unlock();
1644 }
1645 }
1646 }
1647 if (w == null || (w.config & UNREGISTERED) == 0) { // else pre-adjusted
1648 long c; // decrement counts
1649 do {} while (!U.compareAndSwapLong
1650 (this, CTL, c = ctl, ((AC_MASK & (c - AC_UNIT)) |
1651 (TC_MASK & (c - TC_UNIT)) |
1652 (SP_MASK & c))));
1653 }
1654 if (w != null) {
1655 w.currentSteal = null;
1656 w.qlock = -1; // ensure set
1657 w.cancelAll(); // cancel remaining tasks
1658 }
1659 while (tryTerminate(false, false) >= 0) { // possibly replace
1660 WorkQueue[] ws; int wl, sp; long c;
1661 if (w == null || w.array == null ||
1662 (ws = workQueues) == null || (wl = ws.length) <= 0)
1663 break;
1664 else if ((sp = (int)(c = ctl)) != 0) { // wake up replacement
1665 if (tryRelease(c, ws[(wl - 1) & sp], AC_UNIT))
1666 break;
1667 }
1668 else if (ex != null && (c & ADD_WORKER) != 0L) {
1669 tryAddWorker(c); // create replacement
1670 break;
1671 }
1672 else // don't need replacement
1673 break;
1674 }
1675 if (ex == null) // help clean on way out
1676 ForkJoinTask.helpExpungeStaleExceptions();
1677 else // rethrow
1678 ForkJoinTask.rethrow(ex);
1679 }
1680
1681 // Signalling
1682
1683 /**
1684 * Tries to create or activate a worker if too few are active.
1685 */
1686 final void signalWork() {
1687 for (;;) {
1688 long c; int sp, i; WorkQueue v; WorkQueue[] ws;
1689 if ((c = ctl) >= 0L) // enough workers
1690 break;
1691 else if ((sp = (int)c) == 0) { // no idle workers
1692 if ((c & ADD_WORKER) != 0L) // too few workers
1693 tryAddWorker(c);
1694 break;
1695 }
1696 else if ((ws = workQueues) == null)
1697 break; // unstarted/terminated
1698 else if (ws.length <= (i = sp & SMASK))
1699 break; // terminated
1700 else if ((v = ws[i]) == null)
1701 break; // terminating
1702 else {
1703 int ns = sp & ~UNSIGNALLED;
1704 int vs = v.scanState;
1705 long nc = (v.stackPred & SP_MASK) | (UC_MASK & (c + AC_UNIT));
1706 if (sp == vs && U.compareAndSwapLong(this, CTL, c, nc)) {
1707 v.scanState = ns;
1708 LockSupport.unpark(v.parker);
1709 break;
1710 }
1711 }
1712 }
1713 }
1714
1715 /**
1716 * Signals and releases worker v if it is top of idle worker
1717 * stack. This performs a one-shot version of signalWork only if
1718 * there is (apparently) at least one idle worker.
1719 *
1720 * @param c incoming ctl value
1721 * @param v if non-null, a worker
1722 * @param inc the increment to active count (zero when compensating)
1723 * @return true if successful
1724 */
1725 private boolean tryRelease(long c, WorkQueue v, long inc) {
1726 int sp = (int)c, ns = sp & ~UNSIGNALLED;
1727 if (v != null) {
1728 int vs = v.scanState;
1729 long nc = (v.stackPred & SP_MASK) | (UC_MASK & (c + inc));
1730 if (sp == vs && U.compareAndSwapLong(this, CTL, c, nc)) {
1731 v.scanState = ns;
1732 LockSupport.unpark(v.parker);
1733 return true;
1734 }
1735 }
1736 return false;
1737 }
1738
1739 /**
1740 * With approx probability of a missed signal, tries (once) to
1741 * reactivate worker w (or some other worker), failing if stale or
1742 * known to be already active.
1743 *
1744 * @param w the worker
1745 * @param ws the workQueue array to use
1746 * @param r random seed
1747 */
1748 private void tryReactivate(WorkQueue w, WorkQueue[] ws, int r) {
1749 long c; int sp, wl; WorkQueue v;
1750 if ((sp = (int)(c = ctl)) != 0 && w != null &&
1751 ws != null && (wl = ws.length) > 0 &&
1752 ((sp ^ r) & SS_SEQ) == 0 &&
1753 (v = ws[(wl - 1) & sp]) != null) {
1754 long nc = (v.stackPred & SP_MASK) | (UC_MASK & (c + AC_UNIT));
1755 int ns = sp & ~UNSIGNALLED;
1756 if (w.scanState < 0 &&
1757 v.scanState == sp &&
1758 U.compareAndSwapLong(this, CTL, c, nc)) {
1759 v.scanState = ns;
1760 LockSupport.unpark(v.parker);
1761 }
1762 }
1763 }
1764
1765 /**
1766 * If worker w exists and is active, enqueues and sets status to inactive.
1767 *
1768 * @param w the worker
1769 * @param ss current (non-negative) scanState
1770 */
1771 private void inactivate(WorkQueue w, int ss) {
1772 int ns = (ss + SS_SEQ) | UNSIGNALLED;
1773 long lc = ns & SP_MASK, nc, c;
1774 if (w != null) {
1775 w.scanState = ns;
1776 do {
1777 nc = lc | (UC_MASK & ((c = ctl) - AC_UNIT));
1778 w.stackPred = (int)c;
1779 } while (!U.compareAndSwapLong(this, CTL, c, nc));
1780 }
1781 }
1782
1783 /**
1784 * Possibly blocks worker w waiting for signal, or returns
1785 * negative status if the worker should terminate. May return
1786 * without status change if multiple stale unparks and/or
1787 * interrupts occur.
1788 *
1789 * @param w the calling worker
1790 * @return negative if w should terminate
1791 */
1792 private int awaitWork(WorkQueue w) {
1793 int stat = 0;
1794 if (w != null && w.scanState < 0) {
1795 long c = ctl;
1796 if ((int)(c >> AC_SHIFT) + (config & SMASK) <= 0)
1797 stat = timedAwaitWork(w, c); // possibly quiescent
1798 else if ((runState & STOP) != 0)
1799 stat = w.qlock = -1; // pool terminating
1800 else if (w.scanState < 0) {
1801 w.parker = Thread.currentThread();
1802 if (w.scanState < 0) // recheck after write
1803 LockSupport.park(this);
1804 w.parker = null;
1805 if ((runState & STOP) != 0)
1806 stat = w.qlock = -1; // recheck
1807 else if (w.scanState < 0)
1808 Thread.interrupted(); // clear status
1809 }
1810 }
1811 return stat;
1812 }
1813
1814 /**
1815 * Possibly triggers shutdown and tries (once) to block worker
1816 * when pool is (or may be) quiescent. Waits up to a duration
1817 * determined by number of workers. On timeout, if ctl has not
1818 * changed, terminates the worker, which will in turn wake up
1819 * another worker to possibly repeat this process.
1820 *
1821 * @param w the calling worker
1822 * @return negative if w should terminate
1823 */
1824 private int timedAwaitWork(WorkQueue w, long c) {
1825 int stat = 0;
1826 int scale = 1 - (short)(c >>> TC_SHIFT);
1827 long deadline = (((scale <= 0) ? 1 : scale) * IDLE_TIMEOUT_MS +
1828 System.currentTimeMillis());
1829 if ((runState >= 0 || (stat = tryTerminate(false, false)) > 0) &&
1830 w != null && w.scanState < 0) {
1831 int ss; AuxState aux;
1832 w.parker = Thread.currentThread();
1833 if (w.scanState < 0)
1834 LockSupport.parkUntil(this, deadline);
1835 w.parker = null;
1836 if ((runState & STOP) != 0)
1837 stat = w.qlock = -1; // pool terminating
1838 else if ((ss = w.scanState) < 0 && !Thread.interrupted() &&
1839 (int)c == ss && (aux = auxState) != null && ctl == c &&
1840 deadline - System.currentTimeMillis() <= TIMEOUT_SLOP_MS) {
1841 aux.lock();
1842 try { // pre-deregister
1843 WorkQueue[] ws;
1844 int cfg = w.config, idx = cfg & SMASK;
1845 long nc = ((UC_MASK & (c - TC_UNIT)) |
1846 (SP_MASK & w.stackPred));
1847 if ((runState & STOP) == 0 &&
1848 (ws = workQueues) != null &&
1849 idx < ws.length && idx >= 0 && ws[idx] == w &&
1850 U.compareAndSwapLong(this, CTL, c, nc)) {
1851 ws[idx] = null;
1852 w.config = cfg | UNREGISTERED;
1853 stat = w.qlock = -1;
1854 }
1855 } finally {
1856 aux.unlock();
1857 }
1858 }
1859 }
1860 return stat;
1861 }
1862
1863 /**
1864 * If the given worker is a spare with no queued tasks, and there
1865 * are enough existing workers, drops it from ctl counts and sets
1866 * its state to terminated.
1867 *
1868 * @param w the calling worker -- must be a spare
1869 * @return true if dropped (in which case it must not process more tasks)
1870 */
1871 private boolean tryDropSpare(WorkQueue w) {
1872 if (w != null && w.isEmpty()) { // no local tasks
1873 long c; int sp, wl; WorkQueue[] ws; WorkQueue v;
1874 while ((short)((c = ctl) >> TC_SHIFT) > 0 &&
1875 ((sp = (int)c) != 0 || (int)(c >> AC_SHIFT) > 0) &&
1876 (ws = workQueues) != null && (wl = ws.length) > 0) {
1877 boolean dropped, canDrop;
1878 if (sp == 0) { // no queued workers
1879 long nc = ((AC_MASK & (c - AC_UNIT)) |
1880 (TC_MASK & (c - TC_UNIT)) | (SP_MASK & c));
1881 dropped = U.compareAndSwapLong(this, CTL, c, nc);
1882 }
1883 else if (
1884 (v = ws[(wl - 1) & sp]) == null || v.scanState != sp)
1885 dropped = false; // stale; retry
1886 else {
1887 long nc = v.stackPred & SP_MASK;
1888 if (w == v || w.scanState >= 0) {
1889 canDrop = true; // w unqueued or topmost
1890 nc |= ((AC_MASK & c) | // ensure replacement
1891 (TC_MASK & (c - TC_UNIT)));
1892 }
1893 else { // w may be queued
1894 canDrop = false; // help uncover
1895 nc |= ((AC_MASK & (c + AC_UNIT)) |
1896 (TC_MASK & c));
1897 }
1898 if (U.compareAndSwapLong(this, CTL, c, nc)) {
1899 v.scanState = sp & ~UNSIGNALLED;
1900 LockSupport.unpark(v.parker);
1901 dropped = canDrop;
1902 }
1903 else
1904 dropped = false;
1905 }
1906 if (dropped) { // pre-deregister
1907 int cfg = w.config, idx = cfg & SMASK;
1908 if (idx >= 0 && idx < ws.length && ws[idx] == w)
1909 ws[idx] = null;
1910 w.config = cfg | UNREGISTERED;
1911 w.qlock = -1;
1912 return true;
1913 }
1914 }
1915 }
1916 return false;
1917 }
1918
1919 /**
1920 * Top-level runloop for workers, called by ForkJoinWorkerThread.run.
1921 */
1922 final void runWorker(WorkQueue w) {
1923 w.growArray(); // allocate queue
1924 int bound = (w.config & SPARE_WORKER) != 0 ? 0 : POLL_LIMIT;
1925 long seed = w.hint * 0xdaba0b6eb09322e3L; // initial random seed
1926 if ((runState & STOP) == 0) {
1927 for (long r = (seed == 0L) ? 1L : seed;;) { // ensure nonzero
1928 if (bound == 0 && tryDropSpare(w))
1929 break;
1930 // high bits of prev seed for step; current low bits for idx
1931 int step = (int)(r >>> 48) | 1;
1932 r ^= r >>> 12; r ^= r << 25; r ^= r >>> 27; // xorshift
1933 if (scan(w, bound, step, (int)r) < 0 && awaitWork(w) < 0)
1934 break;
1935 }
1936 }
1937 }
1938
1939 // Scanning for tasks
1940
1941 /**
1942 * Repeatedly scans for and tries to steal and execute (via
1943 * workQueue.runTask) a queued task. Each scan traverses queues in
1944 * pseudorandom permutation. Upon finding a non-empty queue, makes
1945 * at most the given bound attempts to re-poll (fewer if
1946 * contended) on the same queue before returning (impossible
1947 * scanState value) 0 to restart scan. Else returns after at least
1948 * 1 and at most 32 full scans.
1949 *
1950 * @param w the worker (via its WorkQueue)
1951 * @param bound repoll bound as bitmask (0 if spare)
1952 * @param step (circular) index increment per iteration (must be odd)
1953 * @param r a random seed for origin index
1954 * @return negative if should await signal
1955 */
1956 private int scan(WorkQueue w, int bound, int step, int r) {
1957 int stat = 0, wl; WorkQueue[] ws;
1958 if ((ws = workQueues) != null && w != null && (wl = ws.length) > 0) {
1959 for (int m = wl - 1,
1960 origin = m & r, idx = origin,
1961 npolls = 0,
1962 ss = w.scanState;;) { // negative if inactive
1963 WorkQueue q; ForkJoinTask<?>[] a; int b, al;
1964 if ((q = ws[idx]) != null && (b = q.base) - q.top < 0 &&
1965 (a = q.array) != null && (al = a.length) > 0) {
1966 int index = (al - 1) & b;
1967 long offset = ((long)index << ASHIFT) + ABASE;
1968 ForkJoinTask<?> t = (ForkJoinTask<?>)
1969 U.getObjectVolatile(a, offset);
1970 if (t == null)
1971 break; // empty or busy
1972 else if (b++ != q.base)
1973 break; // busy
1974 else if (ss < 0) {
1975 tryReactivate(w, ws, r);
1976 break; // retry upon rescan
1977 }
1978 else if (!U.compareAndSwapObject(a, offset, t, null))
1979 break; // contended
1980 else {
1981 q.base = b;
1982 w.currentSteal = t;
1983 if (b != q.top) // propagate signal
1984 signalWork();
1985 w.runTask(t);
1986 if (++npolls > bound)
1987 break;
1988 }
1989 }
1990 else if (npolls != 0) // rescan
1991 break;
1992 else if ((idx = (idx + step) & m) == origin) {
1993 if (ss < 0) { // await signal
1994 stat = ss;
1995 break;
1996 }
1997 else if (r >= 0) {
1998 inactivate(w, ss);
1999 break;
2000 }
2001 else
2002 r <<= 1; // at most 31 rescans
2003 }
2004 }
2005 }
2006 return stat;
2007 }
2008
2009 // Joining tasks
2010
2011 /**
2012 * Tries to steal and run tasks within the target's computation.
2013 * Uses a variant of the top-level algorithm, restricted to tasks
2014 * with the given task as ancestor: It prefers taking and running
2015 * eligible tasks popped from the worker's own queue (via
2016 * popCC). Otherwise it scans others, randomly moving on
2017 * contention or execution, deciding to give up based on a
2018 * checksum (via return codes from pollAndExecCC). The maxTasks
2019 * argument supports external usages; internal calls use zero,
2020 * allowing unbounded steps (external calls trap non-positive
2021 * values).
2022 *
2023 * @param w caller
2024 * @param maxTasks if non-zero, the maximum number of other tasks to run
2025 * @return task status on exit
2026 */
2027 final int helpComplete(WorkQueue w, CountedCompleter<?> task,
2028 int maxTasks) {
2029 WorkQueue[] ws; int s = 0, wl;
2030 if ((ws = workQueues) != null && (wl = ws.length) > 1 &&
2031 task != null && w != null) {
2032 for (int m = wl - 1,
2033 mode = w.config,
2034 r = ~mode, // scanning seed
2035 origin = r & m, k = origin, // first queue to scan
2036 step = 3, // first scan step
2037 h = 1, // 1:ran, >1:contended, <0:hash
2038 oldSum = 0, checkSum = 0;;) {
2039 CountedCompleter<?> p; WorkQueue q; int i;
2040 if ((s = task.status) < 0)
2041 break;
2042 if (h == 1 && (p = w.popCC(task, mode)) != null) {
2043 p.doExec(); // run local task
2044 if (maxTasks != 0 && --maxTasks == 0)
2045 break;
2046 origin = k; // reset
2047 oldSum = checkSum = 0;
2048 }
2049 else { // poll other worker queues
2050 if ((i = k | 1) < 0 || i > m || (q = ws[i]) == null)
2051 h = 0;
2052 else if ((h = q.pollAndExecCC(task)) < 0)
2053 checkSum += h;
2054 if (h > 0) {
2055 if (h == 1 && maxTasks != 0 && --maxTasks == 0)
2056 break;
2057 step = (r >>> 16) | 3;
2058 r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift
2059 k = origin = r & m; // move and restart
2060 oldSum = checkSum = 0;
2061 }
2062 else if ((k = (k + step) & m) == origin) {
2063 if (oldSum == (oldSum = checkSum))
2064 break;
2065 checkSum = 0;
2066 }
2067 }
2068 }
2069 }
2070 return s;
2071 }
2072
2073 /**
2074 * Tries to locate and execute tasks for a stealer of the given
2075 * task, or in turn one of its stealers. Traces currentSteal ->
2076 * currentJoin links looking for a thread working on a descendant
2077 * of the given task and with a non-empty queue to steal back and
2078 * execute tasks from. The first call to this method upon a
2079 * waiting join will often entail scanning/search, (which is OK
2080 * because the joiner has nothing better to do), but this method
2081 * leaves hints in workers to speed up subsequent calls.
2082 *
2083 * @param w caller
2084 * @param task the task to join
2085 */
2086 private void helpStealer(WorkQueue w, ForkJoinTask<?> task) {
2087 if (task != null && w != null) {
2088 ForkJoinTask<?> ps = w.currentSteal;
2089 WorkQueue[] ws; int wl, oldSum = 0;
2090 outer: while (w.tryRemoveAndExec(task) && task.status >= 0 &&
2091 (ws = workQueues) != null && (wl = ws.length) > 0) {
2092 ForkJoinTask<?> subtask;
2093 int m = wl - 1, checkSum = 0; // for stability check
2094 WorkQueue j = w, v; // v is subtask stealer
2095 descent: for (subtask = task; subtask.status >= 0; ) {
2096 for (int h = j.hint | 1, k = 0, i;;) {
2097 if ((v = ws[i = (h + (k << 1)) & m]) != null) {
2098 if (v.currentSteal == subtask) {
2099 j.hint = i;
2100 break;
2101 }
2102 checkSum += v.base;
2103 }
2104 if (++k > m) // can't find stealer
2105 break outer;
2106 }
2107
2108 for (;;) { // help v or descend
2109 ForkJoinTask<?>[] a; int b, al;
2110 if (subtask.status < 0) // too late to help
2111 break descent;
2112 checkSum += (b = v.base);
2113 ForkJoinTask<?> next = v.currentJoin;
2114 ForkJoinTask<?> t = null;
2115 if ((a = v.array) != null && (al = a.length) > 0) {
2116 int index = (al - 1) & b;
2117 long offset = ((long)index << ASHIFT) + ABASE;
2118 t = (ForkJoinTask<?>)
2119 U.getObjectVolatile(a, offset);
2120 if (t != null && b++ == v.base) {
2121 if (j.currentJoin != subtask ||
2122 v.currentSteal != subtask ||
2123 subtask.status < 0)
2124 break descent; // stale
2125 if (U.compareAndSwapObject(a, offset, t, null)) {
2126 v.base = b;
2127 w.currentSteal = t;
2128 for (int top = w.top;;) {
2129 t.doExec(); // help
2130 w.currentSteal = ps;
2131 if (task.status < 0)
2132 break outer;
2133 if (w.top == top)
2134 break; // run local tasks
2135 if ((t = w.pop()) == null)
2136 break descent;
2137 w.currentSteal = t;
2138 }
2139 }
2140 }
2141 }
2142 if (t == null && b == v.base && b - v.top >= 0) {
2143 if ((subtask = next) == null) { // try to descend
2144 if (next == v.currentJoin &&
2145 oldSum == (oldSum = checkSum))
2146 break outer;
2147 break descent;
2148 }
2149 j = v;
2150 break;
2151 }
2152 }
2153 }
2154 }
2155 }
2156 }
2157
2158 /**
2159 * Tries to decrement active count (sometimes implicitly) and
2160 * possibly release or create a compensating worker in preparation
2161 * for blocking. Returns false (retryable by caller), on
2162 * contention, detected staleness, instability, or termination.
2163 *
2164 * @param w caller
2165 */
2166 private boolean tryCompensate(WorkQueue w) {
2167 boolean canBlock; int wl;
2168 long c = ctl;
2169 WorkQueue[] ws = workQueues;
2170 int pc = config & SMASK;
2171 int ac = pc + (int)(c >> AC_SHIFT);
2172 int tc = pc + (short)(c >> TC_SHIFT);
2173 if (w == null || w.qlock < 0 || pc == 0 || // terminating or disabled
2174 ws == null || (wl = ws.length) <= 0)
2175 canBlock = false;
2176 else {
2177 int m = wl - 1, sp;
2178 boolean busy = true; // validate ac
2179 for (int i = 0; i <= m; ++i) {
2180 int k; WorkQueue v;
2181 if ((k = (i << 1) | 1) <= m && k >= 0 && (v = ws[k]) != null &&
2182 v.scanState >= 0 && v.currentSteal == null) {
2183 busy = false;
2184 break;
2185 }
2186 }
2187 if (!busy || ctl != c)
2188 canBlock = false; // unstable or stale
2189 else if ((sp = (int)c) != 0) // release idle worker
2190 canBlock = tryRelease(c, ws[m & sp], 0L);
2191 else if (tc >= pc && ac > 1 && w.isEmpty()) {
2192 long nc = ((AC_MASK & (c - AC_UNIT)) |
2193 (~AC_MASK & c)); // uncompensated
2194 canBlock = U.compareAndSwapLong(this, CTL, c, nc);
2195 }
2196 else if (tc >= MAX_CAP ||
2197 (this == common && tc >= pc + COMMON_MAX_SPARES))
2198 throw new RejectedExecutionException(
2199 "Thread limit exceeded replacing blocked worker");
2200 else { // similar to tryAddWorker
2201 boolean isSpare = (tc >= pc);
2202 long nc = (AC_MASK & c) | (TC_MASK & (c + TC_UNIT));
2203 canBlock = (U.compareAndSwapLong(this, CTL, c, nc) &&
2204 createWorker(isSpare)); // throws on exception
2205 }
2206 }
2207 return canBlock;
2208 }
2209
2210 /**
2211 * Helps and/or blocks until the given task is done or timeout.
2212 *
2213 * @param w caller
2214 * @param task the task
2215 * @param deadline for timed waits, if nonzero
2216 * @return task status on exit
2217 */
2218 final int awaitJoin(WorkQueue w, ForkJoinTask<?> task, long deadline) {
2219 int s = 0;
2220 if (w != null) {
2221 ForkJoinTask<?> prevJoin = w.currentJoin;
2222 if (task != null && (s = task.status) >= 0) {
2223 w.currentJoin = task;
2224 CountedCompleter<?> cc = (task instanceof CountedCompleter) ?
2225 (CountedCompleter<?>)task : null;
2226 for (;;) {
2227 if (cc != null)
2228 helpComplete(w, cc, 0);
2229 else
2230 helpStealer(w, task);
2231 if ((s = task.status) < 0)
2232 break;
2233 long ms, ns;
2234 if (deadline == 0L)
2235 ms = 0L;
2236 else if ((ns = deadline - System.nanoTime()) <= 0L)
2237 break;
2238 else if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) <= 0L)
2239 ms = 1L;
2240 if (tryCompensate(w)) {
2241 task.internalWait(ms);
2242 U.getAndAddLong(this, CTL, AC_UNIT);
2243 }
2244 if ((s = task.status) < 0)
2245 break;
2246 }
2247 w.currentJoin = prevJoin;
2248 }
2249 }
2250 return s;
2251 }
2252
2253 // Specialized scanning
2254
2255 /**
2256 * Returns a (probably) non-empty steal queue, if one is found
2257 * during a scan, else null. This method must be retried by
2258 * caller if, by the time it tries to use the queue, it is empty.
2259 */
2260 private WorkQueue findNonEmptyStealQueue() {
2261 WorkQueue[] ws; int wl; // one-shot version of scan loop
2262 int r = ThreadLocalRandom.nextSecondarySeed();
2263 if ((ws = workQueues) != null && (wl = ws.length) > 0) {
2264 int m = wl - 1, origin = r & m;
2265 for (int k = origin, oldSum = 0, checkSum = 0;;) {
2266 WorkQueue q; int b;
2267 if ((q = ws[k]) != null) {
2268 if ((b = q.base) - q.top < 0)
2269 return q;
2270 checkSum += b;
2271 }
2272 if ((k = (k + 1) & m) == origin) {
2273 if (oldSum == (oldSum = checkSum))
2274 break;
2275 checkSum = 0;
2276 }
2277 }
2278 }
2279 return null;
2280 }
2281
2282 /**
2283 * Runs tasks until {@code isQuiescent()}. We piggyback on
2284 * active count ctl maintenance, but rather than blocking
2285 * when tasks cannot be found, we rescan until all others cannot
2286 * find tasks either.
2287 */
2288 final void helpQuiescePool(WorkQueue w) {
2289 ForkJoinTask<?> ps = w.currentSteal; // save context
2290 int wc = w.config;
2291 for (boolean active = true;;) {
2292 long c; WorkQueue q; ForkJoinTask<?> t;
2293 if (wc >= 0 && (t = w.pop()) != null) { // run locals if LIFO
2294 (w.currentSteal = t).doExec();
2295 w.currentSteal = ps;
2296 }
2297 else if ((q = findNonEmptyStealQueue()) != null) {
2298 if (!active) { // re-establish active count
2299 active = true;
2300 U.getAndAddLong(this, CTL, AC_UNIT);
2301 }
2302 if ((t = q.pollAt(q.base)) != null) {
2303 (w.currentSteal = t).doExec();
2304 w.currentSteal = ps;
2305 if (++w.nsteals < 0)
2306 w.transferStealCount(this);
2307 }
2308 }
2309 else if (active) { // decrement active count without queuing
2310 long nc = (AC_MASK & ((c = ctl) - AC_UNIT)) | (~AC_MASK & c);
2311 if (U.compareAndSwapLong(this, CTL, c, nc))
2312 active = false;
2313 }
2314 else if ((int)((c = ctl) >> AC_SHIFT) + (config & SMASK) <= 0 &&
2315 U.compareAndSwapLong(this, CTL, c, c + AC_UNIT))
2316 break;
2317 }
2318 }
2319
2320 /**
2321 * Gets and removes a local or stolen task for the given worker.
2322 *
2323 * @return a task, if available
2324 */
2325 final ForkJoinTask<?> nextTaskFor(WorkQueue w) {
2326 for (ForkJoinTask<?> t;;) {
2327 WorkQueue q;
2328 if ((t = w.nextLocalTask()) != null)
2329 return t;
2330 if ((q = findNonEmptyStealQueue()) == null)
2331 return null;
2332 if ((t = q.pollAt(q.base)) != null)
2333 return t;
2334 }
2335 }
2336
2337 /**
2338 * Returns a cheap heuristic guide for task partitioning when
2339 * programmers, frameworks, tools, or languages have little or no
2340 * idea about task granularity. In essence, by offering this
2341 * method, we ask users only about tradeoffs in overhead vs
2342 * expected throughput and its variance, rather than how finely to
2343 * partition tasks.
2344 *
2345 * In a steady state strict (tree-structured) computation, each
2346 * thread makes available for stealing enough tasks for other
2347 * threads to remain active. Inductively, if all threads play by
2348 * the same rules, each thread should make available only a
2349 * constant number of tasks.
2350 *
2351 * The minimum useful constant is just 1. But using a value of 1
2352 * would require immediate replenishment upon each steal to
2353 * maintain enough tasks, which is infeasible. Further,
2354 * partitionings/granularities of offered tasks should minimize
2355 * steal rates, which in general means that threads nearer the top
2356 * of computation tree should generate more than those nearer the
2357 * bottom. In perfect steady state, each thread is at
2358 * approximately the same level of computation tree. However,
2359 * producing extra tasks amortizes the uncertainty of progress and
2360 * diffusion assumptions.
2361 *
2362 * So, users will want to use values larger (but not much larger)
2363 * than 1 to both smooth over transient shortages and hedge
2364 * against uneven progress; as traded off against the cost of
2365 * extra task overhead. We leave the user to pick a threshold
2366 * value to compare with the results of this call to guide
2367 * decisions, but recommend values such as 3.
2368 *
2369 * When all threads are active, it is on average OK to estimate
2370 * surplus strictly locally. In steady-state, if one thread is
2371 * maintaining say 2 surplus tasks, then so are others. So we can
2372 * just use estimated queue length. However, this strategy alone
2373 * leads to serious mis-estimates in some non-steady-state
2374 * conditions (ramp-up, ramp-down, other stalls). We can detect
2375 * many of these by further considering the number of "idle"
2376 * threads, that are known to have zero queued tasks, so
2377 * compensate by a factor of (#idle/#active) threads.
2378 */
2379 static int getSurplusQueuedTaskCount() {
2380 Thread t; ForkJoinWorkerThread wt; ForkJoinPool pool; WorkQueue q;
2381 if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
2382 int p = (pool = (wt = (ForkJoinWorkerThread)t).pool).config & SMASK;
2383 int n = (q = wt.workQueue).top - q.base;
2384 int a = (int)(pool.ctl >> AC_SHIFT) + p;
2385 return n - (a > (p >>>= 1) ? 0 :
2386 a > (p >>>= 1) ? 1 :
2387 a > (p >>>= 1) ? 2 :
2388 a > (p >>>= 1) ? 4 :
2389 8);
2390 }
2391 return 0;
2392 }
2393
2394 // Termination
2395
2396 /**
2397 * Possibly initiates and/or completes termination.
2398 *
2399 * @param now if true, unconditionally terminate, else only
2400 * if no work and no active workers
2401 * @param enable if true, terminate when next possible
2402 * @return -1: terminating/terminated, 0: retry if internal caller, else 1
2403 */
2404 private int tryTerminate(boolean now, boolean enable) {
2405 int rs; // 3 phases: try to set SHUTDOWN, then STOP, then TERMINATED
2406
2407 while ((rs = runState) >= 0) {
2408 if (!enable || this == common) // cannot shutdown
2409 return 1;
2410 else if (rs == 0)
2411 tryInitialize(false); // ensure initialized
2412 else
2413 U.compareAndSwapInt(this, RUNSTATE, rs, rs | SHUTDOWN);
2414 }
2415
2416 if ((rs & STOP) == 0) { // try to initiate termination
2417 if (!now) { // check quiescence
2418 for (long oldSum = 0L;;) { // repeat until stable
2419 WorkQueue[] ws; WorkQueue w; int b;
2420 long checkSum = ctl;
2421 if ((int)(checkSum >> AC_SHIFT) + (config & SMASK) > 0)
2422 return 0; // still active workers
2423 if ((ws = workQueues) != null) {
2424 for (int i = 0; i < ws.length; ++i) {
2425 if ((w = ws[i]) != null) {
2426 checkSum += (b = w.base);
2427 if (w.currentSteal != null || b != w.top)
2428 return 0; // retry if internal caller
2429 }
2430 }
2431 }
2432 if (oldSum == (oldSum = checkSum))
2433 break;
2434 }
2435 }
2436 do {} while (!U.compareAndSwapInt(this, RUNSTATE,
2437 rs = runState, rs | STOP));
2438 }
2439
2440 for (long oldSum = 0L;;) { // repeat until stable
2441 WorkQueue[] ws; WorkQueue w; ForkJoinWorkerThread wt;
2442 long checkSum = ctl;
2443 if ((ws = workQueues) != null) { // help terminate others
2444 for (int i = 0; i < ws.length; ++i) {
2445 if ((w = ws[i]) != null) {
2446 w.cancelAll(); // clear queues
2447 checkSum += w.base;
2448 if (w.qlock >= 0) {
2449 w.qlock = -1; // racy set OK
2450 if ((wt = w.owner) != null) {
2451 try { // unblock join or park
2452 wt.interrupt();
2453 } catch (Throwable ignore) {
2454 }
2455 }
2456 }
2457 }
2458 }
2459 }
2460 if (oldSum == (oldSum = checkSum))
2461 break;
2462 }
2463
2464 if ((short)(ctl >>> TC_SHIFT) + (config & SMASK) <= 0) {
2465 runState = (STARTED | SHUTDOWN | STOP | TERMINATED); // final write
2466 synchronized (this) {
2467 notifyAll(); // for awaitTermination
2468 }
2469 }
2470
2471 return -1;
2472 }
2473
2474 // External operations
2475
2476 /**
2477 * Constructs and tries to install a new external queue,
2478 * failing if the workQueues array already has a queue at
2479 * the given index.
2480 *
2481 * @param index the index of the new queue
2482 */
2483 private void tryCreateExternalQueue(int index) {
2484 AuxState aux;
2485 if ((aux = auxState) != null && index >= 0) {
2486 WorkQueue q = new WorkQueue(this, null);
2487 q.config = index;
2488 q.scanState = ~UNSIGNALLED;
2489 q.qlock = 1; // lock queue
2490 boolean installed = false;
2491 aux.lock();
2492 try { // lock pool to install
2493 WorkQueue[] ws;
2494 if ((ws = workQueues) != null && index < ws.length &&
2495 ws[index] == null) {
2496 ws[index] = q; // else throw away
2497 installed = true;
2498 }
2499 } finally {
2500 aux.unlock();
2501 }
2502 if (installed) {
2503 try {
2504 q.growArray();
2505 } finally {
2506 q.qlock = 0;
2507 }
2508 }
2509 }
2510 }
2511
2512 /**
2513 * Adds the given task to a submission queue at submitter's
2514 * current queue. Also performs secondary initialization upon the
2515 * first submission of the first task to the pool, and detects
2516 * first submission by an external thread and creates a new shared
2517 * queue if the one at index if empty or contended.
2518 *
2519 * @param task the task. Caller must ensure non-null.
2520 */
2521 final void externalPush(ForkJoinTask<?> task) {
2522 int r; // initialize caller's probe
2523 if ((r = ThreadLocalRandom.getProbe()) == 0) {
2524 ThreadLocalRandom.localInit();
2525 r = ThreadLocalRandom.getProbe();
2526 }
2527 for (;;) {
2528 WorkQueue q; int wl, k, stat;
2529 int rs = runState;
2530 WorkQueue[] ws = workQueues;
2531 if (rs <= 0 || ws == null || (wl = ws.length) <= 0)
2532 tryInitialize(true);
2533 else if ((q = ws[k = (wl - 1) & r & SQMASK]) == null)
2534 tryCreateExternalQueue(k);
2535 else if ((stat = q.sharedPush(task)) < 0)
2536 break;
2537 else if (stat == 0) {
2538 signalWork();
2539 break;
2540 }
2541 else // move if busy
2542 r = ThreadLocalRandom.advanceProbe(r);
2543 }
2544 }
2545
2546 /**
2547 * Pushes a possibly-external submission.
2548 */
2549 private <T> ForkJoinTask<T> externalSubmit(ForkJoinTask<T> task) {
2550 Thread t; ForkJoinWorkerThread w; WorkQueue q;
2551 if (task == null)
2552 throw new NullPointerException();
2553 if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) &&
2554 (w = (ForkJoinWorkerThread)t).pool == this &&
2555 (q = w.workQueue) != null)
2556 q.push(task);
2557 else
2558 externalPush(task);
2559 return task;
2560 }
2561
2562 /**
2563 * Returns common pool queue for an external thread.
2564 */
2565 static WorkQueue commonSubmitterQueue() {
2566 ForkJoinPool p = common;
2567 int r = ThreadLocalRandom.getProbe();
2568 WorkQueue[] ws; int wl;
2569 return (p != null && (ws = p.workQueues) != null &&
2570 (wl = ws.length) > 0) ?
2571 ws[(wl - 1) & r & SQMASK] : null;
2572 }
2573
2574 /**
2575 * Performs tryUnpush for an external submitter.
2576 */
2577 final boolean tryExternalUnpush(ForkJoinTask<?> task) {
2578 int r = ThreadLocalRandom.getProbe();
2579 WorkQueue[] ws; WorkQueue w; int wl;
2580 return ((ws = workQueues) != null &&
2581 (wl = ws.length) > 0 &&
2582 (w = ws[(wl - 1) & r & SQMASK]) != null &&
2583 w.trySharedUnpush(task));
2584 }
2585
2586 /**
2587 * Performs helpComplete for an external submitter.
2588 */
2589 final int externalHelpComplete(CountedCompleter<?> task, int maxTasks) {
2590 WorkQueue[] ws; int wl;
2591 int r = ThreadLocalRandom.getProbe();
2592 return ((ws = workQueues) != null && (wl = ws.length) > 0) ?
2593 helpComplete(ws[(wl - 1) & r & SQMASK], task, maxTasks) : 0;
2594 }
2595
2596 // Exported methods
2597
2598 // Constructors
2599
2600 /**
2601 * Creates a {@code ForkJoinPool} with parallelism equal to {@link
2602 * java.lang.Runtime#availableProcessors}, using the {@linkplain
2603 * #defaultForkJoinWorkerThreadFactory default thread factory},
2604 * no UncaughtExceptionHandler, and non-async LIFO processing mode.
2605 *
2606 * @throws SecurityException if a security manager exists and
2607 * the caller is not permitted to modify threads
2608 * because it does not hold {@link
2609 * java.lang.RuntimePermission}{@code ("modifyThread")}
2610 */
2611 public ForkJoinPool() {
2612 this(Math.min(MAX_CAP, Runtime.getRuntime().availableProcessors()),
2613 defaultForkJoinWorkerThreadFactory, null, false);
2614 }
2615
2616 /**
2617 * Creates a {@code ForkJoinPool} with the indicated parallelism
2618 * level, the {@linkplain
2619 * #defaultForkJoinWorkerThreadFactory default thread factory},
2620 * no UncaughtExceptionHandler, and non-async LIFO processing mode.
2621 *
2622 * @param parallelism the parallelism level
2623 * @throws IllegalArgumentException if parallelism less than or
2624 * equal to zero, or greater than implementation limit
2625 * @throws SecurityException if a security manager exists and
2626 * the caller is not permitted to modify threads
2627 * because it does not hold {@link
2628 * java.lang.RuntimePermission}{@code ("modifyThread")}
2629 */
2630 public ForkJoinPool(int parallelism) {
2631 this(parallelism, defaultForkJoinWorkerThreadFactory, null, false);
2632 }
2633
2634 /**
2635 * Creates a {@code ForkJoinPool} with the given parameters.
2636 *
2637 * @param parallelism the parallelism level. For default value,
2638 * use {@link java.lang.Runtime#availableProcessors}.
2639 * @param factory the factory for creating new threads. For default value,
2640 * use {@link #defaultForkJoinWorkerThreadFactory}.
2641 * @param handler the handler for internal worker threads that
2642 * terminate due to unrecoverable errors encountered while executing
2643 * tasks. For default value, use {@code null}.
2644 * @param asyncMode if true,
2645 * establishes local first-in-first-out scheduling mode for forked
2646 * tasks that are never joined. This mode may be more appropriate
2647 * than default locally stack-based mode in applications in which
2648 * worker threads only process event-style asynchronous tasks.
2649 * For default value, use {@code false}.
2650 * @throws IllegalArgumentException if parallelism less than or
2651 * equal to zero, or greater than implementation limit
2652 * @throws NullPointerException if the factory is null
2653 * @throws SecurityException if a security manager exists and
2654 * the caller is not permitted to modify threads
2655 * because it does not hold {@link
2656 * java.lang.RuntimePermission}{@code ("modifyThread")}
2657 */
2658 public ForkJoinPool(int parallelism,
2659 ForkJoinWorkerThreadFactory factory,
2660 UncaughtExceptionHandler handler,
2661 boolean asyncMode) {
2662 this(checkParallelism(parallelism),
2663 checkFactory(factory),
2664 handler,
2665 asyncMode ? FIFO_QUEUE : LIFO_QUEUE,
2666 "ForkJoinPool-" + nextPoolId() + "-worker-");
2667 checkPermission();
2668 }
2669
2670 private static int checkParallelism(int parallelism) {
2671 if (parallelism <= 0 || parallelism > MAX_CAP)
2672 throw new IllegalArgumentException();
2673 return parallelism;
2674 }
2675
2676 private static ForkJoinWorkerThreadFactory checkFactory
2677 (ForkJoinWorkerThreadFactory factory) {
2678 if (factory == null)
2679 throw new NullPointerException();
2680 return factory;
2681 }
2682
2683 /**
2684 * Creates a {@code ForkJoinPool} with the given parameters, without
2685 * any security checks or parameter validation. Invoked directly by
2686 * makeCommonPool.
2687 */
2688 private ForkJoinPool(int parallelism,
2689 ForkJoinWorkerThreadFactory factory,
2690 UncaughtExceptionHandler handler,
2691 int mode,
2692 String workerNamePrefix) {
2693 this.workerNamePrefix = workerNamePrefix;
2694 this.factory = factory;
2695 this.ueh = handler;
2696 this.config = (parallelism & SMASK) | mode;
2697 long np = (long)(-parallelism); // offset ctl counts
2698 this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
2699 }
2700
2701 /**
2702 * Returns the common pool instance. This pool is statically
2703 * constructed; its run state is unaffected by attempts to {@link
2704 * #shutdown} or {@link #shutdownNow}. However this pool and any
2705 * ongoing processing are automatically terminated upon program
2706 * {@link System#exit}. Any program that relies on asynchronous
2707 * task processing to complete before program termination should
2708 * invoke {@code commonPool().}{@link #awaitQuiescence awaitQuiescence},
2709 * before exit.
2710 *
2711 * @return the common pool instance
2712 * @since 1.8
2713 */
2714 public static ForkJoinPool commonPool() {
2715 // assert common != null : "static init error";
2716 return common;
2717 }
2718
2719 // Execution methods
2720
2721 /**
2722 * Performs the given task, returning its result upon completion.
2723 * If the computation encounters an unchecked Exception or Error,
2724 * it is rethrown as the outcome of this invocation. Rethrown
2725 * exceptions behave in the same way as regular exceptions, but,
2726 * when possible, contain stack traces (as displayed for example
2727 * using {@code ex.printStackTrace()}) of both the current thread
2728 * as well as the thread actually encountering the exception;
2729 * minimally only the latter.
2730 *
2731 * @param task the task
2732 * @param <T> the type of the task's result
2733 * @return the task's result
2734 * @throws NullPointerException if the task is null
2735 * @throws RejectedExecutionException if the task cannot be
2736 * scheduled for execution
2737 */
2738 public <T> T invoke(ForkJoinTask<T> task) {
2739 if (task == null)
2740 throw new NullPointerException();
2741 externalSubmit(task);
2742 return task.join();
2743 }
2744
2745 /**
2746 * Arranges for (asynchronous) execution of the given task.
2747 *
2748 * @param task the task
2749 * @throws NullPointerException if the task is null
2750 * @throws RejectedExecutionException if the task cannot be
2751 * scheduled for execution
2752 */
2753 public void execute(ForkJoinTask<?> task) {
2754 externalSubmit(task);
2755 }
2756
2757 // AbstractExecutorService methods
2758
2759 /**
2760 * @throws NullPointerException if the task is null
2761 * @throws RejectedExecutionException if the task cannot be
2762 * scheduled for execution
2763 */
2764 public void execute(Runnable task) {
2765 if (task == null)
2766 throw new NullPointerException();
2767 ForkJoinTask<?> job;
2768 if (task instanceof ForkJoinTask<?>) // avoid re-wrap
2769 job = (ForkJoinTask<?>) task;
2770 else
2771 job = new ForkJoinTask.RunnableExecuteAction(task);
2772 externalSubmit(job);
2773 }
2774
2775 /**
2776 * Submits a ForkJoinTask for execution.
2777 *
2778 * @param task the task to submit
2779 * @param <T> the type of the task's result
2780 * @return the task
2781 * @throws NullPointerException if the task is null
2782 * @throws RejectedExecutionException if the task cannot be
2783 * scheduled for execution
2784 */
2785 public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
2786 return externalSubmit(task);
2787 }
2788
2789 /**
2790 * @throws NullPointerException if the task is null
2791 * @throws RejectedExecutionException if the task cannot be
2792 * scheduled for execution
2793 */
2794 public <T> ForkJoinTask<T> submit(Callable<T> task) {
2795 return externalSubmit(new ForkJoinTask.AdaptedCallable<T>(task));
2796 }
2797
2798 /**
2799 * @throws NullPointerException if the task is null
2800 * @throws RejectedExecutionException if the task cannot be
2801 * scheduled for execution
2802 */
2803 public <T> ForkJoinTask<T> submit(Runnable task, T result) {
2804 return externalSubmit(new ForkJoinTask.AdaptedRunnable<T>(task, result));
2805 }
2806
2807 /**
2808 * @throws NullPointerException if the task is null
2809 * @throws RejectedExecutionException if the task cannot be
2810 * scheduled for execution
2811 */
2812 public ForkJoinTask<?> submit(Runnable task) {
2813 if (task == null)
2814 throw new NullPointerException();
2815 ForkJoinTask<?> job;
2816 if (task instanceof ForkJoinTask<?>) // avoid re-wrap
2817 job = (ForkJoinTask<?>) task;
2818 else
2819 job = new ForkJoinTask.AdaptedRunnableAction(task);
2820 return externalSubmit(job);
2821 }
2822
2823 /**
2824 * @throws NullPointerException {@inheritDoc}
2825 * @throws RejectedExecutionException {@inheritDoc}
2826 */
2827 public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
2828 // In previous versions of this class, this method constructed
2829 // a task to run ForkJoinTask.invokeAll, but now external
2830 // invocation of multiple tasks is at least as efficient.
2831 ArrayList<Future<T>> futures = new ArrayList<>(tasks.size());
2832
2833 try {
2834 for (Callable<T> t : tasks) {
2835 ForkJoinTask<T> f = new ForkJoinTask.AdaptedCallable<T>(t);
2836 futures.add(f);
2837 externalSubmit(f);
2838 }
2839 for (int i = 0, size = futures.size(); i < size; i++)
2840 ((ForkJoinTask<?>)futures.get(i)).quietlyJoin();
2841 return futures;
2842 } catch (Throwable t) {
2843 for (int i = 0, size = futures.size(); i < size; i++)
2844 futures.get(i).cancel(false);
2845 throw t;
2846 }
2847 }
2848
2849 /**
2850 * Returns the factory used for constructing new workers.
2851 *
2852 * @return the factory used for constructing new workers
2853 */
2854 public ForkJoinWorkerThreadFactory getFactory() {
2855 return factory;
2856 }
2857
2858 /**
2859 * Returns the handler for internal worker threads that terminate
2860 * due to unrecoverable errors encountered while executing tasks.
2861 *
2862 * @return the handler, or {@code null} if none
2863 */
2864 public UncaughtExceptionHandler getUncaughtExceptionHandler() {
2865 return ueh;
2866 }
2867
2868 /**
2869 * Returns the targeted parallelism level of this pool.
2870 *
2871 * @return the targeted parallelism level of this pool
2872 */
2873 public int getParallelism() {
2874 int par;
2875 return ((par = config & SMASK) > 0) ? par : 1;
2876 }
2877
2878 /**
2879 * Returns the targeted parallelism level of the common pool.
2880 *
2881 * @return the targeted parallelism level of the common pool
2882 * @since 1.8
2883 */
2884 public static int getCommonPoolParallelism() {
2885 return COMMON_PARALLELISM;
2886 }
2887
2888 /**
2889 * Returns the number of worker threads that have started but not
2890 * yet terminated. The result returned by this method may differ
2891 * from {@link #getParallelism} when threads are created to
2892 * maintain parallelism when others are cooperatively blocked.
2893 *
2894 * @return the number of worker threads
2895 */
2896 public int getPoolSize() {
2897 return (config & SMASK) + (short)(ctl >>> TC_SHIFT);
2898 }
2899
2900 /**
2901 * Returns {@code true} if this pool uses local first-in-first-out
2902 * scheduling mode for forked tasks that are never joined.
2903 *
2904 * @return {@code true} if this pool uses async mode
2905 */
2906 public boolean getAsyncMode() {
2907 return (config & FIFO_QUEUE) != 0;
2908 }
2909
2910 /**
2911 * Returns an estimate of the number of worker threads that are
2912 * not blocked waiting to join tasks or for other managed
2913 * synchronization. This method may overestimate the
2914 * number of running threads.
2915 *
2916 * @return the number of worker threads
2917 */
2918 public int getRunningThreadCount() {
2919 int rc = 0;
2920 WorkQueue[] ws; WorkQueue w;
2921 if ((ws = workQueues) != null) {
2922 for (int i = 1; i < ws.length; i += 2) {
2923 if ((w = ws[i]) != null && w.isApparentlyUnblocked())
2924 ++rc;
2925 }
2926 }
2927 return rc;
2928 }
2929
2930 /**
2931 * Returns an estimate of the number of threads that are currently
2932 * stealing or executing tasks. This method may overestimate the
2933 * number of active threads.
2934 *
2935 * @return the number of active threads
2936 */
2937 public int getActiveThreadCount() {
2938 int r = (config & SMASK) + (int)(ctl >> AC_SHIFT);
2939 return (r <= 0) ? 0 : r; // suppress momentarily negative values
2940 }
2941
2942 /**
2943 * Returns {@code true} if all worker threads are currently idle.
2944 * An idle worker is one that cannot obtain a task to execute
2945 * because none are available to steal from other threads, and
2946 * there are no pending submissions to the pool. This method is
2947 * conservative; it might not return {@code true} immediately upon
2948 * idleness of all threads, but will eventually become true if
2949 * threads remain inactive.
2950 *
2951 * @return {@code true} if all threads are currently idle
2952 */
2953 public boolean isQuiescent() {
2954 return (config & SMASK) + (int)(ctl >> AC_SHIFT) <= 0;
2955 }
2956
2957 /**
2958 * Returns an estimate of the total number of tasks stolen from
2959 * one thread's work queue by another. The reported value
2960 * underestimates the actual total number of steals when the pool
2961 * is not quiescent. This value may be useful for monitoring and
2962 * tuning fork/join programs: in general, steal counts should be
2963 * high enough to keep threads busy, but low enough to avoid
2964 * overhead and contention across threads.
2965 *
2966 * @return the number of steals
2967 */
2968 public long getStealCount() {
2969 AuxState sc = auxState;
2970 long count = (sc == null) ? 0L : sc.stealCount;
2971 WorkQueue[] ws; WorkQueue w;
2972 if ((ws = workQueues) != null) {
2973 for (int i = 1; i < ws.length; i += 2) {
2974 if ((w = ws[i]) != null)
2975 count += w.nsteals;
2976 }
2977 }
2978 return count;
2979 }
2980
2981 /**
2982 * Returns an estimate of the total number of tasks currently held
2983 * in queues by worker threads (but not including tasks submitted
2984 * to the pool that have not begun executing). This value is only
2985 * an approximation, obtained by iterating across all threads in
2986 * the pool. This method may be useful for tuning task
2987 * granularities.
2988 *
2989 * @return the number of queued tasks
2990 */
2991 public long getQueuedTaskCount() {
2992 long count = 0;
2993 WorkQueue[] ws; WorkQueue w;
2994 if ((ws = workQueues) != null) {
2995 for (int i = 1; i < ws.length; i += 2) {
2996 if ((w = ws[i]) != null)
2997 count += w.queueSize();
2998 }
2999 }
3000 return count;
3001 }
3002
3003 /**
3004 * Returns an estimate of the number of tasks submitted to this
3005 * pool that have not yet begun executing. This method may take
3006 * time proportional to the number of submissions.
3007 *
3008 * @return the number of queued submissions
3009 */
3010 public int getQueuedSubmissionCount() {
3011 int count = 0;
3012 WorkQueue[] ws; WorkQueue w;
3013 if ((ws = workQueues) != null) {
3014 for (int i = 0; i < ws.length; i += 2) {
3015 if ((w = ws[i]) != null)
3016 count += w.queueSize();
3017 }
3018 }
3019 return count;
3020 }
3021
3022 /**
3023 * Returns {@code true} if there are any tasks submitted to this
3024 * pool that have not yet begun executing.
3025 *
3026 * @return {@code true} if there are any queued submissions
3027 */
3028 public boolean hasQueuedSubmissions() {
3029 WorkQueue[] ws; WorkQueue w;
3030 if ((ws = workQueues) != null) {
3031 for (int i = 0; i < ws.length; i += 2) {
3032 if ((w = ws[i]) != null && !w.isEmpty())
3033 return true;
3034 }
3035 }
3036 return false;
3037 }
3038
3039 /**
3040 * Removes and returns the next unexecuted submission if one is
3041 * available. This method may be useful in extensions to this
3042 * class that re-assign work in systems with multiple pools.
3043 *
3044 * @return the next submission, or {@code null} if none
3045 */
3046 protected ForkJoinTask<?> pollSubmission() {
3047 WorkQueue[] ws; int wl; WorkQueue w; ForkJoinTask<?> t;
3048 int r = ThreadLocalRandom.nextSecondarySeed();
3049 if ((ws = workQueues) != null && (wl = ws.length) > 0) {
3050 for (int m = wl - 1, i = 0; i < wl; ++i) {
3051 if ((w = ws[(i << 1) & m]) != null && (t = w.poll()) != null)
3052 return t;
3053 }
3054 }
3055 return null;
3056 }
3057
3058 /**
3059 * Removes all available unexecuted submitted and forked tasks
3060 * from scheduling queues and adds them to the given collection,
3061 * without altering their execution status. These may include
3062 * artificially generated or wrapped tasks. This method is
3063 * designed to be invoked only when the pool is known to be
3064 * quiescent. Invocations at other times may not remove all
3065 * tasks. A failure encountered while attempting to add elements
3066 * to collection {@code c} may result in elements being in
3067 * neither, either or both collections when the associated
3068 * exception is thrown. The behavior of this operation is
3069 * undefined if the specified collection is modified while the
3070 * operation is in progress.
3071 *
3072 * @param c the collection to transfer elements into
3073 * @return the number of elements transferred
3074 */
3075 protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
3076 int count = 0;
3077 WorkQueue[] ws; WorkQueue w; ForkJoinTask<?> t;
3078 if ((ws = workQueues) != null) {
3079 for (int i = 0; i < ws.length; ++i) {
3080 if ((w = ws[i]) != null) {
3081 while ((t = w.poll()) != null) {
3082 c.add(t);
3083 ++count;
3084 }
3085 }
3086 }
3087 }
3088 return count;
3089 }
3090
3091 /**
3092 * Returns a string identifying this pool, as well as its state,
3093 * including indications of run state, parallelism level, and
3094 * worker and task counts.
3095 *
3096 * @return a string identifying this pool, as well as its state
3097 */
3098 public String toString() {
3099 // Use a single pass through workQueues to collect counts
3100 long qt = 0L, qs = 0L; int rc = 0;
3101 AuxState sc = auxState;
3102 long st = (sc == null) ? 0L : sc.stealCount;
3103 long c = ctl;
3104 WorkQueue[] ws; WorkQueue w;
3105 if ((ws = workQueues) != null) {
3106 for (int i = 0; i < ws.length; ++i) {
3107 if ((w = ws[i]) != null) {
3108 int size = w.queueSize();
3109 if ((i & 1) == 0)
3110 qs += size;
3111 else {
3112 qt += size;
3113 st += w.nsteals;
3114 if (w.isApparentlyUnblocked())
3115 ++rc;
3116 }
3117 }
3118 }
3119 }
3120 int pc = (config & SMASK);
3121 int tc = pc + (short)(c >>> TC_SHIFT);
3122 int ac = pc + (int)(c >> AC_SHIFT);
3123 if (ac < 0) // ignore transient negative
3124 ac = 0;
3125 int rs = runState;
3126 String level = ((rs & TERMINATED) != 0 ? "Terminated" :
3127 (rs & STOP) != 0 ? "Terminating" :
3128 (rs & SHUTDOWN) != 0 ? "Shutting down" :
3129 "Running");
3130 return super.toString() +
3131 "[" + level +
3132 ", parallelism = " + pc +
3133 ", size = " + tc +
3134 ", active = " + ac +
3135 ", running = " + rc +
3136 ", steals = " + st +
3137 ", tasks = " + qt +
3138 ", submissions = " + qs +
3139 "]";
3140 }
3141
3142 /**
3143 * Possibly initiates an orderly shutdown in which previously
3144 * submitted tasks are executed, but no new tasks will be
3145 * accepted. Invocation has no effect on execution state if this
3146 * is the {@link #commonPool()}, and no additional effect if
3147 * already shut down. Tasks that are in the process of being
3148 * submitted concurrently during the course of this method may or
3149 * may not be rejected.
3150 *
3151 * @throws SecurityException if a security manager exists and
3152 * the caller is not permitted to modify threads
3153 * because it does not hold {@link
3154 * java.lang.RuntimePermission}{@code ("modifyThread")}
3155 */
3156 public void shutdown() {
3157 checkPermission();
3158 tryTerminate(false, true);
3159 }
3160
3161 /**
3162 * Possibly attempts to cancel and/or stop all tasks, and reject
3163 * all subsequently submitted tasks. Invocation has no effect on
3164 * execution state if this is the {@link #commonPool()}, and no
3165 * additional effect if already shut down. Otherwise, tasks that
3166 * are in the process of being submitted or executed concurrently
3167 * during the course of this method may or may not be
3168 * rejected. This method cancels both existing and unexecuted
3169 * tasks, in order to permit termination in the presence of task
3170 * dependencies. So the method always returns an empty list
3171 * (unlike the case for some other Executors).
3172 *
3173 * @return an empty list
3174 * @throws SecurityException if a security manager exists and
3175 * the caller is not permitted to modify threads
3176 * because it does not hold {@link
3177 * java.lang.RuntimePermission}{@code ("modifyThread")}
3178 */
3179 public List<Runnable> shutdownNow() {
3180 checkPermission();
3181 tryTerminate(true, true);
3182 return Collections.emptyList();
3183 }
3184
3185 /**
3186 * Returns {@code true} if all tasks have completed following shut down.
3187 *
3188 * @return {@code true} if all tasks have completed following shut down
3189 */
3190 public boolean isTerminated() {
3191 return (runState & TERMINATED) != 0;
3192 }
3193
3194 /**
3195 * Returns {@code true} if the process of termination has
3196 * commenced but not yet completed. This method may be useful for
3197 * debugging. A return of {@code true} reported a sufficient
3198 * period after shutdown may indicate that submitted tasks have
3199 * ignored or suppressed interruption, or are waiting for I/O,
3200 * causing this executor not to properly terminate. (See the
3201 * advisory notes for class {@link ForkJoinTask} stating that
3202 * tasks should not normally entail blocking operations. But if
3203 * they do, they must abort them on interrupt.)
3204 *
3205 * @return {@code true} if terminating but not yet terminated
3206 */
3207 public boolean isTerminating() {
3208 int rs = runState;
3209 return (rs & STOP) != 0 && (rs & TERMINATED) == 0;
3210 }
3211
3212 /**
3213 * Returns {@code true} if this pool has been shut down.
3214 *
3215 * @return {@code true} if this pool has been shut down
3216 */
3217 public boolean isShutdown() {
3218 return (runState & SHUTDOWN) != 0;
3219 }
3220
3221 /**
3222 * Blocks until all tasks have completed execution after a
3223 * shutdown request, or the timeout occurs, or the current thread
3224 * is interrupted, whichever happens first. Because the {@link
3225 * #commonPool()} never terminates until program shutdown, when
3226 * applied to the common pool, this method is equivalent to {@link
3227 * #awaitQuiescence(long, TimeUnit)} but always returns {@code false}.
3228 *
3229 * @param timeout the maximum time to wait
3230 * @param unit the time unit of the timeout argument
3231 * @return {@code true} if this executor terminated and
3232 * {@code false} if the timeout elapsed before termination
3233 * @throws InterruptedException if interrupted while waiting
3234 */
3235 public boolean awaitTermination(long timeout, TimeUnit unit)
3236 throws InterruptedException {
3237 if (Thread.interrupted())
3238 throw new InterruptedException();
3239 if (this == common) {
3240 awaitQuiescence(timeout, unit);
3241 return false;
3242 }
3243 long nanos = unit.toNanos(timeout);
3244 if (isTerminated())
3245 return true;
3246 if (nanos <= 0L)
3247 return false;
3248 long deadline = System.nanoTime() + nanos;
3249 synchronized (this) {
3250 for (;;) {
3251 if (isTerminated())
3252 return true;
3253 if (nanos <= 0L)
3254 return false;
3255 long millis = TimeUnit.NANOSECONDS.toMillis(nanos);
3256 wait(millis > 0L ? millis : 1L);
3257 nanos = deadline - System.nanoTime();
3258 }
3259 }
3260 }
3261
3262 /**
3263 * If called by a ForkJoinTask operating in this pool, equivalent
3264 * in effect to {@link ForkJoinTask#helpQuiesce}. Otherwise,
3265 * waits and/or attempts to assist performing tasks until this
3266 * pool {@link #isQuiescent} or the indicated timeout elapses.
3267 *
3268 * @param timeout the maximum time to wait
3269 * @param unit the time unit of the timeout argument
3270 * @return {@code true} if quiescent; {@code false} if the
3271 * timeout elapsed.
3272 */
3273 public boolean awaitQuiescence(long timeout, TimeUnit unit) {
3274 long nanos = unit.toNanos(timeout);
3275 ForkJoinWorkerThread wt;
3276 Thread thread = Thread.currentThread();
3277 if ((thread instanceof ForkJoinWorkerThread) &&
3278 (wt = (ForkJoinWorkerThread)thread).pool == this) {
3279 helpQuiescePool(wt.workQueue);
3280 return true;
3281 }
3282 long startTime = System.nanoTime();
3283 WorkQueue[] ws;
3284 int r = 0, wl;
3285 boolean found = true;
3286 while (!isQuiescent() && (ws = workQueues) != null &&
3287 (wl = ws.length) > 0) {
3288 if (!found) {
3289 if ((System.nanoTime() - startTime) > nanos)
3290 return false;
3291 Thread.yield(); // cannot block
3292 }
3293 found = false;
3294 for (int m = wl - 1, j = (m + 1) << 2; j >= 0; --j) {
3295 ForkJoinTask<?> t; WorkQueue q; int b, k;
3296 if ((k = r++ & m) <= m && k >= 0 && (q = ws[k]) != null &&
3297 (b = q.base) - q.top < 0) {
3298 found = true;
3299 if ((t = q.pollAt(b)) != null)
3300 t.doExec();
3301 break;
3302 }
3303 }
3304 }
3305 return true;
3306 }
3307
3308 /**
3309 * Waits and/or attempts to assist performing tasks indefinitely
3310 * until the {@link #commonPool()} {@link #isQuiescent}.
3311 */
3312 static void quiesceCommonPool() {
3313 common.awaitQuiescence(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
3314 }
3315
3316 /**
3317 * Interface for extending managed parallelism for tasks running
3318 * in {@link ForkJoinPool}s.
3319 *
3320 * <p>A {@code ManagedBlocker} provides two methods. Method
3321 * {@link #isReleasable} must return {@code true} if blocking is
3322 * not necessary. Method {@link #block} blocks the current thread
3323 * if necessary (perhaps internally invoking {@code isReleasable}
3324 * before actually blocking). These actions are performed by any
3325 * thread invoking {@link ForkJoinPool#managedBlock(ManagedBlocker)}.
3326 * The unusual methods in this API accommodate synchronizers that
3327 * may, but don't usually, block for long periods. Similarly, they
3328 * allow more efficient internal handling of cases in which
3329 * additional workers may be, but usually are not, needed to
3330 * ensure sufficient parallelism. Toward this end,
3331 * implementations of method {@code isReleasable} must be amenable
3332 * to repeated invocation.
3333 *
3334 * <p>For example, here is a ManagedBlocker based on a
3335 * ReentrantLock:
3336 * <pre> {@code
3337 * class ManagedLocker implements ManagedBlocker {
3338 * final ReentrantLock lock;
3339 * boolean hasLock = false;
3340 * ManagedLocker(ReentrantLock lock) { this.lock = lock; }
3341 * public boolean block() {
3342 * if (!hasLock)
3343 * lock.lock();
3344 * return true;
3345 * }
3346 * public boolean isReleasable() {
3347 * return hasLock || (hasLock = lock.tryLock());
3348 * }
3349 * }}</pre>
3350 *
3351 * <p>Here is a class that possibly blocks waiting for an
3352 * item on a given queue:
3353 * <pre> {@code
3354 * class QueueTaker<E> implements ManagedBlocker {
3355 * final BlockingQueue<E> queue;
3356 * volatile E item = null;
3357 * QueueTaker(BlockingQueue<E> q) { this.queue = q; }
3358 * public boolean block() throws InterruptedException {
3359 * if (item == null)
3360 * item = queue.take();
3361 * return true;
3362 * }
3363 * public boolean isReleasable() {
3364 * return item != null || (item = queue.poll()) != null;
3365 * }
3366 * public E getItem() { // call after pool.managedBlock completes
3367 * return item;
3368 * }
3369 * }}</pre>
3370 */
3371 public static interface ManagedBlocker {
3372 /**
3373 * Possibly blocks the current thread, for example waiting for
3374 * a lock or condition.
3375 *
3376 * @return {@code true} if no additional blocking is necessary
3377 * (i.e., if isReleasable would return true)
3378 * @throws InterruptedException if interrupted while waiting
3379 * (the method is not required to do so, but is allowed to)
3380 */
3381 boolean block() throws InterruptedException;
3382
3383 /**
3384 * Returns {@code true} if blocking is unnecessary.
3385 * @return {@code true} if blocking is unnecessary
3386 */
3387 boolean isReleasable();
3388 }
3389
3390 /**
3391 * Runs the given possibly blocking task. When {@linkplain
3392 * ForkJoinTask#inForkJoinPool() running in a ForkJoinPool}, this
3393 * method possibly arranges for a spare thread to be activated if
3394 * necessary to ensure sufficient parallelism while the current
3395 * thread is blocked in {@link ManagedBlocker#block blocker.block()}.
3396 *
3397 * <p>This method repeatedly calls {@code blocker.isReleasable()} and
3398 * {@code blocker.block()} until either method returns {@code true}.
3399 * Every call to {@code blocker.block()} is preceded by a call to
3400 * {@code blocker.isReleasable()} that returned {@code false}.
3401 *
3402 * <p>If not running in a ForkJoinPool, this method is
3403 * behaviorally equivalent to
3404 * <pre> {@code
3405 * while (!blocker.isReleasable())
3406 * if (blocker.block())
3407 * break;}</pre>
3408 *
3409 * If running in a ForkJoinPool, the pool may first be expanded to
3410 * ensure sufficient parallelism available during the call to
3411 * {@code blocker.block()}.
3412 *
3413 * @param blocker the blocker task
3414 * @throws InterruptedException if {@code blocker.block()} did so
3415 */
3416 public static void managedBlock(ManagedBlocker blocker)
3417 throws InterruptedException {
3418 ForkJoinPool p;
3419 ForkJoinWorkerThread wt;
3420 Thread t = Thread.currentThread();
3421 if ((t instanceof ForkJoinWorkerThread) &&
3422 (p = (wt = (ForkJoinWorkerThread)t).pool) != null) {
3423 WorkQueue w = wt.workQueue;
3424 while (!blocker.isReleasable()) {
3425 if (p.tryCompensate(w)) {
3426 try {
3427 do {} while (!blocker.isReleasable() &&
3428 !blocker.block());
3429 } finally {
3430 U.getAndAddLong(p, CTL, AC_UNIT);
3431 }
3432 break;
3433 }
3434 }
3435 }
3436 else {
3437 do {} while (!blocker.isReleasable() &&
3438 !blocker.block());
3439 }
3440 }
3441
3442 // AbstractExecutorService overrides. These rely on undocumented
3443 // fact that ForkJoinTask.adapt returns ForkJoinTasks that also
3444 // implement RunnableFuture.
3445
3446 protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
3447 return new ForkJoinTask.AdaptedRunnable<T>(runnable, value);
3448 }
3449
3450 protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
3451 return new ForkJoinTask.AdaptedCallable<T>(callable);
3452 }
3453
3454 // Unsafe mechanics
3455 private static final jdk.internal.misc.Unsafe U = jdk.internal.misc.Unsafe.getUnsafe();
3456 private static final long CTL;
3457 private static final long RUNSTATE;
3458 private static final int ABASE;
3459 private static final int ASHIFT;
3460
3461 static {
3462 try {
3463 CTL = U.objectFieldOffset
3464 (ForkJoinPool.class.getDeclaredField("ctl"));
3465 RUNSTATE = U.objectFieldOffset
3466 (ForkJoinPool.class.getDeclaredField("runState"));
3467 ABASE = U.arrayBaseOffset(ForkJoinTask[].class);
3468 int scale = U.arrayIndexScale(ForkJoinTask[].class);
3469 if ((scale & (scale - 1)) != 0)
3470 throw new Error("array index scale not a power of two");
3471 ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
3472 } catch (ReflectiveOperationException e) {
3473 throw new Error(e);
3474 }
3475
3476 // Reduce the risk of rare disastrous classloading in first call to
3477 // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
3478 Class<?> ensureLoaded = LockSupport.class;
3479
3480 int commonMaxSpares = DEFAULT_COMMON_MAX_SPARES;
3481 try {
3482 String p = System.getProperty
3483 ("java.util.concurrent.ForkJoinPool.common.maximumSpares");
3484 if (p != null)
3485 commonMaxSpares = Integer.parseInt(p);
3486 } catch (Exception ignore) {}
3487 COMMON_MAX_SPARES = commonMaxSpares;
3488
3489 defaultForkJoinWorkerThreadFactory =
3490 new DefaultForkJoinWorkerThreadFactory();
3491 modifyThreadPermission = new RuntimePermission("modifyThread");
3492
3493 common = java.security.AccessController.doPrivileged
3494 (new java.security.PrivilegedAction<ForkJoinPool>() {
3495 public ForkJoinPool run() { return makeCommonPool(); }});
3496
3497 // report 1 even if threads disabled
3498 COMMON_PARALLELISM = Math.max(common.config & SMASK, 1);
3499 }
3500
3501 /**
3502 * Creates and returns the common pool, respecting user settings
3503 * specified via system properties.
3504 */
3505 static ForkJoinPool makeCommonPool() {
3506 int parallelism = -1;
3507 ForkJoinWorkerThreadFactory factory = null;
3508 UncaughtExceptionHandler handler = null;
3509 try { // ignore exceptions in accessing/parsing properties
3510 String pp = System.getProperty
3511 ("java.util.concurrent.ForkJoinPool.common.parallelism");
3512 String fp = System.getProperty
3513 ("java.util.concurrent.ForkJoinPool.common.threadFactory");
3514 String hp = System.getProperty
3515 ("java.util.concurrent.ForkJoinPool.common.exceptionHandler");
3516 if (pp != null)
3517 parallelism = Integer.parseInt(pp);
3518 if (fp != null)
3519 factory = ((ForkJoinWorkerThreadFactory)ClassLoader.
3520 getSystemClassLoader().loadClass(fp).newInstance());
3521 if (hp != null)
3522 handler = ((UncaughtExceptionHandler)ClassLoader.
3523 getSystemClassLoader().loadClass(hp).newInstance());
3524 } catch (Exception ignore) {
3525 }
3526 if (factory == null) {
3527 if (System.getSecurityManager() == null)
3528 factory = defaultForkJoinWorkerThreadFactory;
3529 else // use security-managed default
3530 factory = new InnocuousForkJoinWorkerThreadFactory();
3531 }
3532 if (parallelism < 0 && // default 1 less than #cores
3533 (parallelism = Runtime.getRuntime().availableProcessors() - 1) <= 0)
3534 parallelism = 1;
3535 if (parallelism > MAX_CAP)
3536 parallelism = MAX_CAP;
3537 return new ForkJoinPool(parallelism, factory, handler, LIFO_QUEUE,
3538 "ForkJoinPool.commonPool-worker-");
3539 }
3540
3541 /**
3542 * Factory for innocuous worker threads.
3543 */
3544 private static final class InnocuousForkJoinWorkerThreadFactory
3545 implements ForkJoinWorkerThreadFactory {
3546
3547 /**
3548 * An ACC to restrict permissions for the factory itself.
3549 * The constructed workers have no permissions set.
3550 */
3551 private static final AccessControlContext innocuousAcc;
3552 static {
3553 Permissions innocuousPerms = new Permissions();
3554 innocuousPerms.add(modifyThreadPermission);
3555 innocuousPerms.add(new RuntimePermission(
3556 "enableContextClassLoaderOverride"));
3557 innocuousPerms.add(new RuntimePermission(
3558 "modifyThreadGroup"));
3559 innocuousAcc = new AccessControlContext(new ProtectionDomain[] {
3560 new ProtectionDomain(null, innocuousPerms)
3561 });
3562 }
3563
3564 public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
3565 return java.security.AccessController.doPrivileged(
3566 new java.security.PrivilegedAction<ForkJoinWorkerThread>() {
3567 public ForkJoinWorkerThread run() {
3568 return new ForkJoinWorkerThread.
3569 InnocuousForkJoinWorkerThread(pool);
3570 }}, innocuousAcc);
3571 }
3572 }
3573
3574 }