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.lang.invoke.MethodHandles;
40 import java.lang.invoke.VarHandle;
41 import java.security.AccessController;
42 import java.security.AccessControlContext;
43 import java.security.Permission;
44 import java.security.Permissions;
45 import java.security.PrivilegedAction;
46 import java.security.ProtectionDomain;
47 import java.util.ArrayList;
48 import java.util.Collection;
49 import java.util.Collections;
50 import java.util.List;
51 import java.util.function.Predicate;
52 import java.util.concurrent.atomic.AtomicInteger;
53 import java.util.concurrent.locks.LockSupport;
54 import java.util.concurrent.locks.ReentrantLock;
55 import java.util.concurrent.locks.Condition;
56
57 /**
58 * An {@link ExecutorService} for running {@link ForkJoinTask}s.
59 * A {@code ForkJoinPool} provides the entry point for submissions
60 * from non-{@code ForkJoinTask} clients, as well as management and
61 * monitoring operations.
62 *
63 * <p>A {@code ForkJoinPool} differs from other kinds of {@link
64 * ExecutorService} mainly by virtue of employing
65 * <em>work-stealing</em>: all threads in the pool attempt to find and
66 * execute tasks submitted to the pool and/or created by other active
67 * tasks (eventually blocking waiting for work if none exist). This
68 * enables efficient processing when most tasks spawn other subtasks
69 * (as do most {@code ForkJoinTask}s), as well as when many small
70 * tasks are submitted to the pool from external clients. Especially
71 * when setting <em>asyncMode</em> to true in constructors, {@code
72 * ForkJoinPool}s may also be appropriate for use with event-style
73 * tasks that are never joined. All worker threads are initialized
74 * with {@link Thread#isDaemon} set {@code true}.
75 *
76 * <p>A static {@link #commonPool()} is available and appropriate for
77 * most applications. The common pool is used by any ForkJoinTask that
78 * is not explicitly submitted to a specified pool. Using the common
79 * pool normally reduces resource usage (its threads are slowly
80 * reclaimed during periods of non-use, and reinstated upon subsequent
81 * use).
82 *
83 * <p>For applications that require separate or custom pools, a {@code
84 * ForkJoinPool} may be constructed with a given target parallelism
85 * level; by default, equal to the number of available processors.
86 * The pool attempts to maintain enough active (or available) threads
87 * by dynamically adding, suspending, or resuming internal worker
88 * threads, even if some tasks are stalled waiting to join others.
89 * However, no such adjustments are guaranteed in the face of blocked
90 * I/O or other unmanaged synchronization. The nested {@link
91 * ManagedBlocker} interface enables extension of the kinds of
92 * synchronization accommodated. The default policies may be
93 * overridden using a constructor with parameters corresponding to
94 * those documented in class {@link ThreadPoolExecutor}.
95 *
96 * <p>In addition to execution and lifecycle control methods, this
97 * class provides status check methods (for example
98 * {@link #getStealCount}) that are intended to aid in developing,
99 * tuning, and monitoring fork/join applications. Also, method
100 * {@link #toString} returns indications of pool state in a
101 * convenient form for informal monitoring.
102 *
103 * <p>As is the case with other ExecutorServices, there are three
104 * main task execution methods summarized in the following table.
105 * These are designed to be used primarily by clients not already
106 * engaged in fork/join computations in the current pool. The main
107 * forms of these methods accept instances of {@code ForkJoinTask},
108 * but overloaded forms also allow mixed execution of plain {@code
109 * Runnable}- or {@code Callable}- based activities as well. However,
110 * tasks that are already executing in a pool should normally instead
111 * use the within-computation forms listed in the table unless using
112 * async event-style tasks that are not usually joined, in which case
113 * there is little difference among choice of methods.
114 *
115 * <table class="plain">
116 * <caption>Summary of task execution methods</caption>
117 * <tr>
118 * <td></td>
119 * <th scope="col"> Call from non-fork/join clients</th>
120 * <th scope="col"> Call from within fork/join computations</th>
121 * </tr>
122 * <tr>
123 * <th scope="row" style="text-align:left"> Arrange async execution</th>
124 * <td> {@link #execute(ForkJoinTask)}</td>
125 * <td> {@link ForkJoinTask#fork}</td>
126 * </tr>
127 * <tr>
128 * <th scope="row" style="text-align:left"> Await and obtain result</th>
129 * <td> {@link #invoke(ForkJoinTask)}</td>
130 * <td> {@link ForkJoinTask#invoke}</td>
131 * </tr>
132 * <tr>
133 * <th scope="row" style="text-align:left"> Arrange exec and obtain Future</th>
134 * <td> {@link #submit(ForkJoinTask)}</td>
135 * <td> {@link ForkJoinTask#fork} (ForkJoinTasks <em>are</em> Futures)</td>
136 * </tr>
137 * </table>
138 *
139 * <p>The parameters used to construct the common pool may be controlled by
140 * setting the following {@linkplain System#getProperty system properties}:
141 * <ul>
142 * <li>{@systemProperty java.util.concurrent.ForkJoinPool.common.parallelism}
143 * - the parallelism level, a non-negative integer
144 * <li>{@systemProperty java.util.concurrent.ForkJoinPool.common.threadFactory}
145 * - the class name of a {@link ForkJoinWorkerThreadFactory}.
146 * The {@linkplain ClassLoader#getSystemClassLoader() system class loader}
147 * is used to load this class.
148 * <li>{@systemProperty java.util.concurrent.ForkJoinPool.common.exceptionHandler}
149 * - the class name of a {@link UncaughtExceptionHandler}.
150 * The {@linkplain ClassLoader#getSystemClassLoader() system class loader}
151 * is used to load this class.
152 * <li>{@systemProperty java.util.concurrent.ForkJoinPool.common.maximumSpares}
153 * - the maximum number of allowed extra threads to maintain target
154 * parallelism (default 256).
155 * </ul>
156 * If no thread factory is supplied via a system property, then the
157 * common pool uses a factory that uses the system class loader as the
158 * {@linkplain Thread#getContextClassLoader() thread context class loader}.
159 * In addition, if a {@link SecurityManager} is present, then
160 * the common pool uses a factory supplying threads that have no
161 * {@link Permissions} enabled.
162 *
163 * Upon any error in establishing these settings, default parameters
164 * are used. It is possible to disable or limit the use of threads in
165 * the common pool by setting the parallelism property to zero, and/or
166 * using a factory that may return {@code null}. However doing so may
167 * cause unjoined tasks to never be executed.
168 *
169 * <p><b>Implementation notes:</b> This implementation restricts the
170 * maximum number of running threads to 32767. Attempts to create
171 * pools with greater than the maximum number result in
172 * {@code IllegalArgumentException}.
173 *
174 * <p>This implementation rejects submitted tasks (that is, by throwing
175 * {@link RejectedExecutionException}) only when the pool is shut down
176 * or internal resources have been exhausted.
177 *
178 * @since 1.7
179 * @author Doug Lea
180 */
181 public class ForkJoinPool extends AbstractExecutorService {
182
183 /*
184 * Implementation Overview
185 *
186 * This class and its nested classes provide the main
187 * functionality and control for a set of worker threads:
188 * Submissions from non-FJ threads enter into submission queues.
189 * Workers take these tasks and typically split them into subtasks
190 * that may be stolen by other workers. Work-stealing based on
191 * randomized scans generally leads to better throughput than
192 * "work dealing" in which producers assign tasks to idle threads,
193 * in part because threads that have finished other tasks before
194 * the signalled thread wakes up (which can be a long time) can
195 * take the task instead. Preference rules give first priority to
196 * processing tasks from their own queues (LIFO or FIFO, depending
197 * on mode), then to randomized FIFO steals of tasks in other
198 * queues. This framework began as vehicle for supporting
199 * tree-structured parallelism using work-stealing. Over time,
200 * its scalability advantages led to extensions and changes to
201 * better support more diverse usage contexts. Because most
202 * internal methods and nested classes are interrelated, their
203 * main rationale and descriptions are presented here; individual
204 * methods and nested classes contain only brief comments about
205 * details.
206 *
207 * WorkQueues
208 * ==========
209 *
210 * Most operations occur within work-stealing queues (in nested
211 * class WorkQueue). These are special forms of Deques that
212 * support only three of the four possible end-operations -- push,
213 * pop, and poll (aka steal), under the further constraints that
214 * push and pop are called only from the owning thread (or, as
215 * extended here, under a lock), while poll may be called from
216 * other threads. (If you are unfamiliar with them, you probably
217 * want to read Herlihy and Shavit's book "The Art of
218 * Multiprocessor programming", chapter 16 describing these in
219 * more detail before proceeding.) The main work-stealing queue
220 * design is roughly similar to those in the papers "Dynamic
221 * Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005
222 * (http://research.sun.com/scalable/pubs/index.html) and
223 * "Idempotent work stealing" by Michael, Saraswat, and Vechev,
224 * PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186).
225 * The main differences ultimately stem from GC requirements that
226 * we null out taken slots as soon as we can, to maintain as small
227 * a footprint as possible even in programs generating huge
228 * numbers of tasks. To accomplish this, we shift the CAS
229 * arbitrating pop vs poll (steal) from being on the indices
230 * ("base" and "top") to the slots themselves.
231 *
232 * Adding tasks then takes the form of a classic array push(task)
233 * in a circular buffer:
234 * q.array[q.top++ % length] = task;
235 *
236 * The actual code needs to null-check and size-check the array,
237 * uses masking, not mod, for indexing a power-of-two-sized array,
238 * enforces memory ordering, supports resizing, and possibly
239 * signals waiting workers to start scanning -- see below.
240 *
241 * The pop operation (always performed by owner) is of the form:
242 * if ((task = getAndSet(q.array, (q.top-1) % length, null)) != null)
243 * decrement top and return task;
244 * If this fails, the queue is empty.
245 *
246 * The poll operation by another stealer thread is, basically:
247 * if (CAS nonnull task at q.array[q.base % length] to null)
248 * increment base and return task;
249 *
250 * This may fail due to contention, and may be retried.
251 * Implementations must ensure a consistent snapshot of the base
252 * index and the task (by looping or trying elsewhere) before
253 * trying CAS. There isn't actually a method of this form,
254 * because failure due to inconsistency or contention is handled
255 * in different ways in different contexts, normally by first
256 * trying other queues. (For the most straightforward example, see
257 * method pollScan.) There are further variants for cases
258 * requiring inspection of elements before extracting them, so
259 * must interleave these with variants of this code. Also, a more
260 * efficient version (nextLocalTask) is used for polls by owners.
261 * It avoids some overhead because the queue cannot be growing
262 * during call.
263 *
264 * Memory ordering. See "Correct and Efficient Work-Stealing for
265 * Weak Memory Models" by Le, Pop, Cohen, and Nardelli, PPoPP 2013
266 * (http://www.di.ens.fr/~zappa/readings/ppopp13.pdf) for an
267 * analysis of memory ordering requirements in work-stealing
268 * algorithms similar to the one used here. Inserting and
269 * extracting tasks in array slots via volatile or atomic accesses
270 * or explicit fences provides primary synchronization.
271 *
272 * Operations on deque elements require reads and writes of both
273 * indices and slots. When possible, we allow these to occur in
274 * any order. Because the base and top indices (along with other
275 * pool or array fields accessed in many methods) only imprecisely
276 * guide where to extract from, we let accesses other than the
277 * element getAndSet/CAS/setVolatile appear in any order, using
278 * plain mode. But we must still preface some methods (mainly
279 * those that may be accessed externally) with an acquireFence to
280 * avoid unbounded staleness. This is equivalent to acting as if
281 * callers use an acquiring read of the reference to the pool or
282 * queue when invoking the method, even when they do not. We use
283 * explicit acquiring reads (getSlot) rather than plain array
284 * access when acquire mode is required but not otherwise ensured
285 * by context. To reduce stalls by other stealers, we encourage
286 * timely writes to the base index by immediately following
287 * updates with a write of a volatile field that must be updated
288 * anyway, or an Opaque-mode write if there is no such
289 * opportunity.
290 *
291 * Because indices and slot contents cannot always be consistent,
292 * the emptiness check base == top is only quiescently accurate
293 * (and so used where this suffices). Otherwise, it may err on the
294 * side of possibly making the queue appear nonempty when a push,
295 * pop, or poll have not fully committed, or making it appear
296 * empty when an update of top or base has not yet been seen.
297 * Similarly, the check in push for the queue array being full may
298 * trigger when not completely full, causing a resize earlier than
299 * required.
300 *
301 * Mainly because of these potential inconsistencies among slots
302 * vs indices, the poll operation, considered individually, is not
303 * wait-free. One thief cannot successfully continue until another
304 * in-progress one (or, if previously empty, a push) visibly
305 * completes. This can stall threads when required to consume
306 * from a given queue (which may spin). However, in the
307 * aggregate, we ensure probabilistic non-blockingness at least
308 * until checking quiescence (which is intrinsically blocking):
309 * If an attempted steal fails, a scanning thief chooses a
310 * different victim target to try next. So, in order for one thief
311 * to progress, it suffices for any in-progress poll or new push
312 * on any empty queue to complete. The worst cases occur when many
313 * threads are looking for tasks being produced by a stalled
314 * producer.
315 *
316 * This approach also enables support of a user mode in which
317 * local task processing is in FIFO, not LIFO order, simply by
318 * using poll rather than pop. This can be useful in
319 * message-passing frameworks in which tasks are never joined,
320 * although with increased contention among task producers and
321 * consumers.
322 *
323 * WorkQueues are also used in a similar way for tasks submitted
324 * to the pool. We cannot mix these tasks in the same queues used
325 * by workers. Instead, we randomly associate submission queues
326 * with submitting threads, using a form of hashing. The
327 * ThreadLocalRandom probe value serves as a hash code for
328 * choosing existing queues, and may be randomly repositioned upon
329 * contention with other submitters. In essence, submitters act
330 * like workers except that they are restricted to executing local
331 * tasks that they submitted (or when known, subtasks thereof).
332 * Insertion of tasks in shared mode requires a lock. We use only
333 * a simple spinlock (using field "source"), because submitters
334 * encountering a busy queue move to a different position to use
335 * or create other queues. They block only when registering new
336 * queues.
337 *
338 * Management
339 * ==========
340 *
341 * The main throughput advantages of work-stealing stem from
342 * decentralized control -- workers mostly take tasks from
343 * themselves or each other, at rates that can exceed a billion
344 * per second. Most non-atomic control is performed by some form
345 * of scanning across or within queues. The pool itself creates,
346 * activates (enables scanning for and running tasks),
347 * deactivates, blocks, and terminates threads, all with minimal
348 * central information. There are only a few properties that we
349 * can globally track or maintain, so we pack them into a small
350 * number of variables, often maintaining atomicity without
351 * blocking or locking. Nearly all essentially atomic control
352 * state is held in a few volatile variables that are by far most
353 * often read (not written) as status and consistency checks. We
354 * pack as much information into them as we can.
355 *
356 * Field "ctl" contains 64 bits holding information needed to
357 * atomically decide to add, enqueue (on an event queue), and
358 * dequeue and release workers. To enable this packing, we
359 * restrict maximum parallelism to (1<<15)-1 (which is far in
360 * excess of normal operating range) to allow ids, counts, and
361 * their negations (used for thresholding) to fit into 16bit
362 * subfields.
363 *
364 * Field "mode" holds configuration parameters as well as lifetime
365 * status, atomically and monotonically setting SHUTDOWN, STOP,
366 * and finally TERMINATED bits. It is updated only via bitwise
367 * atomics (getAndBitwiseOr).
368 *
369 * Array "queues" holds references to WorkQueues. It is updated
370 * (only during worker creation and termination) under the
371 * registrationLock, but is otherwise concurrently readable, and
372 * accessed directly (although always prefaced by acquireFences or
373 * other acquiring reads). To simplify index-based operations, the
374 * array size is always a power of two, and all readers must
375 * tolerate null slots. Worker queues are at odd indices. Worker
376 * ids masked with SMASK match their index. Shared (submission)
377 * queues are at even indices. Grouping them together in this way
378 * simplifies and speeds up task scanning.
379 *
380 * All worker thread creation is on-demand, triggered by task
381 * submissions, replacement of terminated workers, and/or
382 * compensation for blocked workers. However, all other support
383 * code is set up to work with other policies. To ensure that we
384 * do not hold on to worker or task references that would prevent
385 * GC, all accesses to workQueues are via indices into the
386 * queues array (which is one source of some of the messy code
387 * constructions here). In essence, the queues array serves as
388 * a weak reference mechanism. Thus for example the stack top
389 * subfield of ctl stores indices, not references.
390 *
391 * Queuing Idle Workers. Unlike HPC work-stealing frameworks, we
392 * cannot let workers spin indefinitely scanning for tasks when
393 * none can be found immediately, and we cannot start/resume
394 * workers unless there appear to be tasks available. On the
395 * other hand, we must quickly prod them into action when new
396 * tasks are submitted or generated. These latencies are mainly a
397 * function of JVM park/unpark (and underlying OS) performance,
398 * which can be slow and variable. In many usages, ramp-up time
399 * is the main limiting factor in overall performance, which is
400 * compounded at program start-up by JIT compilation and
401 * allocation. On the other hand, throughput degrades when too
402 * many threads poll for too few tasks.
403 *
404 * The "ctl" field atomically maintains total and "released"
405 * worker counts, plus the head of the available worker queue
406 * (actually stack, represented by the lower 32bit subfield of
407 * ctl). Released workers are those known to be scanning for
408 * and/or running tasks. Unreleased ("available") workers are
409 * recorded in the ctl stack. These workers are made available for
410 * signalling by enqueuing in ctl (see method awaitWork). The
411 * "queue" is a form of Treiber stack. This is ideal for
412 * activating threads in most-recently used order, and improves
413 * performance and locality, outweighing the disadvantages of
414 * being prone to contention and inability to release a worker
415 * unless it is topmost on stack. The top stack state holds the
416 * value of the "phase" field of the worker: its index and status,
417 * plus a version counter that, in addition to the count subfields
418 * (also serving as version stamps) provide protection against
419 * Treiber stack ABA effects.
420 *
421 * Creating workers. To create a worker, we pre-increment counts
422 * (serving as a reservation), and attempt to construct a
423 * ForkJoinWorkerThread via its factory. On starting, the new
424 * thread first invokes registerWorker, where it constructs a
425 * WorkQueue and is assigned an index in the queues array
426 * (expanding the array if necessary). Upon any exception across
427 * these steps, or null return from factory, deregisterWorker
428 * adjusts counts and records accordingly. If a null return, the
429 * pool continues running with fewer than the target number
430 * workers. If exceptional, the exception is propagated, generally
431 * to some external caller.
432 *
433 * WorkQueue field "phase" is used by both workers and the pool to
434 * manage and track whether a worker is UNSIGNALLED (possibly
435 * blocked waiting for a signal). When a worker is enqueued its
436 * phase field is set negative. Note that phase field updates lag
437 * queue CAS releases; seeing a negative phase does not guarantee
438 * that the worker is available. When queued, the lower 16 bits of
439 * its phase must hold its pool index. So we place the index there
440 * upon initialization and never modify these bits.
441 *
442 * The ctl field also serves as the basis for memory
443 * synchronization surrounding activation. This uses a more
444 * efficient version of a Dekker-like rule that task producers and
445 * consumers sync with each other by both writing/CASing ctl (even
446 * if to its current value). However, rather than CASing ctl to
447 * its current value in the common case where no action is
448 * required, we reduce write contention by ensuring that
449 * signalWork invocations are prefaced with a full-volatile memory
450 * access (which is usually needed anyway).
451 *
452 * Signalling. Signals (in signalWork) cause new or reactivated
453 * workers to scan for tasks. Method signalWork and its callers
454 * try to approximate the unattainable goal of having the right
455 * number of workers activated for the tasks at hand, but must err
456 * on the side of too many workers vs too few to avoid stalls. If
457 * computations are purely tree structured, it suffices for every
458 * worker to activate another when it pushes a task into an empty
459 * queue, resulting in O(log(#threads)) steps to full activation.
460 * If instead, tasks come in serially from only a single producer,
461 * each worker taking its first (since the last quiescence) task
462 * from a queue should signal another if there are more tasks in
463 * that queue. This is equivalent to, but generally faster than,
464 * arranging the stealer take two tasks, re-pushing one on its own
465 * queue, and signalling (because its queue is empty), also
466 * resulting in logarithmic full activation time. Because we don't
467 * know about usage patterns (or most commonly, mixtures), we use
468 * both approaches. We approximate the second rule by arranging
469 * that workers in scan() do not repeat signals when repeatedly
470 * taking tasks from any given queue, by remembering the previous
471 * one. There are narrow windows in which both rules may apply,
472 * leading to duplicate or unnecessary signals. Despite such
473 * limitations, these rules usually avoid slowdowns that otherwise
474 * occur when too many workers contend to take too few tasks, or
475 * when producers waste most of their time resignalling. However,
476 * contention and overhead effects may still occur during ramp-up,
477 * ramp-down, and small computations involving only a few workers.
478 *
479 * Scanning. Method scan performs top-level scanning for (and
480 * execution of) tasks. Scans by different workers and/or at
481 * different times are unlikely to poll queues in the same
482 * order. Each scan traverses and tries to poll from each queue in
483 * a pseudorandom permutation order by starting at a random index,
484 * and using a constant cyclically exhaustive stride; restarting
485 * upon contention. (Non-top-level scans; for example in
486 * helpJoin, use simpler linear probes because they do not
487 * systematically contend with top-level scans.) The pseudorandom
488 * generator need not have high-quality statistical properties in
489 * the long term. We use Marsaglia XorShifts, seeded with the Weyl
490 * sequence from ThreadLocalRandom probes, which are cheap and
491 * suffice. Scans do not otherwise explicitly take into account
492 * core affinities, loads, cache localities, etc, However, they do
493 * exploit temporal locality (which usually approximates these) by
494 * preferring to re-poll from the same queue after a successful
495 * poll before trying others (see method topLevelExec). This
496 * reduces fairness, which is partially counteracted by using a
497 * one-shot form of poll (tryPoll) that may lose to other workers.
498 *
499 * Deactivation. Method scan returns a sentinel when no tasks are
500 * found, leading to deactivation (see awaitWork). The count
501 * fields in ctl allow accurate discovery of quiescent states
502 * (i.e., when all workers are idle) after deactivation. However,
503 * this may also race with new (external) submissions, so a
504 * recheck is also needed to determine quiescence. Upon apparently
505 * triggering quiescence, awaitWork re-scans and self-signals if
506 * it may have missed a signal. In other cases, a missed signal
507 * may transiently lower parallelism because deactivation does not
508 * necessarily mean that there is no more work, only that that
509 * there were no tasks not taken by other workers. But more
510 * signals are generated (see above) to eventually reactivate if
511 * needed.
512 *
513 * Trimming workers. To release resources after periods of lack of
514 * use, a worker starting to wait when the pool is quiescent will
515 * time out and terminate if the pool has remained quiescent for
516 * period given by field keepAlive.
517 *
518 * Shutdown and Termination. A call to shutdownNow invokes
519 * tryTerminate to atomically set a mode bit. The calling thread,
520 * as well as every other worker thereafter terminating, helps
521 * terminate others by cancelling their unprocessed tasks, and
522 * waking them up. Calls to non-abrupt shutdown() preface this by
523 * checking isQuiescent before triggering the "STOP" phase of
524 * termination.
525 *
526 * Joining Tasks
527 * =============
528 *
529 * Normally, the first option when joining a task that is not done
530 * is to try to unfork it from local queue and run it. Otherwise,
531 * any of several actions may be taken when one worker is waiting
532 * to join a task stolen (or always held) by another. Because we
533 * are multiplexing many tasks on to a pool of workers, we can't
534 * always just let them block (as in Thread.join). We also cannot
535 * just reassign the joiner's run-time stack with another and
536 * replace it later, which would be a form of "continuation", that
537 * even if possible is not necessarily a good idea since we may
538 * need both an unblocked task and its continuation to progress.
539 * Instead we combine two tactics:
540 *
541 * Helping: Arranging for the joiner to execute some task that it
542 * could be running if the steal had not occurred.
543 *
544 * Compensating: Unless there are already enough live threads,
545 * method tryCompensate() may create or re-activate a spare
546 * thread to compensate for blocked joiners until they unblock.
547 *
548 * A third form (implemented via tryRemove) amounts to helping a
549 * hypothetical compensator: If we can readily tell that a
550 * possible action of a compensator is to steal and execute the
551 * task being joined, the joining thread can do so directly,
552 * without the need for a compensation thread; although with a
553 * (rare) possibility of reduced parallelism because of a
554 * transient gap in the queue array.
555 *
556 * Other intermediate forms available for specific task types (for
557 * example helpAsyncBlocker) often avoid or postpone the need for
558 * blocking or compensation.
559 *
560 * The ManagedBlocker extension API can't use helping so relies
561 * only on compensation in method awaitBlocker.
562 *
563 * The algorithm in helpJoin entails a form of "linear helping".
564 * Each worker records (in field "source") the id of the queue
565 * from which it last stole a task. The scan in method helpJoin
566 * uses these markers to try to find a worker to help (i.e., steal
567 * back a task from and execute it) that could hasten completion
568 * of the actively joined task. Thus, the joiner executes a task
569 * that would be on its own local deque if the to-be-joined task
570 * had not been stolen. This is a conservative variant of the
571 * approach described in Wagner & Calder "Leapfrogging: a portable
572 * technique for implementing efficient futures" SIGPLAN Notices,
573 * 1993 (http://portal.acm.org/citation.cfm?id=155354). It differs
574 * mainly in that we only record queue ids, not full dependency
575 * links. This requires a linear scan of the queues array to
576 * locate stealers, but isolates cost to when it is needed, rather
577 * than adding to per-task overhead. Also, searches are limited to
578 * direct and at most two levels of indirect stealers, after which
579 * there are rapidly diminishing returns on increased overhead.
580 * Searches can fail to locate stealers when stalls delay
581 * recording sources. Further, even when accurately identified,
582 * stealers might not ever produce a task that the joiner can in
583 * turn help with. So, compensation is tried upon failure to find
584 * tasks to run.
585 *
586 * Joining CountedCompleters (see helpComplete) differs from (and
587 * is generally more efficient than) other cases because task
588 * eligibility is determined by checking completion chains rather
589 * than tracking stealers.
590 *
591 * Joining under timeouts (ForkJoinTask timed get) uses a
592 * constrained mixture of helping and compensating in part because
593 * pools (actually, only the common pool) may not have any
594 * available threads: If the pool is saturated (all available
595 * workers are busy), the caller tries to remove and otherwise
596 * help; else it blocks under compensation so that it may time out
597 * independently of any tasks.
598 *
599 * Compensation does not by default aim to keep exactly the target
600 * parallelism number of unblocked threads running at any given
601 * time. Some previous versions of this class employed immediate
602 * compensations for any blocked join. However, in practice, the
603 * vast majority of blockages are transient byproducts of GC and
604 * other JVM or OS activities that are made worse by replacement
605 * when they cause longer-term oversubscription. Rather than
606 * impose arbitrary policies, we allow users to override the
607 * default of only adding threads upon apparent starvation. The
608 * compensation mechanism may also be bounded. Bounds for the
609 * commonPool (see COMMON_MAX_SPARES) better enable JVMs to cope
610 * with programming errors and abuse before running out of
611 * resources to do so.
612 *
613 * Common Pool
614 * ===========
615 *
616 * The static common pool always exists after static
617 * initialization. Since it (or any other created pool) need
618 * never be used, we minimize initial construction overhead and
619 * footprint to the setup of about a dozen fields.
620 *
621 * When external threads submit to the common pool, they can
622 * perform subtask processing (see helpComplete and related
623 * methods) upon joins. This caller-helps policy makes it
624 * sensible to set common pool parallelism level to one (or more)
625 * less than the total number of available cores, or even zero for
626 * pure caller-runs. We do not need to record whether external
627 * submissions are to the common pool -- if not, external help
628 * methods return quickly. These submitters would otherwise be
629 * blocked waiting for completion, so the extra effort (with
630 * liberally sprinkled task status checks) in inapplicable cases
631 * amounts to an odd form of limited spin-wait before blocking in
632 * ForkJoinTask.join.
633 *
634 * As a more appropriate default in managed environments, unless
635 * overridden by system properties, we use workers of subclass
636 * InnocuousForkJoinWorkerThread when there is a SecurityManager
637 * present. These workers have no permissions set, do not belong
638 * to any user-defined ThreadGroup, and erase all ThreadLocals
639 * after executing any top-level task. The associated mechanics
640 * may be JVM-dependent and must access particular Thread class
641 * fields to achieve this effect.
642 *
643 * Interrupt handling
644 * ==================
645 *
646 * The framework is designed to manage task cancellation
647 * (ForkJoinTask.cancel) independently from the interrupt status
648 * of threads running tasks. (See the public ForkJoinTask
649 * documentation for rationale.) Interrupts are issued only in
650 * tryTerminate, when workers should be terminating and tasks
651 * should be cancelled anyway. Interrupts are cleared only when
652 * necessary to ensure that calls to LockSupport.park do not loop
653 * indefinitely (park returns immediately if the current thread is
654 * interrupted). If so, interruption is reinstated after blocking
655 * if status could be visible during the scope of any task. For
656 * cases in which task bodies are specified or desired to
657 * interrupt upon cancellation, ForkJoinTask.cancel can be
658 * overridden to do so (as is done for invoke{Any,All}).
659 *
660 * Memory placement
661 * ================
662 *
663 * Performance can be very sensitive to placement of instances of
664 * ForkJoinPool and WorkQueues and their queue arrays. To reduce
665 * false-sharing impact, the @Contended annotation isolates the
666 * ForkJoinPool.ctl field as well as the most heavily written
667 * WorkQueue fields. These mainly reduce cache traffic by scanners.
668 * WorkQueue arrays are presized large enough to avoid resizing
669 * (which transiently reduces throughput) in most tree-like
670 * computations, although not in some streaming usages. Initial
671 * sizes are not large enough to avoid secondary contention
672 * effects (especially for GC cardmarks) when queues are placed
673 * near each other in memory. This is common, but has different
674 * impact in different collectors and remains incompletely
675 * addressed.
676 *
677 * Style notes
678 * ===========
679 *
680 * Memory ordering relies mainly on atomic operations (CAS,
681 * getAndSet, getAndAdd) along with explicit fences. This can be
682 * awkward and ugly, but also reflects the need to control
683 * outcomes across the unusual cases that arise in very racy code
684 * with very few invariants. All fields are read into locals
685 * before use, and null-checked if they are references, even if
686 * they can never be null under current usages. Array accesses
687 * using masked indices include checks (that are always true) that
688 * the array length is non-zero to avoid compilers inserting more
689 * expensive traps. This is usually done in a "C"-like style of
690 * listing declarations at the heads of methods or blocks, and
691 * using inline assignments on first encounter. Nearly all
692 * explicit checks lead to bypass/return, not exception throws,
693 * because they may legitimately arise during shutdown.
694 *
695 * There is a lot of representation-level coupling among classes
696 * ForkJoinPool, ForkJoinWorkerThread, and ForkJoinTask. The
697 * fields of WorkQueue maintain data structures managed by
698 * ForkJoinPool, so are directly accessed. There is little point
699 * trying to reduce this, since any associated future changes in
700 * representations will need to be accompanied by algorithmic
701 * changes anyway. Several methods intrinsically sprawl because
702 * they must accumulate sets of consistent reads of fields held in
703 * local variables. Some others are artificially broken up to
704 * reduce producer/consumer imbalances due to dynamic compilation.
705 * There are also other coding oddities (including several
706 * unnecessary-looking hoisted null checks) that help some methods
707 * perform reasonably even when interpreted (not compiled).
708 *
709 * The order of declarations in this file is (with a few exceptions):
710 * (1) Static utility functions
711 * (2) Nested (static) classes
712 * (3) Static fields
713 * (4) Fields, along with constants used when unpacking some of them
714 * (5) Internal control methods
715 * (6) Callbacks and other support for ForkJoinTask methods
716 * (7) Exported methods
717 * (8) Static block initializing statics in minimally dependent order
718 *
719 * Revision notes
720 * ==============
721 *
722 * The main sources of differences of January 2020 ForkJoin
723 * classes from previous version are:
724 *
725 * * ForkJoinTask now uses field "aux" to support blocking joins
726 * and/or record exceptions, replacing reliance on builtin
727 * monitors and side tables.
728 * * Scans probe slots (vs compare indices), along with related
729 * changes that reduce performance differences across most
730 * garbage collectors, and reduce contention.
731 * * Refactoring for better integration of special task types and
732 * other capabilities that had been incrementally tacked on. Plus
733 * many minor reworkings to improve consistency.
734 */
735
736 // Static utilities
737
738 /**
739 * If there is a security manager, makes sure caller has
740 * permission to modify threads.
741 */
742 private static void checkPermission() {
743 SecurityManager security = System.getSecurityManager();
744 if (security != null)
745 security.checkPermission(modifyThreadPermission);
746 }
747
748 static AccessControlContext contextWithPermissions(Permission ... perms) {
749 Permissions permissions = new Permissions();
750 for (Permission perm : perms)
751 permissions.add(perm);
752 return new AccessControlContext(
753 new ProtectionDomain[] { new ProtectionDomain(null, permissions) });
754 }
755
756 // Nested classes
757
758 /**
759 * Factory for creating new {@link ForkJoinWorkerThread}s.
760 * A {@code ForkJoinWorkerThreadFactory} must be defined and used
761 * for {@code ForkJoinWorkerThread} subclasses that extend base
762 * functionality or initialize threads with different contexts.
763 */
764 public static interface ForkJoinWorkerThreadFactory {
765 /**
766 * Returns a new worker thread operating in the given pool.
767 * Returning null or throwing an exception may result in tasks
768 * never being executed. If this method throws an exception,
769 * it is relayed to the caller of the method (for example
770 * {@code execute}) causing attempted thread creation. If this
771 * method returns null or throws an exception, it is not
772 * retried until the next attempted creation (for example
773 * another call to {@code execute}).
774 *
775 * @param pool the pool this thread works in
776 * @return the new worker thread, or {@code null} if the request
777 * to create a thread is rejected
778 * @throws NullPointerException if the pool is null
779 */
780 public ForkJoinWorkerThread newThread(ForkJoinPool pool);
781 }
782
783 /**
784 * Default ForkJoinWorkerThreadFactory implementation; creates a
785 * new ForkJoinWorkerThread using the system class loader as the
786 * thread context class loader.
787 */
788 static final class DefaultForkJoinWorkerThreadFactory
789 implements ForkJoinWorkerThreadFactory {
790 // ACC for access to the factory
791 private static final AccessControlContext ACC = contextWithPermissions(
792 new RuntimePermission("getClassLoader"),
793 new RuntimePermission("setContextClassLoader"));
794 public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
795 return AccessController.doPrivileged(
796 new PrivilegedAction<>() {
797 public ForkJoinWorkerThread run() {
798 return new ForkJoinWorkerThread(null, pool, true, false);
799 }},
800 ACC);
801 }
802 }
803
804 /**
805 * Factory for CommonPool unless overridden by System property.
806 * Creates InnocuousForkJoinWorkerThreads if a security manager is
807 * present at time of invocation. Support requires that we break
808 * quite a lot of encapsulation (some via helper methods in
809 * ThreadLocalRandom) to access and set Thread fields.
810 */
811 static final class DefaultCommonPoolForkJoinWorkerThreadFactory
812 implements ForkJoinWorkerThreadFactory {
813 private static final AccessControlContext ACC = contextWithPermissions(
814 modifyThreadPermission,
815 new RuntimePermission("enableContextClassLoaderOverride"),
816 new RuntimePermission("modifyThreadGroup"),
817 new RuntimePermission("getClassLoader"),
818 new RuntimePermission("setContextClassLoader"));
819
820 public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
821 return AccessController.doPrivileged(
822 new PrivilegedAction<>() {
823 public ForkJoinWorkerThread run() {
824 return System.getSecurityManager() == null ?
825 new ForkJoinWorkerThread(null, pool, true, true):
826 new ForkJoinWorkerThread.
827 InnocuousForkJoinWorkerThread(pool); }},
828 ACC);
829 }
830 }
831
832 // Constants shared across ForkJoinPool and WorkQueue
833
834 // Bounds
835 static final int SWIDTH = 16; // width of short
836 static final int SMASK = 0xffff; // short bits == max index
837 static final int MAX_CAP = 0x7fff; // max #workers - 1
838
839 // Masks and units for WorkQueue.phase and ctl sp subfield
840 static final int UNSIGNALLED = 1 << 31; // must be negative
841 static final int SS_SEQ = 1 << 16; // version count
842
843 // Mode bits and sentinels, some also used in WorkQueue fields
844 static final int FIFO = 1 << 16; // fifo queue or access mode
845 static final int SRC = 1 << 17; // set for valid queue ids
846 static final int INNOCUOUS = 1 << 18; // set for Innocuous workers
847 static final int QUIET = 1 << 19; // quiescing phase or source
848 static final int SHUTDOWN = 1 << 24;
849 static final int TERMINATED = 1 << 25;
850 static final int STOP = 1 << 31; // must be negative
851 static final int UNCOMPENSATE = 1 << 16; // tryCompensate return
852
853 /**
854 * Initial capacity of work-stealing queue array. Must be a power
855 * of two, at least 2. See above.
856 */
857 static final int INITIAL_QUEUE_CAPACITY = 1 << 8;
858
859 /**
860 * Queues supporting work-stealing as well as external task
861 * submission. See above for descriptions and algorithms.
862 */
863 static final class WorkQueue {
864 volatile int phase; // versioned, negative if inactive
865 int stackPred; // pool stack (ctl) predecessor link
866 int config; // index, mode, ORed with SRC after init
867 int base; // index of next slot for poll
868 ForkJoinTask<?>[] array; // the queued tasks; power of 2 size
869 final ForkJoinWorkerThread owner; // owning thread or null if shared
870
871 // segregate fields frequently updated but not read by scans or steals
872 @jdk.internal.vm.annotation.Contended("w")
873 int top; // index of next slot for push
874 @jdk.internal.vm.annotation.Contended("w")
875 volatile int source; // source queue id, lock, or sentinel
876 @jdk.internal.vm.annotation.Contended("w")
877 int nsteals; // number of steals from other queues
878
879 // Support for atomic operations
880 private static final VarHandle QA; // for array slots
881 private static final VarHandle SOURCE;
882 private static final VarHandle BASE;
883 static final ForkJoinTask<?> getSlot(ForkJoinTask<?>[] a, int i) {
884 return (ForkJoinTask<?>)QA.getAcquire(a, i);
885 }
886 static final ForkJoinTask<?> getAndClearSlot(ForkJoinTask<?>[] a,
887 int i) {
888 return (ForkJoinTask<?>)QA.getAndSet(a, i, null);
889 }
890 static final void setSlotVolatile(ForkJoinTask<?>[] a, int i,
891 ForkJoinTask<?> v) {
892 QA.setVolatile(a, i, v);
893 }
894 static final boolean casSlotToNull(ForkJoinTask<?>[] a, int i,
895 ForkJoinTask<?> c) {
896 return QA.weakCompareAndSet(a, i, c, null);
897 }
898 final boolean tryLock() {
899 return SOURCE.compareAndSet(this, 0, 1);
900 }
901 final void setBaseOpaque(int b) {
902 BASE.setOpaque(this, b);
903 }
904
905 /**
906 * Constructor used by ForkJoinWorkerThreads. Most fields
907 * are initialized upon thread start, in pool.registerWorker.
908 */
909 WorkQueue(ForkJoinWorkerThread owner, boolean isInnocuous) {
910 this.config = (isInnocuous) ? INNOCUOUS : 0;
911 this.owner = owner;
912 }
913
914 /**
915 * Constructor used for external queues.
916 */
917 WorkQueue(int config) {
918 array = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
919 this.config = config;
920 owner = null;
921 phase = -1;
922 }
923
924 /**
925 * Returns an exportable index (used by ForkJoinWorkerThread).
926 */
927 final int getPoolIndex() {
928 return (config & 0xffff) >>> 1; // ignore odd/even tag bit
929 }
930
931 /**
932 * Returns the approximate number of tasks in the queue.
933 */
934 final int queueSize() {
935 VarHandle.acquireFence(); // ensure fresh reads by external callers
936 int n = top - base;
937 return (n < 0) ? 0 : n; // ignore transient negative
938 }
939
940 /**
941 * Provides a more conservative estimate of whether this queue
942 * has any tasks than does queueSize.
943 */
944 final boolean isEmpty() {
945 return !((source != 0 && owner == null) || top - base > 0);
946 }
947
948 /**
949 * Pushes a task. Call only by owner in unshared queues.
950 *
951 * @param task the task. Caller must ensure non-null.
952 * @param pool (no-op if null)
953 * @throws RejectedExecutionException if array cannot be resized
954 */
955 final void push(ForkJoinTask<?> task, ForkJoinPool pool) {
956 ForkJoinTask<?>[] a = array;
957 int s = top++, d = s - base, cap, m; // skip insert if disabled
958 if (a != null && pool != null && (cap = a.length) > 0) {
959 setSlotVolatile(a, (m = cap - 1) & s, task);
960 if (d == m)
961 growArray();
962 if (d == m || a[m & (s - 1)] == null)
963 pool.signalWork(); // signal if was empty or resized
964 }
965 }
966
967 /**
968 * Pushes task to a shared queue with lock already held, and unlocks.
969 *
970 * @return true if caller should signal work
971 */
972 final boolean lockedPush(ForkJoinTask<?> task) {
973 ForkJoinTask<?>[] a = array;
974 int s = top++, d = s - base, cap, m;
975 if (a != null && (cap = a.length) > 0) {
976 a[(m = cap - 1) & s] = task;
977 if (d == m)
978 growArray();
979 source = 0; // unlock
980 if (d == m || a[m & (s - 1)] == null)
981 return true;
982 }
983 return false;
984 }
985
986 /**
987 * Doubles the capacity of array. Called by owner or with lock
988 * held after pre-incrementing top, which is reverted on
989 * allocation failure.
990 */
991 final void growArray() {
992 ForkJoinTask<?>[] oldArray = array, newArray;
993 int s = top - 1, oldCap, newCap;
994 if (oldArray != null && (oldCap = oldArray.length) > 0 &&
995 (newCap = oldCap << 1) > 0) { // skip if disabled
996 try {
997 newArray = new ForkJoinTask<?>[newCap];
998 } catch (Throwable ex) {
999 top = s;
1000 if (owner == null)
1001 source = 0; // unlock
1002 throw new RejectedExecutionException(
1003 "Queue capacity exceeded");
1004 }
1005 int newMask = newCap - 1, oldMask = oldCap - 1;
1006 for (int k = oldCap; k > 0; --k, --s) {
1007 ForkJoinTask<?> x; // poll old, push to new
1008 if ((x = getAndClearSlot(oldArray, s & oldMask)) == null)
1009 break; // others already taken
1010 newArray[s & newMask] = x;
1011 }
1012 VarHandle.releaseFence(); // fill before publish
1013 array = newArray;
1014 }
1015 }
1016
1017 // Variants of pop
1018
1019 /**
1020 * Pops and returns task, or null if empty. Called only by owner.
1021 */
1022 private ForkJoinTask<?> pop() {
1023 ForkJoinTask<?> t = null;
1024 int s = top, cap; ForkJoinTask<?>[] a;
1025 if ((a = array) != null && (cap = a.length) > 0 && base != s-- &&
1026 (t = getAndClearSlot(a, (cap - 1) & s)) != null)
1027 top = s;
1028 return t;
1029 }
1030
1031 /**
1032 * Pops the given task for owner only if it is at the current top.
1033 */
1034 final boolean tryUnpush(ForkJoinTask<?> task) {
1035 int s = top, cap; ForkJoinTask<?>[] a;
1036 if ((a = array) != null && (cap = a.length) > 0 && base != s-- &&
1037 casSlotToNull(a, (cap - 1) & s, task)) {
1038 top = s;
1039 return true;
1040 }
1041 return false;
1042 }
1043
1044 /**
1045 * Locking version of tryUnpush.
1046 */
1047 final boolean externalTryUnpush(ForkJoinTask<?> task) {
1048 boolean taken = false;
1049 int s = top, cap, k; ForkJoinTask<?>[] a;
1050 if ((a = array) != null && (cap = a.length) > 0 &&
1051 a[k = (cap - 1) & (s - 1)] == task && tryLock()) {
1052 if (top == s && array == a &&
1053 (taken = casSlotToNull(a, k, task)))
1054 top = s - 1;
1055 source = 0; // release lock
1056 }
1057 return taken;
1058 }
1059
1060 /**
1061 * Deep form of tryUnpush: Traverses from top and removes task if
1062 * present, shifting others to fill gap.
1063 */
1064 final boolean tryRemove(ForkJoinTask<?> task, boolean owned) {
1065 boolean taken = false;
1066 int p = top, cap; ForkJoinTask<?>[] a; ForkJoinTask<?> t;
1067 if ((a = array) != null && task != null && (cap = a.length) > 0) {
1068 int m = cap - 1, s = p - 1, d = p - base;
1069 for (int i = s, k; d > 0; --i, --d) {
1070 if ((t = a[k = i & m]) == task) {
1071 if (owned || tryLock()) {
1072 if ((owned || (array == a && top == p)) &&
1073 (taken = casSlotToNull(a, k, t))) {
1074 for (int j = i; j != s; ) // shift down
1075 a[j & m] = getAndClearSlot(a, ++j & m);
1076 top = s;
1077 }
1078 if (!owned)
1079 source = 0;
1080 }
1081 break;
1082 }
1083 }
1084 }
1085 return taken;
1086 }
1087
1088 // variants of poll
1089
1090 /**
1091 * Tries once to poll next task in FIFO order, failing on
1092 * inconsistency or contention.
1093 */
1094 final ForkJoinTask<?> tryPoll() {
1095 int cap, b, k; ForkJoinTask<?>[] a;
1096 if ((a = array) != null && (cap = a.length) > 0) {
1097 ForkJoinTask<?> t = getSlot(a, k = (cap - 1) & (b = base));
1098 if (base == b++ && t != null && casSlotToNull(a, k, t)) {
1099 setBaseOpaque(b);
1100 return t;
1101 }
1102 }
1103 return null;
1104 }
1105
1106 /**
1107 * Takes next task, if one exists, in order specified by mode.
1108 */
1109 final ForkJoinTask<?> nextLocalTask(int cfg) {
1110 ForkJoinTask<?> t = null;
1111 int s = top, cap; ForkJoinTask<?>[] a;
1112 if ((a = array) != null && (cap = a.length) > 0) {
1113 for (int b, d;;) {
1114 if ((d = s - (b = base)) <= 0)
1115 break;
1116 if (d == 1 || (cfg & FIFO) == 0) {
1117 if ((t = getAndClearSlot(a, --s & (cap - 1))) != null)
1118 top = s;
1119 break;
1120 }
1121 if ((t = getAndClearSlot(a, b++ & (cap - 1))) != null) {
1122 setBaseOpaque(b);
1123 break;
1124 }
1125 }
1126 }
1127 return t;
1128 }
1129
1130 /**
1131 * Takes next task, if one exists, using configured mode.
1132 */
1133 final ForkJoinTask<?> nextLocalTask() {
1134 return nextLocalTask(config);
1135 }
1136
1137 /**
1138 * Returns next task, if one exists, in order specified by mode.
1139 */
1140 final ForkJoinTask<?> peek() {
1141 VarHandle.acquireFence();
1142 int cap; ForkJoinTask<?>[] a;
1143 return ((a = array) != null && (cap = a.length) > 0) ?
1144 a[(cap - 1) & ((config & FIFO) != 0 ? base : top - 1)] : null;
1145 }
1146
1147 // specialized execution methods
1148
1149 /**
1150 * Runs the given (stolen) task if nonnull, as well as
1151 * remaining local tasks and/or others available from the
1152 * given queue.
1153 */
1154 final void topLevelExec(ForkJoinTask<?> task, WorkQueue q) {
1155 int cfg = config, nstolen = 1;
1156 while (task != null) {
1157 task.doExec();
1158 if ((task = nextLocalTask(cfg)) == null &&
1159 q != null && (task = q.tryPoll()) != null)
1160 ++nstolen;
1161 }
1162 nsteals += nstolen;
1163 source = 0;
1164 if ((cfg & INNOCUOUS) != 0)
1165 ThreadLocalRandom.eraseThreadLocals(Thread.currentThread());
1166 }
1167
1168 /**
1169 * Tries to pop and run tasks within the target's computation
1170 * until done, not found, or limit exceeded.
1171 *
1172 * @param task root of CountedCompleter computation
1173 * @param owned true if owned by a ForkJoinWorkerThread
1174 * @param limit max runs, or zero for no limit
1175 * @return task status on exit
1176 */
1177 final int helpComplete(ForkJoinTask<?> task, boolean owned, int limit) {
1178 int status = 0, cap, k, p, s; ForkJoinTask<?>[] a; ForkJoinTask<?> t;
1179 while (task != null && (status = task.status) >= 0 &&
1180 (a = array) != null && (cap = a.length) > 0 &&
1181 (t = a[k = (cap - 1) & (s = (p = top) - 1)])
1182 instanceof CountedCompleter) {
1183 CountedCompleter<?> f = (CountedCompleter<?>)t;
1184 boolean taken = false;
1185 for (;;) { // exec if root task is a completer of t
1186 if (f == task) {
1187 if (owned) {
1188 if ((taken = casSlotToNull(a, k, t)))
1189 top = s;
1190 }
1191 else if (tryLock()) {
1192 if (top == p && array == a &&
1193 (taken = casSlotToNull(a, k, t)))
1194 top = s;
1195 source = 0;
1196 }
1197 break;
1198 }
1199 else if ((f = f.completer) == null)
1200 break;
1201 }
1202 if (!taken)
1203 break;
1204 t.doExec();
1205 if (limit != 0 && --limit == 0)
1206 break;
1207 }
1208 return status;
1209 }
1210
1211 /**
1212 * Tries to poll and run AsynchronousCompletionTasks until
1213 * none found or blocker is released.
1214 *
1215 * @param blocker the blocker
1216 */
1217 final void helpAsyncBlocker(ManagedBlocker blocker) {
1218 int cap, b, d, k; ForkJoinTask<?>[] a; ForkJoinTask<?> t;
1219 while (blocker != null && (d = top - (b = base)) > 0 &&
1220 (a = array) != null && (cap = a.length) > 0 &&
1221 (((t = getSlot(a, k = (cap - 1) & b)) == null && d > 1) ||
1222 t instanceof
1223 CompletableFuture.AsynchronousCompletionTask) &&
1224 !blocker.isReleasable()) {
1225 if (t != null && base == b++ && casSlotToNull(a, k, t)) {
1226 setBaseOpaque(b);
1227 t.doExec();
1228 }
1229 }
1230 }
1231
1232 // misc
1233
1234 /** AccessControlContext for innocuous workers, created on 1st use. */
1235 private static AccessControlContext INNOCUOUS_ACC;
1236
1237 /**
1238 * Initializes (upon registration) InnocuousForkJoinWorkerThreads.
1239 */
1240 final void initializeInnocuousWorker() {
1241 AccessControlContext acc; // racy construction OK
1242 if ((acc = INNOCUOUS_ACC) == null)
1243 INNOCUOUS_ACC = acc = new AccessControlContext(
1244 new ProtectionDomain[] { new ProtectionDomain(null, null) });
1245 Thread t = Thread.currentThread();
1246 ThreadLocalRandom.setInheritedAccessControlContext(t, acc);
1247 ThreadLocalRandom.eraseThreadLocals(t);
1248 }
1249
1250 /**
1251 * Returns true if owned by a worker thread and not known to be blocked.
1252 */
1253 final boolean isApparentlyUnblocked() {
1254 Thread wt; Thread.State s;
1255 return ((wt = owner) != null &&
1256 (s = wt.getState()) != Thread.State.BLOCKED &&
1257 s != Thread.State.WAITING &&
1258 s != Thread.State.TIMED_WAITING);
1259 }
1260
1261 static {
1262 try {
1263 QA = MethodHandles.arrayElementVarHandle(ForkJoinTask[].class);
1264 MethodHandles.Lookup l = MethodHandles.lookup();
1265 SOURCE = l.findVarHandle(WorkQueue.class, "source", int.class);
1266 BASE = l.findVarHandle(WorkQueue.class, "base", int.class);
1267 } catch (ReflectiveOperationException e) {
1268 throw new ExceptionInInitializerError(e);
1269 }
1270 }
1271 }
1272
1273 // static fields (initialized in static initializer below)
1274
1275 /**
1276 * Creates a new ForkJoinWorkerThread. This factory is used unless
1277 * overridden in ForkJoinPool constructors.
1278 */
1279 public static final ForkJoinWorkerThreadFactory
1280 defaultForkJoinWorkerThreadFactory;
1281
1282 /**
1283 * Permission required for callers of methods that may start or
1284 * kill threads.
1285 */
1286 static final RuntimePermission modifyThreadPermission;
1287
1288 /**
1289 * Common (static) pool. Non-null for public use unless a static
1290 * construction exception, but internal usages null-check on use
1291 * to paranoically avoid potential initialization circularities
1292 * as well as to simplify generated code.
1293 */
1294 static final ForkJoinPool common;
1295
1296 /**
1297 * Common pool parallelism. To allow simpler use and management
1298 * when common pool threads are disabled, we allow the underlying
1299 * common.parallelism field to be zero, but in that case still report
1300 * parallelism as 1 to reflect resulting caller-runs mechanics.
1301 */
1302 static final int COMMON_PARALLELISM;
1303
1304 /**
1305 * Limit on spare thread construction in tryCompensate.
1306 */
1307 private static final int COMMON_MAX_SPARES;
1308
1309 /**
1310 * Sequence number for creating worker names
1311 */
1312 private static volatile int poolIds;
1313
1314 // static configuration constants
1315
1316 /**
1317 * Default idle timeout value (in milliseconds) for the thread
1318 * triggering quiescence to park waiting for new work
1319 */
1320 private static final long DEFAULT_KEEPALIVE = 60_000L;
1321
1322 /**
1323 * Undershoot tolerance for idle timeouts
1324 */
1325 private static final long TIMEOUT_SLOP = 20L;
1326
1327 /**
1328 * The default value for COMMON_MAX_SPARES. Overridable using the
1329 * "java.util.concurrent.ForkJoinPool.common.maximumSpares" system
1330 * property. The default value is far in excess of normal
1331 * requirements, but also far short of MAX_CAP and typical OS
1332 * thread limits, so allows JVMs to catch misuse/abuse before
1333 * running out of resources needed to do so.
1334 */
1335 private static final int DEFAULT_COMMON_MAX_SPARES = 256;
1336
1337 /*
1338 * Bits and masks for field ctl, packed with 4 16 bit subfields:
1339 * RC: Number of released (unqueued) workers minus target parallelism
1340 * TC: Number of total workers minus target parallelism
1341 * SS: version count and status of top waiting thread
1342 * ID: poolIndex of top of Treiber stack of waiters
1343 *
1344 * When convenient, we can extract the lower 32 stack top bits
1345 * (including version bits) as sp=(int)ctl. The offsets of counts
1346 * by the target parallelism and the positionings of fields makes
1347 * it possible to perform the most common checks via sign tests of
1348 * fields: When ac is negative, there are not enough unqueued
1349 * workers, when tc is negative, there are not enough total
1350 * workers. When sp is non-zero, there are waiting workers. To
1351 * deal with possibly negative fields, we use casts in and out of
1352 * "short" and/or signed shifts to maintain signedness.
1353 *
1354 * Because it occupies uppermost bits, we can add one release
1355 * count using getAndAdd of RC_UNIT, rather than CAS, when
1356 * returning from a blocked join. Other updates entail multiple
1357 * subfields and masking, requiring CAS.
1358 *
1359 * The limits packed in field "bounds" are also offset by the
1360 * parallelism level to make them comparable to the ctl rc and tc
1361 * fields.
1362 */
1363
1364 // Lower and upper word masks
1365 private static final long SP_MASK = 0xffffffffL;
1366 private static final long UC_MASK = ~SP_MASK;
1367
1368 // Release counts
1369 private static final int RC_SHIFT = 48;
1370 private static final long RC_UNIT = 0x0001L << RC_SHIFT;
1371 private static final long RC_MASK = 0xffffL << RC_SHIFT;
1372
1373 // Total counts
1374 private static final int TC_SHIFT = 32;
1375 private static final long TC_UNIT = 0x0001L << TC_SHIFT;
1376 private static final long TC_MASK = 0xffffL << TC_SHIFT;
1377 private static final long ADD_WORKER = 0x0001L << (TC_SHIFT + 15); // sign
1378
1379 // Instance fields
1380
1381 final long keepAlive; // milliseconds before dropping if idle
1382 volatile long stealCount; // collects worker nsteals
1383 int scanRover; // advances across pollScan calls
1384 volatile int threadIds; // for worker thread names
1385 final int bounds; // min, max threads packed as shorts
1386 volatile int mode; // parallelism, runstate, queue mode
1387 WorkQueue[] queues; // main registry
1388 final ReentrantLock registrationLock;
1389 Condition termination; // lazily constructed
1390 final String workerNamePrefix; // null for common pool
1391 final ForkJoinWorkerThreadFactory factory;
1392 final UncaughtExceptionHandler ueh; // per-worker UEH
1393 final Predicate<? super ForkJoinPool> saturate;
1394
1395 @jdk.internal.vm.annotation.Contended("fjpctl") // segregate
1396 volatile long ctl; // main pool control
1397
1398 // Support for atomic operations
1399 private static final VarHandle CTL;
1400 private static final VarHandle MODE;
1401 private static final VarHandle THREADIDS;
1402 private static final VarHandle POOLIDS;
1403 private boolean compareAndSetCtl(long c, long v) {
1404 return CTL.compareAndSet(this, c, v);
1405 }
1406 private long compareAndExchangeCtl(long c, long v) {
1407 return (long)CTL.compareAndExchange(this, c, v);
1408 }
1409 private long getAndAddCtl(long v) {
1410 return (long)CTL.getAndAdd(this, v);
1411 }
1412 private int getAndBitwiseOrMode(int v) {
1413 return (int)MODE.getAndBitwiseOr(this, v);
1414 }
1415 private int getAndAddThreadIds(int x) {
1416 return (int)THREADIDS.getAndAdd(this, x);
1417 }
1418 private static int getAndAddPoolIds(int x) {
1419 return (int)POOLIDS.getAndAdd(x);
1420 }
1421
1422 // Creating, registering and deregistering workers
1423
1424 /**
1425 * Tries to construct and start one worker. Assumes that total
1426 * count has already been incremented as a reservation. Invokes
1427 * deregisterWorker on any failure.
1428 *
1429 * @return true if successful
1430 */
1431 private boolean createWorker() {
1432 ForkJoinWorkerThreadFactory fac = factory;
1433 Throwable ex = null;
1434 ForkJoinWorkerThread wt = null;
1435 try {
1436 if (fac != null && (wt = fac.newThread(this)) != null) {
1437 wt.start();
1438 return true;
1439 }
1440 } catch (Throwable rex) {
1441 ex = rex;
1442 }
1443 deregisterWorker(wt, ex);
1444 return false;
1445 }
1446
1447 /**
1448 * Provides a name for ForkJoinWorkerThread constructor.
1449 */
1450 final String nextWorkerThreadName() {
1451 String prefix = workerNamePrefix;
1452 int tid = getAndAddThreadIds(1) + 1;
1453 if (prefix == null) // commonPool has no prefix
1454 prefix = "ForkJoinPool.commonPool-worker-";
1455 return prefix.concat(Integer.toString(tid));
1456 }
1457
1458 /**
1459 * Finishes initializing and records owned queue.
1460 *
1461 * @param w caller's WorkQueue
1462 */
1463 final void registerWorker(WorkQueue w) {
1464 ReentrantLock lock = registrationLock;
1465 ThreadLocalRandom.localInit();
1466 int seed = ThreadLocalRandom.getProbe();
1467 if (w != null && lock != null) {
1468 int modebits = (mode & FIFO) | w.config;
1469 w.array = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
1470 w.stackPred = seed; // stash for runWorker
1471 if ((modebits & INNOCUOUS) != 0)
1472 w.initializeInnocuousWorker();
1473 int id = (seed << 1) | 1; // initial index guess
1474 lock.lock();
1475 try {
1476 WorkQueue[] qs; int n; // find queue index
1477 if ((qs = queues) != null && (n = qs.length) > 0) {
1478 int k = n, m = n - 1;
1479 for (; qs[id &= m] != null && k > 0; id -= 2, k -= 2);
1480 if (k == 0)
1481 id = n | 1; // resize below
1482 w.phase = w.config = id | modebits; // now publishable
1483
1484 if (id < n)
1485 qs[id] = w;
1486 else { // expand array
1487 int an = n << 1, am = an - 1;
1488 WorkQueue[] as = new WorkQueue[an];
1489 as[id & am] = w;
1490 for (int j = 1; j < n; j += 2)
1491 as[j] = qs[j];
1492 for (int j = 0; j < n; j += 2) {
1493 WorkQueue q;
1494 if ((q = qs[j]) != null) // shared queues may move
1495 as[q.config & am] = q;
1496 }
1497 VarHandle.releaseFence(); // fill before publish
1498 queues = as;
1499 }
1500 }
1501 } finally {
1502 lock.unlock();
1503 }
1504 }
1505 }
1506
1507 /**
1508 * Final callback from terminating worker, as well as upon failure
1509 * to construct or start a worker. Removes record of worker from
1510 * array, and adjusts counts. If pool is shutting down, tries to
1511 * complete termination.
1512 *
1513 * @param wt the worker thread, or null if construction failed
1514 * @param ex the exception causing failure, or null if none
1515 */
1516 final void deregisterWorker(ForkJoinWorkerThread wt, Throwable ex) {
1517 ReentrantLock lock = registrationLock;
1518 WorkQueue w = null;
1519 int cfg = 0;
1520 if (wt != null && (w = wt.workQueue) != null && lock != null) {
1521 WorkQueue[] qs; int n, i;
1522 cfg = w.config;
1523 long ns = w.nsteals & 0xffffffffL;
1524 lock.lock(); // remove index from array
1525 if ((qs = queues) != null && (n = qs.length) > 0 &&
1526 qs[i = cfg & (n - 1)] == w)
1527 qs[i] = null;
1528 stealCount += ns; // accumulate steals
1529 lock.unlock();
1530 long c = ctl;
1531 if ((cfg & QUIET) == 0) // unless self-signalled, decrement counts
1532 do {} while (c != (c = compareAndExchangeCtl(
1533 c, ((RC_MASK & (c - RC_UNIT)) |
1534 (TC_MASK & (c - TC_UNIT)) |
1535 (SP_MASK & c)))));
1536 else if ((int)c == 0) // was dropped on timeout
1537 cfg = 0; // suppress signal if last
1538 for (ForkJoinTask<?> t; (t = w.pop()) != null; )
1539 ForkJoinTask.cancelIgnoringExceptions(t); // cancel tasks
1540 }
1541
1542 if (!tryTerminate(false, false) && w != null && (cfg & SRC) != 0)
1543 signalWork(); // possibly replace worker
1544 if (ex != null)
1545 ForkJoinTask.rethrow(ex);
1546 }
1547
1548 /*
1549 * Tries to create or release a worker if too few are running.
1550 */
1551 final void signalWork() {
1552 for (long c = ctl; c < 0L;) {
1553 int sp, i; WorkQueue[] qs; WorkQueue v;
1554 if ((sp = (int)c & ~UNSIGNALLED) == 0) { // no idle workers
1555 if ((c & ADD_WORKER) == 0L) // enough total workers
1556 break;
1557 if (c == (c = compareAndExchangeCtl(
1558 c, ((RC_MASK & (c + RC_UNIT)) |
1559 (TC_MASK & (c + TC_UNIT)))))) {
1560 createWorker();
1561 break;
1562 }
1563 }
1564 else if ((qs = queues) == null)
1565 break; // unstarted/terminated
1566 else if (qs.length <= (i = sp & SMASK))
1567 break; // terminated
1568 else if ((v = qs[i]) == null)
1569 break; // terminating
1570 else {
1571 long nc = (v.stackPred & SP_MASK) | (UC_MASK & (c + RC_UNIT));
1572 Thread vt = v.owner;
1573 if (c == (c = compareAndExchangeCtl(c, nc))) {
1574 v.phase = sp;
1575 LockSupport.unpark(vt); // release idle worker
1576 break;
1577 }
1578 }
1579 }
1580 }
1581
1582 /**
1583 * Top-level runloop for workers, called by ForkJoinWorkerThread.run.
1584 * See above for explanation.
1585 *
1586 * @param w caller's WorkQueue (may be null on failed initialization)
1587 */
1588 final void runWorker(WorkQueue w) {
1589 if (w != null) { // skip on failed init
1590 w.config |= SRC; // mark as valid source
1591 int r = w.stackPred, src = 0; // use seed from registerWorker
1592 do {
1593 r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift
1594 } while ((src = scan(w, src, r)) >= 0 ||
1595 (src = awaitWork(w)) == 0);
1596 }
1597 }
1598
1599 /**
1600 * Scans for and if found executes top-level tasks: Tries to poll
1601 * each queue starting at a random index with random stride,
1602 * returning source id or retry indicator if contended or
1603 * inconsistent.
1604 *
1605 * @param w caller's WorkQueue
1606 * @param prevSrc the previous queue stolen from in current phase, or 0
1607 * @param r random seed
1608 * @return id of queue if taken, negative if none found, prevSrc for retry
1609 */
1610 private int scan(WorkQueue w, int prevSrc, int r) {
1611 WorkQueue[] qs = queues;
1612 int n = (w == null || qs == null) ? 0 : qs.length;
1613 for (int step = (r >>> 16) | 1, i = n; i > 0; --i, r += step) {
1614 int j, cap, b; WorkQueue q; ForkJoinTask<?>[] a;
1615 if ((q = qs[j = r & (n - 1)]) != null && // poll at qs[j].array[k]
1616 (a = q.array) != null && (cap = a.length) > 0) {
1617 int k = (cap - 1) & (b = q.base), nextBase = b + 1;
1618 int nextIndex = (cap - 1) & nextBase, src = j | SRC;
1619 ForkJoinTask<?> t = WorkQueue.getSlot(a, k);
1620 if (q.base != b) // inconsistent
1621 return prevSrc;
1622 else if (t != null && WorkQueue.casSlotToNull(a, k, t)) {
1623 q.base = nextBase;
1624 ForkJoinTask<?> next = a[nextIndex];
1625 if ((w.source = src) != prevSrc && next != null)
1626 signalWork(); // propagate
1627 w.topLevelExec(t, q);
1628 return src;
1629 }
1630 else if (a[nextIndex] != null) // revisit
1631 return prevSrc;
1632 }
1633 }
1634 return (queues != qs) ? prevSrc: -1; // possibly resized
1635 }
1636
1637 /**
1638 * Advances worker phase, pushes onto ctl stack, and awaits signal
1639 * or reports termination.
1640 *
1641 * @return negative if terminated, else 0
1642 */
1643 private int awaitWork(WorkQueue w) {
1644 if (w == null)
1645 return -1; // already terminated
1646 int phase = (w.phase + SS_SEQ) & ~UNSIGNALLED;
1647 w.phase = phase | UNSIGNALLED; // advance phase
1648 long prevCtl = ctl, c; // enqueue
1649 do {
1650 w.stackPred = (int)prevCtl;
1651 c = ((prevCtl - RC_UNIT) & UC_MASK) | (phase & SP_MASK);
1652 } while (prevCtl != (prevCtl = compareAndExchangeCtl(prevCtl, c)));
1653
1654 Thread.interrupted(); // clear status
1655 LockSupport.setCurrentBlocker(this); // prepare to block (exit also OK)
1656 long deadline = 0L; // nonzero if possibly quiescent
1657 int ac = (int)(c >> RC_SHIFT), md;
1658 if ((md = mode) < 0) // pool is terminating
1659 return -1;
1660 else if ((md & SMASK) + ac <= 0) {
1661 boolean checkTermination = (md & SHUTDOWN) != 0;
1662 if ((deadline = System.currentTimeMillis() + keepAlive) == 0L)
1663 deadline = 1L; // avoid zero
1664 WorkQueue[] qs = queues; // check for racing submission
1665 int n = (qs == null) ? 0 : qs.length;
1666 for (int i = 0; i < n; i += 2) {
1667 WorkQueue q; ForkJoinTask<?>[] a; int cap, b;
1668 if (ctl != c) { // already signalled
1669 checkTermination = false;
1670 break;
1671 }
1672 else if ((q = qs[i]) != null &&
1673 (a = q.array) != null && (cap = a.length) > 0 &&
1674 ((b = q.base) != q.top || a[(cap - 1) & b] != null ||
1675 q.source != 0)) {
1676 if (compareAndSetCtl(c, prevCtl))
1677 w.phase = phase; // self-signal
1678 checkTermination = false;
1679 break;
1680 }
1681 }
1682 if (checkTermination && tryTerminate(false, false))
1683 return -1; // trigger quiescent termination
1684 }
1685
1686 for (boolean alt = false;;) { // await activation or termination
1687 if (w.phase >= 0)
1688 break;
1689 else if (mode < 0)
1690 return -1;
1691 else if ((c = ctl) == prevCtl)
1692 Thread.onSpinWait(); // signal in progress
1693 else if (!(alt = !alt)) // check between park calls
1694 Thread.interrupted();
1695 else if (deadline == 0L)
1696 LockSupport.park();
1697 else if (deadline - System.currentTimeMillis() > TIMEOUT_SLOP)
1698 LockSupport.parkUntil(deadline);
1699 else if (((int)c & SMASK) == (w.config & SMASK) &&
1700 compareAndSetCtl(c, ((UC_MASK & (c - TC_UNIT)) |
1701 (prevCtl & SP_MASK)))) {
1702 w.config |= QUIET; // sentinel for deregisterWorker
1703 return -1; // drop on timeout
1704 }
1705 else if ((deadline += keepAlive) == 0L)
1706 deadline = 1L; // not at head; restart timer
1707 }
1708 return 0;
1709 }
1710
1711 // Utilities used by ForkJoinTask
1712
1713 /**
1714 * Returns true if all workers are busy, possibly creating one if allowed
1715 */
1716 final boolean isSaturated() {
1717 int maxTotal = bounds >>> SWIDTH;
1718 for (long c;;) {
1719 if (((int)(c = ctl) & ~UNSIGNALLED) != 0)
1720 return false;
1721 if ((short)(c >>> TC_SHIFT) >= maxTotal)
1722 return true;
1723 long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK);
1724 if (compareAndSetCtl(c, nc))
1725 return !createWorker();
1726 }
1727 }
1728
1729 /**
1730 * Returns true if can start terminating if enabled, or already terminated
1731 */
1732 final boolean canStop() {
1733 outer: for (long oldSum = 0L;;) { // repeat until stable
1734 int md; WorkQueue[] qs; long c;
1735 if ((qs = queues) == null || ((md = mode) & STOP) != 0)
1736 return true;
1737 if ((md & SMASK) + (int)((c = ctl) >> RC_SHIFT) > 0)
1738 break;
1739 long checkSum = c;
1740 for (int i = 1; i < qs.length; i += 2) { // scan submitters
1741 WorkQueue q; ForkJoinTask<?>[] a; int s = 0, cap;
1742 if ((q = qs[i]) != null && (a = q.array) != null &&
1743 (cap = a.length) > 0 &&
1744 ((s = q.top) != q.base || a[(cap - 1) & s] != null ||
1745 q.source != 0))
1746 break outer;
1747 checkSum += (((long)i) << 32) ^ s;
1748 }
1749 if (oldSum == (oldSum = checkSum) && queues == qs)
1750 return true;
1751 }
1752 return (mode & STOP) != 0; // recheck mode on false return
1753 }
1754
1755 /**
1756 * Tries to decrement counts (sometimes implicitly) and possibly
1757 * arrange for a compensating worker in preparation for
1758 * blocking. May fail due to interference, in which case -1 is
1759 * returned so caller may retry. A zero return value indicates
1760 * that the caller doesn't need to re-adjust counts when later
1761 * unblocked.
1762 *
1763 * @param c incoming ctl value
1764 * @return UNCOMPENSATE: block then adjust, 0: block, -1 : retry
1765 */
1766 private int tryCompensate(long c) {
1767 Predicate<? super ForkJoinPool> sat;
1768 int b = bounds; // counts are signed; centered at parallelism level == 0
1769 int minActive = (short)(b & SMASK),
1770 maxTotal = b >>> SWIDTH,
1771 active = (int)(c >> RC_SHIFT),
1772 total = (short)(c >>> TC_SHIFT),
1773 sp = (int)c & ~UNSIGNALLED;
1774 if (total >= 0) {
1775 if (sp != 0) { // activate idle worker
1776 WorkQueue[] qs; int n; WorkQueue v;
1777 if ((qs = queues) != null && (n = qs.length) > 0 &&
1778 (v = qs[sp & (n - 1)]) != null) {
1779 Thread vt = v.owner;
1780 long nc = ((long)v.stackPred & SP_MASK) | (UC_MASK & c);
1781 if (compareAndSetCtl(c, nc)) {
1782 v.phase = sp;
1783 LockSupport.unpark(vt);
1784 return UNCOMPENSATE;
1785 }
1786 }
1787 return -1; // retry
1788 }
1789 else if (active > minActive) { // reduce parallelism
1790 long nc = ((RC_MASK & (c - RC_UNIT)) | (~RC_MASK & c));
1791 return compareAndSetCtl(c, nc) ? UNCOMPENSATE : -1;
1792 }
1793 }
1794 if (total < maxTotal) { // expand pool
1795 long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK);
1796 return (!compareAndSetCtl(c, nc) ? -1 :
1797 !createWorker() ? 0 : UNCOMPENSATE);
1798 }
1799 else if (!compareAndSetCtl(c, c)) // validate
1800 return -1;
1801 else if ((sat = saturate) != null && sat.test(this))
1802 return 0;
1803 else
1804 throw new RejectedExecutionException(
1805 "Thread limit exceeded replacing blocked worker");
1806 }
1807
1808 /**
1809 * Readjusts RC count; called from ForkJoinTask after blocking.
1810 */
1811 final void uncompensate() {
1812 getAndAddCtl(RC_UNIT);
1813 }
1814
1815 /**
1816 * Helps if possible until the given task is done. Scans other
1817 * queues for a task produced by one of w's stealers; returning
1818 * compensated blocking sentinel if none are found.
1819 *
1820 * @param task the task
1821 * @param w caller's WorkQueue
1822 * @return task status on exit, or UNCOMPENSATE for compensated blocking
1823 */
1824 final int helpJoin(ForkJoinTask<?> task, WorkQueue w) {
1825 int s = 0;
1826 if (task != null && w != null) {
1827 int wsrc = w.source, wid = w.config & SMASK, r = wid + 2;
1828 boolean scan = true;
1829 long c = 0L; // track ctl stability
1830 outer: for (;;) {
1831 if ((s = task.status) < 0)
1832 break;
1833 else if (scan = !scan) { // previous scan was empty
1834 if (mode < 0)
1835 ForkJoinTask.cancelIgnoringExceptions(task);
1836 else if (c == (c = ctl) && (s = tryCompensate(c)) >= 0)
1837 break; // block
1838 }
1839 else { // scan for subtasks
1840 WorkQueue[] qs = queues;
1841 int n = (qs == null) ? 0 : qs.length, m = n - 1;
1842 for (int i = n; i > 0; i -= 2, r += 2) {
1843 int j; WorkQueue q, x, y; ForkJoinTask<?>[] a;
1844 if ((q = qs[j = r & m]) != null) {
1845 int sq = q.source & SMASK, cap, b;
1846 if ((a = q.array) != null && (cap = a.length) > 0) {
1847 int k = (cap - 1) & (b = q.base);
1848 int nextBase = b + 1, src = j | SRC, sx;
1849 ForkJoinTask<?> t = WorkQueue.getSlot(a, k);
1850 boolean eligible = sq == wid ||
1851 ((x = qs[sq & m]) != null && // indirect
1852 ((sx = (x.source & SMASK)) == wid ||
1853 ((y = qs[sx & m]) != null && // 2-indirect
1854 (y.source & SMASK) == wid)));
1855 if ((s = task.status) < 0)
1856 break outer;
1857 else if ((q.source & SMASK) != sq ||
1858 q.base != b)
1859 scan = true; // inconsistent
1860 else if (t == null)
1861 scan |= (a[nextBase & (cap - 1)] != null ||
1862 q.top != b); // lagging
1863 else if (eligible) {
1864 if (WorkQueue.casSlotToNull(a, k, t)) {
1865 q.base = nextBase;
1866 w.source = src;
1867 t.doExec();
1868 w.source = wsrc;
1869 }
1870 scan = true;
1871 break;
1872 }
1873 }
1874 }
1875 }
1876 }
1877 }
1878 }
1879 return s;
1880 }
1881
1882 /**
1883 * Extra helpJoin steps for CountedCompleters. Scans for and runs
1884 * subtasks of the given root task, returning if none are found.
1885 *
1886 * @param task root of CountedCompleter computation
1887 * @param w caller's WorkQueue
1888 * @param owned true if owned by a ForkJoinWorkerThread
1889 * @return task status on exit
1890 */
1891 final int helpComplete(ForkJoinTask<?> task, WorkQueue w, boolean owned) {
1892 int s = 0;
1893 if (task != null && w != null) {
1894 int r = w.config;
1895 boolean scan = true, locals = true;
1896 long c = 0L;
1897 outer: for (;;) {
1898 if (locals) { // try locals before scanning
1899 if ((s = w.helpComplete(task, owned, 0)) < 0)
1900 break;
1901 locals = false;
1902 }
1903 else if ((s = task.status) < 0)
1904 break;
1905 else if (scan = !scan) {
1906 if (c == (c = ctl))
1907 break;
1908 }
1909 else { // scan for subtasks
1910 WorkQueue[] qs = queues;
1911 int n = (qs == null) ? 0 : qs.length;
1912 for (int i = n; i > 0; --i, ++r) {
1913 int j, cap, b; WorkQueue q; ForkJoinTask<?>[] a;
1914 boolean eligible = false;
1915 if ((q = qs[j = r & (n - 1)]) != null &&
1916 (a = q.array) != null && (cap = a.length) > 0) {
1917 int k = (cap - 1) & (b = q.base), nextBase = b + 1;
1918 ForkJoinTask<?> t = WorkQueue.getSlot(a, k);
1919 if (t instanceof CountedCompleter) {
1920 CountedCompleter<?> f = (CountedCompleter<?>)t;
1921 do {} while (!(eligible = (f == task)) &&
1922 (f = f.completer) != null);
1923 }
1924 if ((s = task.status) < 0)
1925 break outer;
1926 else if (q.base != b)
1927 scan = true; // inconsistent
1928 else if (t == null)
1929 scan |= (a[nextBase & (cap - 1)] != null ||
1930 q.top != b);
1931 else if (eligible) {
1932 if (WorkQueue.casSlotToNull(a, k, t)) {
1933 q.setBaseOpaque(nextBase);
1934 t.doExec();
1935 locals = true;
1936 }
1937 scan = true;
1938 break;
1939 }
1940 }
1941 }
1942 }
1943 }
1944 }
1945 return s;
1946 }
1947
1948 /**
1949 * Scans for and returns a polled task, if available. Used only
1950 * for untracked polls. Begins scan at an index (scanRover)
1951 * advanced on each call, to avoid systematic unfairness.
1952 *
1953 * @param submissionsOnly if true, only scan submission queues
1954 */
1955 private ForkJoinTask<?> pollScan(boolean submissionsOnly) {
1956 VarHandle.acquireFence();
1957 int r = scanRover += 0x61c88647; // Weyl increment; raciness OK
1958 if (submissionsOnly) // even indices only
1959 r &= ~1;
1960 int step = (submissionsOnly) ? 2 : 1;
1961 WorkQueue[] qs; int n;
1962 while ((qs = queues) != null && (n = qs.length) > 0) {
1963 boolean scan = false;
1964 for (int i = 0; i < n; i += step) {
1965 int j, cap, b; WorkQueue q; ForkJoinTask<?>[] a;
1966 if ((q = qs[j = (n - 1) & (r + i)]) != null &&
1967 (a = q.array) != null && (cap = a.length) > 0) {
1968 int k = (cap - 1) & (b = q.base), nextBase = b + 1;
1969 ForkJoinTask<?> t = WorkQueue.getSlot(a, k);
1970 if (q.base != b)
1971 scan = true;
1972 else if (t == null)
1973 scan |= (q.top != b || a[nextBase & (cap - 1)] != null);
1974 else if (!WorkQueue.casSlotToNull(a, k, t))
1975 scan = true;
1976 else {
1977 q.setBaseOpaque(nextBase);
1978 return t;
1979 }
1980 }
1981 }
1982 if (!scan && queues == qs)
1983 break;
1984 }
1985 return null;
1986 }
1987
1988 /**
1989 * Runs tasks until {@code isQuiescent()}. Rather than blocking
1990 * when tasks cannot be found, rescans until all others cannot
1991 * find tasks either.
1992 *
1993 * @param nanos max wait time (Long.MAX_VALUE if effectively untimed)
1994 * @param interruptible true if return on interrupt
1995 * @return positive if quiescent, negative if interrupted, else 0
1996 */
1997 final int helpQuiescePool(WorkQueue w, long nanos, boolean interruptible) {
1998 if (w == null)
1999 return 0;
2000 long startTime = System.nanoTime(), parkTime = 0L;
2001 int prevSrc = w.source, wsrc = prevSrc, cfg = w.config, r = cfg + 1;
2002 for (boolean active = true, locals = true;;) {
2003 boolean busy = false, scan = false;
2004 if (locals) { // run local tasks before (re)polling
2005 locals = false;
2006 for (ForkJoinTask<?> u; (u = w.nextLocalTask(cfg)) != null;)
2007 u.doExec();
2008 }
2009 WorkQueue[] qs = queues;
2010 int n = (qs == null) ? 0 : qs.length;
2011 for (int i = n; i > 0; --i, ++r) {
2012 int j, b, cap; WorkQueue q; ForkJoinTask<?>[] a;
2013 if ((q = qs[j = (n - 1) & r]) != null && q != w &&
2014 (a = q.array) != null && (cap = a.length) > 0) {
2015 int k = (cap - 1) & (b = q.base);
2016 int nextBase = b + 1, src = j | SRC;
2017 ForkJoinTask<?> t = WorkQueue.getSlot(a, k);
2018 if (q.base != b)
2019 busy = scan = true;
2020 else if (t != null) {
2021 busy = scan = true;
2022 if (!active) { // increment before taking
2023 active = true;
2024 getAndAddCtl(RC_UNIT);
2025 }
2026 if (WorkQueue.casSlotToNull(a, k, t)) {
2027 q.base = nextBase;
2028 w.source = src;
2029 t.doExec();
2030 w.source = wsrc = prevSrc;
2031 locals = true;
2032 }
2033 break;
2034 }
2035 else if (!busy) {
2036 if (q.top != b || a[nextBase & (cap - 1)] != null)
2037 busy = scan = true;
2038 else if (q.source != QUIET && q.phase >= 0)
2039 busy = true;
2040 }
2041 }
2042 }
2043 VarHandle.acquireFence();
2044 if (!scan && queues == qs) {
2045 boolean interrupted;
2046 if (!busy) {
2047 w.source = prevSrc;
2048 if (!active)
2049 getAndAddCtl(RC_UNIT);
2050 return 1;
2051 }
2052 if (wsrc != QUIET)
2053 w.source = wsrc = QUIET;
2054 if (active) { // decrement
2055 active = false;
2056 parkTime = 0L;
2057 getAndAddCtl(RC_MASK & -RC_UNIT);
2058 }
2059 else if (parkTime == 0L) {
2060 parkTime = 1L << 10; // initially about 1 usec
2061 Thread.yield();
2062 }
2063 else if ((interrupted = interruptible && Thread.interrupted()) ||
2064 System.nanoTime() - startTime > nanos) {
2065 getAndAddCtl(RC_UNIT);
2066 return interrupted ? -1 : 0;
2067 }
2068 else {
2069 LockSupport.parkNanos(this, parkTime);
2070 if (parkTime < nanos >>> 8 && parkTime < 1L << 20)
2071 parkTime <<= 1; // max sleep approx 1 sec or 1% nanos
2072 }
2073 }
2074 }
2075 }
2076
2077 /**
2078 * Helps quiesce from external caller until done, interrupted, or timeout
2079 *
2080 * @param nanos max wait time (Long.MAX_VALUE if effectively untimed)
2081 * @param interruptible true if return on interrupt
2082 * @return positive if quiescent, negative if interrupted, else 0
2083 */
2084 final int externalHelpQuiescePool(long nanos, boolean interruptible) {
2085 for (long startTime = System.nanoTime(), parkTime = 0L;;) {
2086 ForkJoinTask<?> t;
2087 if ((t = pollScan(false)) != null) {
2088 t.doExec();
2089 parkTime = 0L;
2090 }
2091 else if (canStop())
2092 return 1;
2093 else if (parkTime == 0L) {
2094 parkTime = 1L << 10;
2095 Thread.yield();
2096 }
2097 else if ((System.nanoTime() - startTime) > nanos)
2098 return 0;
2099 else if (interruptible && Thread.interrupted())
2100 return -1;
2101 else {
2102 LockSupport.parkNanos(this, parkTime);
2103 if (parkTime < nanos >>> 8 && parkTime < 1L << 20)
2104 parkTime <<= 1;
2105 }
2106 }
2107 }
2108
2109 /**
2110 * Gets and removes a local or stolen task for the given worker.
2111 *
2112 * @return a task, if available
2113 */
2114 final ForkJoinTask<?> nextTaskFor(WorkQueue w) {
2115 ForkJoinTask<?> t;
2116 if (w == null || (t = w.nextLocalTask(w.config)) == null)
2117 t = pollScan(false);
2118 return t;
2119 }
2120
2121 // External operations
2122
2123 /**
2124 * Finds and locks a WorkQueue for an external submitter, or
2125 * returns null if shutdown or terminating.
2126 */
2127 final WorkQueue submissionQueue() {
2128 int r;
2129 if ((r = ThreadLocalRandom.getProbe()) == 0) {
2130 ThreadLocalRandom.localInit(); // initialize caller's probe
2131 r = ThreadLocalRandom.getProbe();
2132 }
2133 for (int id = r << 1;;) { // even indices only
2134 int md = mode, n, i; WorkQueue q; ReentrantLock lock;
2135 WorkQueue[] qs = queues;
2136 if ((md & SHUTDOWN) != 0 || qs == null || (n = qs.length) <= 0)
2137 return null;
2138 else if ((q = qs[i = (n - 1) & id]) == null) {
2139 if ((lock = registrationLock) != null) {
2140 WorkQueue w = new WorkQueue(id | SRC);
2141 lock.lock(); // install under lock
2142 if (qs[i] == null)
2143 qs[i] = w; // else lost race; discard
2144 lock.unlock();
2145 }
2146 }
2147 else if (!q.tryLock()) // move and restart
2148 id = (r = ThreadLocalRandom.advanceProbe(r)) << 1;
2149 else
2150 return q;
2151 }
2152 }
2153
2154 /**
2155 * Adds the given task to an external submission queue, or throws
2156 * exception if shutdown or terminating.
2157 *
2158 * @param task the task. Caller must ensure non-null.
2159 */
2160 final void externalPush(ForkJoinTask<?> task) {
2161 WorkQueue q;
2162 if ((q = submissionQueue()) == null)
2163 throw new RejectedExecutionException(); // shutdown or disabled
2164 else if (q.lockedPush(task))
2165 signalWork();
2166 }
2167
2168 /**
2169 * Pushes a possibly-external submission.
2170 */
2171 private <T> ForkJoinTask<T> externalSubmit(ForkJoinTask<T> task) {
2172 Thread t; ForkJoinWorkerThread wt; WorkQueue q;
2173 if (task == null)
2174 throw new NullPointerException();
2175 if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) &&
2176 (q = (wt = (ForkJoinWorkerThread)t).workQueue) != null &&
2177 wt.pool == this)
2178 q.push(task, this);
2179 else
2180 externalPush(task);
2181 return task;
2182 }
2183
2184 /**
2185 * Returns common pool queue for an external thread that has
2186 * possibly ever submitted a common pool task (nonzero probe), or
2187 * null if none.
2188 */
2189 static WorkQueue commonQueue() {
2190 ForkJoinPool p; WorkQueue[] qs;
2191 int r = ThreadLocalRandom.getProbe(), n;
2192 return ((p = common) != null && (qs = p.queues) != null &&
2193 (n = qs.length) > 0 && r != 0) ?
2194 qs[(n - 1) & (r << 1)] : null;
2195 }
2196
2197 /**
2198 * If the given executor is a ForkJoinPool, poll and execute
2199 * AsynchronousCompletionTasks from worker's queue until none are
2200 * available or blocker is released.
2201 */
2202 static void helpAsyncBlocker(Executor e, ManagedBlocker blocker) {
2203 WorkQueue w = null; Thread t; ForkJoinWorkerThread wt;
2204 if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
2205 if ((wt = (ForkJoinWorkerThread)t).pool == e)
2206 w = wt.workQueue;
2207 }
2208 else if (e == common)
2209 w = commonQueue();
2210 if (w != null)
2211 w.helpAsyncBlocker(blocker);
2212 }
2213
2214 /**
2215 * Returns a cheap heuristic guide for task partitioning when
2216 * programmers, frameworks, tools, or languages have little or no
2217 * idea about task granularity. In essence, by offering this
2218 * method, we ask users only about tradeoffs in overhead vs
2219 * expected throughput and its variance, rather than how finely to
2220 * partition tasks.
2221 *
2222 * In a steady state strict (tree-structured) computation, each
2223 * thread makes available for stealing enough tasks for other
2224 * threads to remain active. Inductively, if all threads play by
2225 * the same rules, each thread should make available only a
2226 * constant number of tasks.
2227 *
2228 * The minimum useful constant is just 1. But using a value of 1
2229 * would require immediate replenishment upon each steal to
2230 * maintain enough tasks, which is infeasible. Further,
2231 * partitionings/granularities of offered tasks should minimize
2232 * steal rates, which in general means that threads nearer the top
2233 * of computation tree should generate more than those nearer the
2234 * bottom. In perfect steady state, each thread is at
2235 * approximately the same level of computation tree. However,
2236 * producing extra tasks amortizes the uncertainty of progress and
2237 * diffusion assumptions.
2238 *
2239 * So, users will want to use values larger (but not much larger)
2240 * than 1 to both smooth over transient shortages and hedge
2241 * against uneven progress; as traded off against the cost of
2242 * extra task overhead. We leave the user to pick a threshold
2243 * value to compare with the results of this call to guide
2244 * decisions, but recommend values such as 3.
2245 *
2246 * When all threads are active, it is on average OK to estimate
2247 * surplus strictly locally. In steady-state, if one thread is
2248 * maintaining say 2 surplus tasks, then so are others. So we can
2249 * just use estimated queue length. However, this strategy alone
2250 * leads to serious mis-estimates in some non-steady-state
2251 * conditions (ramp-up, ramp-down, other stalls). We can detect
2252 * many of these by further considering the number of "idle"
2253 * threads, that are known to have zero queued tasks, so
2254 * compensate by a factor of (#idle/#active) threads.
2255 */
2256 static int getSurplusQueuedTaskCount() {
2257 Thread t; ForkJoinWorkerThread wt; ForkJoinPool pool; WorkQueue q;
2258 if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) &&
2259 (pool = (wt = (ForkJoinWorkerThread)t).pool) != null &&
2260 (q = wt.workQueue) != null) {
2261 int p = pool.mode & SMASK;
2262 int a = p + (int)(pool.ctl >> RC_SHIFT);
2263 int n = q.top - q.base;
2264 return n - (a > (p >>>= 1) ? 0 :
2265 a > (p >>>= 1) ? 1 :
2266 a > (p >>>= 1) ? 2 :
2267 a > (p >>>= 1) ? 4 :
2268 8);
2269 }
2270 return 0;
2271 }
2272
2273 // Termination
2274
2275 /**
2276 * Possibly initiates and/or completes termination.
2277 *
2278 * @param now if true, unconditionally terminate, else only
2279 * if no work and no active workers
2280 * @param enable if true, terminate when next possible
2281 * @return true if terminating or terminated
2282 */
2283 private boolean tryTerminate(boolean now, boolean enable) {
2284 int md; // try to set SHUTDOWN, then STOP, then help terminate
2285 if (((md = mode) & SHUTDOWN) == 0) {
2286 if (!enable)
2287 return false;
2288 md = getAndBitwiseOrMode(SHUTDOWN);
2289 }
2290 if ((md & STOP) == 0) {
2291 if (!now && !canStop())
2292 return false;
2293 md = getAndBitwiseOrMode(STOP);
2294 }
2295 for (int k = 0; k < 2; ++k) { // twice in case of lagging qs updates
2296 for (ForkJoinTask<?> t; (t = pollScan(false)) != null; )
2297 ForkJoinTask.cancelIgnoringExceptions(t); // help cancel
2298 WorkQueue[] qs; int n; WorkQueue q; Thread thread;
2299 if ((qs = queues) != null && (n = qs.length) > 0) {
2300 for (int j = 1; j < n; j += 2) { // unblock other workers
2301 if ((q = qs[j]) != null && (thread = q.owner) != null &&
2302 !thread.isInterrupted()) {
2303 try {
2304 thread.interrupt();
2305 } catch (Throwable ignore) {
2306 }
2307 }
2308 }
2309 }
2310 ReentrantLock lock; Condition cond; // signal when no workers
2311 if (((md = mode) & TERMINATED) == 0 &&
2312 (md & SMASK) + (short)(ctl >>> TC_SHIFT) <= 0 &&
2313 (getAndBitwiseOrMode(TERMINATED) & TERMINATED) == 0 &&
2314 (lock = registrationLock) != null) {
2315 lock.lock();
2316 if ((cond = termination) != null)
2317 cond.signalAll();
2318 lock.unlock();
2319 }
2320 }
2321 return true;
2322 }
2323
2324 // Exported methods
2325
2326 // Constructors
2327
2328 /**
2329 * Creates a {@code ForkJoinPool} with parallelism equal to {@link
2330 * java.lang.Runtime#availableProcessors}, using defaults for all
2331 * other parameters (see {@link #ForkJoinPool(int,
2332 * ForkJoinWorkerThreadFactory, UncaughtExceptionHandler, boolean,
2333 * int, int, int, Predicate, long, TimeUnit)}).
2334 *
2335 * @throws SecurityException if a security manager exists and
2336 * the caller is not permitted to modify threads
2337 * because it does not hold {@link
2338 * java.lang.RuntimePermission}{@code ("modifyThread")}
2339 */
2340 public ForkJoinPool() {
2341 this(Math.min(MAX_CAP, Runtime.getRuntime().availableProcessors()),
2342 defaultForkJoinWorkerThreadFactory, null, false,
2343 0, MAX_CAP, 1, null, DEFAULT_KEEPALIVE, TimeUnit.MILLISECONDS);
2344 }
2345
2346 /**
2347 * Creates a {@code ForkJoinPool} with the indicated parallelism
2348 * level, using defaults for all other parameters (see {@link
2349 * #ForkJoinPool(int, ForkJoinWorkerThreadFactory,
2350 * UncaughtExceptionHandler, boolean, int, int, int, Predicate,
2351 * long, TimeUnit)}).
2352 *
2353 * @param parallelism the parallelism level
2354 * @throws IllegalArgumentException if parallelism less than or
2355 * equal to zero, or greater than implementation limit
2356 * @throws SecurityException if a security manager exists and
2357 * the caller is not permitted to modify threads
2358 * because it does not hold {@link
2359 * java.lang.RuntimePermission}{@code ("modifyThread")}
2360 */
2361 public ForkJoinPool(int parallelism) {
2362 this(parallelism, defaultForkJoinWorkerThreadFactory, null, false,
2363 0, MAX_CAP, 1, null, DEFAULT_KEEPALIVE, TimeUnit.MILLISECONDS);
2364 }
2365
2366 /**
2367 * Creates a {@code ForkJoinPool} with the given parameters (using
2368 * defaults for others -- see {@link #ForkJoinPool(int,
2369 * ForkJoinWorkerThreadFactory, UncaughtExceptionHandler, boolean,
2370 * int, int, int, Predicate, long, TimeUnit)}).
2371 *
2372 * @param parallelism the parallelism level. For default value,
2373 * use {@link java.lang.Runtime#availableProcessors}.
2374 * @param factory the factory for creating new threads. For default value,
2375 * use {@link #defaultForkJoinWorkerThreadFactory}.
2376 * @param handler the handler for internal worker threads that
2377 * terminate due to unrecoverable errors encountered while executing
2378 * tasks. For default value, use {@code null}.
2379 * @param asyncMode if true,
2380 * establishes local first-in-first-out scheduling mode for forked
2381 * tasks that are never joined. This mode may be more appropriate
2382 * than default locally stack-based mode in applications in which
2383 * worker threads only process event-style asynchronous tasks.
2384 * For default value, use {@code false}.
2385 * @throws IllegalArgumentException if parallelism less than or
2386 * equal to zero, or greater than implementation limit
2387 * @throws NullPointerException if the factory is null
2388 * @throws SecurityException if a security manager exists and
2389 * the caller is not permitted to modify threads
2390 * because it does not hold {@link
2391 * java.lang.RuntimePermission}{@code ("modifyThread")}
2392 */
2393 public ForkJoinPool(int parallelism,
2394 ForkJoinWorkerThreadFactory factory,
2395 UncaughtExceptionHandler handler,
2396 boolean asyncMode) {
2397 this(parallelism, factory, handler, asyncMode,
2398 0, MAX_CAP, 1, null, DEFAULT_KEEPALIVE, TimeUnit.MILLISECONDS);
2399 }
2400
2401 /**
2402 * Creates a {@code ForkJoinPool} with the given parameters.
2403 *
2404 * @param parallelism the parallelism level. For default value,
2405 * use {@link java.lang.Runtime#availableProcessors}.
2406 *
2407 * @param factory the factory for creating new threads. For
2408 * default value, use {@link #defaultForkJoinWorkerThreadFactory}.
2409 *
2410 * @param handler the handler for internal worker threads that
2411 * terminate due to unrecoverable errors encountered while
2412 * executing tasks. For default value, use {@code null}.
2413 *
2414 * @param asyncMode if true, establishes local first-in-first-out
2415 * scheduling mode for forked tasks that are never joined. This
2416 * mode may be more appropriate than default locally stack-based
2417 * mode in applications in which worker threads only process
2418 * event-style asynchronous tasks. For default value, use {@code
2419 * false}.
2420 *
2421 * @param corePoolSize the number of threads to keep in the pool
2422 * (unless timed out after an elapsed keep-alive). Normally (and
2423 * by default) this is the same value as the parallelism level,
2424 * but may be set to a larger value to reduce dynamic overhead if
2425 * tasks regularly block. Using a smaller value (for example
2426 * {@code 0}) has the same effect as the default.
2427 *
2428 * @param maximumPoolSize the maximum number of threads allowed.
2429 * When the maximum is reached, attempts to replace blocked
2430 * threads fail. (However, because creation and termination of
2431 * different threads may overlap, and may be managed by the given
2432 * thread factory, this value may be transiently exceeded.) To
2433 * arrange the same value as is used by default for the common
2434 * pool, use {@code 256} plus the {@code parallelism} level. (By
2435 * default, the common pool allows a maximum of 256 spare
2436 * threads.) Using a value (for example {@code
2437 * Integer.MAX_VALUE}) larger than the implementation's total
2438 * thread limit has the same effect as using this limit (which is
2439 * the default).
2440 *
2441 * @param minimumRunnable the minimum allowed number of core
2442 * threads not blocked by a join or {@link ManagedBlocker}. To
2443 * ensure progress, when too few unblocked threads exist and
2444 * unexecuted tasks may exist, new threads are constructed, up to
2445 * the given maximumPoolSize. For the default value, use {@code
2446 * 1}, that ensures liveness. A larger value might improve
2447 * throughput in the presence of blocked activities, but might
2448 * not, due to increased overhead. A value of zero may be
2449 * acceptable when submitted tasks cannot have dependencies
2450 * requiring additional threads.
2451 *
2452 * @param saturate if non-null, a predicate invoked upon attempts
2453 * to create more than the maximum total allowed threads. By
2454 * default, when a thread is about to block on a join or {@link
2455 * ManagedBlocker}, but cannot be replaced because the
2456 * maximumPoolSize would be exceeded, a {@link
2457 * RejectedExecutionException} is thrown. But if this predicate
2458 * returns {@code true}, then no exception is thrown, so the pool
2459 * continues to operate with fewer than the target number of
2460 * runnable threads, which might not ensure progress.
2461 *
2462 * @param keepAliveTime the elapsed time since last use before
2463 * a thread is terminated (and then later replaced if needed).
2464 * For the default value, use {@code 60, TimeUnit.SECONDS}.
2465 *
2466 * @param unit the time unit for the {@code keepAliveTime} argument
2467 *
2468 * @throws IllegalArgumentException if parallelism is less than or
2469 * equal to zero, or is greater than implementation limit,
2470 * or if maximumPoolSize is less than parallelism,
2471 * of if the keepAliveTime is less than or equal to zero.
2472 * @throws NullPointerException if the factory is null
2473 * @throws SecurityException if a security manager exists and
2474 * the caller is not permitted to modify threads
2475 * because it does not hold {@link
2476 * java.lang.RuntimePermission}{@code ("modifyThread")}
2477 * @since 9
2478 */
2479 public ForkJoinPool(int parallelism,
2480 ForkJoinWorkerThreadFactory factory,
2481 UncaughtExceptionHandler handler,
2482 boolean asyncMode,
2483 int corePoolSize,
2484 int maximumPoolSize,
2485 int minimumRunnable,
2486 Predicate<? super ForkJoinPool> saturate,
2487 long keepAliveTime,
2488 TimeUnit unit) {
2489 checkPermission();
2490 int p = parallelism;
2491 if (p <= 0 || p > MAX_CAP || p > maximumPoolSize || keepAliveTime <= 0L)
2492 throw new IllegalArgumentException();
2493 if (factory == null || unit == null)
2494 throw new NullPointerException();
2495 this.factory = factory;
2496 this.ueh = handler;
2497 this.saturate = saturate;
2498 this.keepAlive = Math.max(unit.toMillis(keepAliveTime), TIMEOUT_SLOP);
2499 int size = 1 << (33 - Integer.numberOfLeadingZeros(p - 1));
2500 int corep = Math.min(Math.max(corePoolSize, p), MAX_CAP);
2501 int maxSpares = Math.min(maximumPoolSize, MAX_CAP) - p;
2502 int minAvail = Math.min(Math.max(minimumRunnable, 0), MAX_CAP);
2503 this.bounds = ((minAvail - p) & SMASK) | (maxSpares << SWIDTH);
2504 this.mode = p | (asyncMode ? FIFO : 0);
2505 this.ctl = ((((long)(-corep) << TC_SHIFT) & TC_MASK) |
2506 (((long)(-p) << RC_SHIFT) & RC_MASK));
2507 this.registrationLock = new ReentrantLock();
2508 this.queues = new WorkQueue[size];
2509 String pid = Integer.toString(getAndAddPoolIds(1) + 1);
2510 this.workerNamePrefix = "ForkJoinPool-" + pid + "-worker-";
2511 }
2512
2513 // helper method for commonPool constructor
2514 private static Object newInstanceFromSystemProperty(String property)
2515 throws ReflectiveOperationException {
2516 String className = System.getProperty(property);
2517 return (className == null)
2518 ? null
2519 : ClassLoader.getSystemClassLoader().loadClass(className)
2520 .getConstructor().newInstance();
2521 }
2522
2523 /**
2524 * Constructor for common pool using parameters possibly
2525 * overridden by system properties
2526 */
2527 private ForkJoinPool(byte forCommonPoolOnly) {
2528 int parallelism = Runtime.getRuntime().availableProcessors() - 1;
2529 ForkJoinWorkerThreadFactory fac = null;
2530 UncaughtExceptionHandler handler = null;
2531 try { // ignore exceptions in accessing/parsing properties
2532 fac = (ForkJoinWorkerThreadFactory) newInstanceFromSystemProperty(
2533 "java.util.concurrent.ForkJoinPool.common.threadFactory");
2534 handler = (UncaughtExceptionHandler) newInstanceFromSystemProperty(
2535 "java.util.concurrent.ForkJoinPool.common.exceptionHandler");
2536 String pp = System.getProperty
2537 ("java.util.concurrent.ForkJoinPool.common.parallelism");
2538 if (pp != null)
2539 parallelism = Integer.parseInt(pp);
2540 } catch (Exception ignore) {
2541 }
2542 int p = this.mode = Math.min(Math.max(parallelism, 0), MAX_CAP);
2543 int size = 1 << (33 - Integer.numberOfLeadingZeros(p > 0 ? p - 1 : 1));
2544 this.factory = (fac != null) ? fac :
2545 new DefaultCommonPoolForkJoinWorkerThreadFactory();
2546 this.ueh = handler;
2547 this.keepAlive = DEFAULT_KEEPALIVE;
2548 this.saturate = null;
2549 this.workerNamePrefix = null;
2550 this.bounds = ((1 - p) & SMASK) | (COMMON_MAX_SPARES << SWIDTH);
2551 this.ctl = ((((long)(-p) << TC_SHIFT) & TC_MASK) |
2552 (((long)(-p) << RC_SHIFT) & RC_MASK));
2553 this.queues = new WorkQueue[size];
2554 this.registrationLock = new ReentrantLock();
2555 }
2556
2557 /**
2558 * Returns the common pool instance. This pool is statically
2559 * constructed; its run state is unaffected by attempts to {@link
2560 * #shutdown} or {@link #shutdownNow}. However this pool and any
2561 * ongoing processing are automatically terminated upon program
2562 * {@link System#exit}. Any program that relies on asynchronous
2563 * task processing to complete before program termination should
2564 * invoke {@code commonPool().}{@link #awaitQuiescence awaitQuiescence},
2565 * before exit.
2566 *
2567 * @return the common pool instance
2568 * @since 1.8
2569 */
2570 public static ForkJoinPool commonPool() {
2571 // assert common != null : "static init error";
2572 return common;
2573 }
2574
2575 // Execution methods
2576
2577 /**
2578 * Performs the given task, returning its result upon completion.
2579 * If the computation encounters an unchecked Exception or Error,
2580 * it is rethrown as the outcome of this invocation. Rethrown
2581 * exceptions behave in the same way as regular exceptions, but,
2582 * when possible, contain stack traces (as displayed for example
2583 * using {@code ex.printStackTrace()}) of both the current thread
2584 * as well as the thread actually encountering the exception;
2585 * minimally only the latter.
2586 *
2587 * @param task the task
2588 * @param <T> the type of the task's result
2589 * @return the task's result
2590 * @throws NullPointerException if the task is null
2591 * @throws RejectedExecutionException if the task cannot be
2592 * scheduled for execution
2593 */
2594 public <T> T invoke(ForkJoinTask<T> task) {
2595 externalSubmit(task);
2596 return task.join();
2597 }
2598
2599 /**
2600 * Arranges for (asynchronous) execution of the given task.
2601 *
2602 * @param task the task
2603 * @throws NullPointerException if the task is null
2604 * @throws RejectedExecutionException if the task cannot be
2605 * scheduled for execution
2606 */
2607 public void execute(ForkJoinTask<?> task) {
2608 externalSubmit(task);
2609 }
2610
2611 // AbstractExecutorService methods
2612
2613 /**
2614 * @throws NullPointerException if the task is null
2615 * @throws RejectedExecutionException if the task cannot be
2616 * scheduled for execution
2617 */
2618 @Override
2619 @SuppressWarnings("unchecked")
2620 public void execute(Runnable task) {
2621 externalSubmit((task instanceof ForkJoinTask<?>)
2622 ? (ForkJoinTask<Void>) task // avoid re-wrap
2623 : new ForkJoinTask.RunnableExecuteAction(task));
2624 }
2625
2626 /**
2627 * Submits a ForkJoinTask for execution.
2628 *
2629 * @param task the task to submit
2630 * @param <T> the type of the task's result
2631 * @return the task
2632 * @throws NullPointerException if the task is null
2633 * @throws RejectedExecutionException if the task cannot be
2634 * scheduled for execution
2635 */
2636 public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
2637 return externalSubmit(task);
2638 }
2639
2640 /**
2641 * @throws NullPointerException if the task is null
2642 * @throws RejectedExecutionException if the task cannot be
2643 * scheduled for execution
2644 */
2645 @Override
2646 public <T> ForkJoinTask<T> submit(Callable<T> task) {
2647 return externalSubmit(new ForkJoinTask.AdaptedCallable<T>(task));
2648 }
2649
2650 /**
2651 * @throws NullPointerException if the task is null
2652 * @throws RejectedExecutionException if the task cannot be
2653 * scheduled for execution
2654 */
2655 @Override
2656 public <T> ForkJoinTask<T> submit(Runnable task, T result) {
2657 return externalSubmit(new ForkJoinTask.AdaptedRunnable<T>(task, result));
2658 }
2659
2660 /**
2661 * @throws NullPointerException if the task is null
2662 * @throws RejectedExecutionException if the task cannot be
2663 * scheduled for execution
2664 */
2665 @Override
2666 @SuppressWarnings("unchecked")
2667 public ForkJoinTask<?> submit(Runnable task) {
2668 return externalSubmit((task instanceof ForkJoinTask<?>)
2669 ? (ForkJoinTask<Void>) task // avoid re-wrap
2670 : new ForkJoinTask.AdaptedRunnableAction(task));
2671 }
2672
2673 /**
2674 * @throws NullPointerException {@inheritDoc}
2675 * @throws RejectedExecutionException {@inheritDoc}
2676 */
2677 @Override
2678 public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
2679 ArrayList<Future<T>> futures = new ArrayList<>(tasks.size());
2680 try {
2681 for (Callable<T> t : tasks) {
2682 ForkJoinTask<T> f =
2683 new ForkJoinTask.AdaptedInterruptibleCallable<T>(t);
2684 futures.add(f);
2685 externalSubmit(f);
2686 }
2687 for (int i = futures.size() - 1; i >= 0; --i)
2688 ((ForkJoinTask<?>)futures.get(i)).quietlyJoin();
2689 return futures;
2690 } catch (Throwable t) {
2691 for (Future<T> e : futures)
2692 ForkJoinTask.cancelIgnoringExceptions(e);
2693 throw t;
2694 }
2695 }
2696
2697 @Override
2698 public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
2699 long timeout, TimeUnit unit)
2700 throws InterruptedException {
2701 long nanos = unit.toNanos(timeout);
2702 ArrayList<Future<T>> futures = new ArrayList<>(tasks.size());
2703 try {
2704 for (Callable<T> t : tasks) {
2705 ForkJoinTask<T> f =
2706 new ForkJoinTask.AdaptedInterruptibleCallable<T>(t);
2707 futures.add(f);
2708 externalSubmit(f);
2709 }
2710 long startTime = System.nanoTime(), ns = nanos;
2711 boolean timedOut = (ns < 0L);
2712 for (int i = futures.size() - 1; i >= 0; --i) {
2713 Future<T> f = futures.get(i);
2714 if (!f.isDone()) {
2715 if (timedOut)
2716 ForkJoinTask.cancelIgnoringExceptions(f);
2717 else {
2718 try {
2719 f.get(ns, TimeUnit.NANOSECONDS);
2720 } catch (CancellationException | TimeoutException |
2721 ExecutionException ok) {
2722 }
2723 if ((ns = nanos - (System.nanoTime() - startTime)) < 0L)
2724 timedOut = true;
2725 }
2726 }
2727 }
2728 return futures;
2729 } catch (Throwable t) {
2730 for (Future<T> e : futures)
2731 ForkJoinTask.cancelIgnoringExceptions(e);
2732 throw t;
2733 }
2734 }
2735
2736 // Task to hold results from InvokeAnyTasks
2737 static final class InvokeAnyRoot<E> extends ForkJoinTask<E> {
2738 private static final long serialVersionUID = 2838392045355241008L;
2739 @SuppressWarnings("serial") // Conditionally serializable
2740 volatile E result;
2741 final AtomicInteger count; // in case all throw
2742 final ForkJoinPool pool; // to check shutdown while collecting
2743 InvokeAnyRoot(int n, ForkJoinPool p) {
2744 pool = p;
2745 count = new AtomicInteger(n);
2746 }
2747 final void tryComplete(Callable<E> c) { // called by InvokeAnyTasks
2748 Throwable ex = null;
2749 boolean failed = (c == null || isCancelled() ||
2750 (pool != null && pool.mode < 0));
2751 if (!failed && !isDone()) {
2752 try {
2753 complete(c.call());
2754 } catch (Throwable tx) {
2755 ex = tx;
2756 failed = true;
2757 }
2758 }
2759 if ((pool != null && pool.mode < 0) ||
2760 (failed && count.getAndDecrement() <= 1))
2761 trySetThrown(ex != null ? ex : new CancellationException());
2762 }
2763 public final boolean exec() { return false; } // never forked
2764 public final E getRawResult() { return result; }
2765 public final void setRawResult(E v) { result = v; }
2766 }
2767
2768 // Variant of AdaptedInterruptibleCallable with results in InvokeAnyRoot
2769 static final class InvokeAnyTask<E> extends ForkJoinTask<E> {
2770 private static final long serialVersionUID = 2838392045355241008L;
2771 final InvokeAnyRoot<E> root;
2772 @SuppressWarnings("serial") // Conditionally serializable
2773 final Callable<E> callable;
2774 transient volatile Thread runner;
2775 InvokeAnyTask(InvokeAnyRoot<E> root, Callable<E> callable) {
2776 this.root = root;
2777 this.callable = callable;
2778 }
2779 public final boolean exec() {
2780 Thread.interrupted();
2781 runner = Thread.currentThread();
2782 root.tryComplete(callable);
2783 runner = null;
2784 Thread.interrupted();
2785 return true;
2786 }
2787 public final boolean cancel(boolean mayInterruptIfRunning) {
2788 Thread t;
2789 boolean stat = super.cancel(false);
2790 if (mayInterruptIfRunning && (t = runner) != null) {
2791 try {
2792 t.interrupt();
2793 } catch (Throwable ignore) {
2794 }
2795 }
2796 return stat;
2797 }
2798 public final void setRawResult(E v) {} // unused
2799 public final E getRawResult() { return null; }
2800 }
2801
2802 @Override
2803 public <T> T invokeAny(Collection<? extends Callable<T>> tasks)
2804 throws InterruptedException, ExecutionException {
2805 int n = tasks.size();
2806 if (n <= 0)
2807 throw new IllegalArgumentException();
2808 InvokeAnyRoot<T> root = new InvokeAnyRoot<T>(n, this);
2809 ArrayList<InvokeAnyTask<T>> fs = new ArrayList<>(n);
2810 try {
2811 for (Callable<T> c : tasks) {
2812 if (c == null)
2813 throw new NullPointerException();
2814 InvokeAnyTask<T> f = new InvokeAnyTask<T>(root, c);
2815 fs.add(f);
2816 externalSubmit(f);
2817 if (root.isDone())
2818 break;
2819 }
2820 return root.get();
2821 } finally {
2822 for (InvokeAnyTask<T> f : fs)
2823 ForkJoinTask.cancelIgnoringExceptions(f);
2824 }
2825 }
2826
2827 @Override
2828 public <T> T invokeAny(Collection<? extends Callable<T>> tasks,
2829 long timeout, TimeUnit unit)
2830 throws InterruptedException, ExecutionException, TimeoutException {
2831 long nanos = unit.toNanos(timeout);
2832 int n = tasks.size();
2833 if (n <= 0)
2834 throw new IllegalArgumentException();
2835 InvokeAnyRoot<T> root = new InvokeAnyRoot<T>(n, this);
2836 ArrayList<InvokeAnyTask<T>> fs = new ArrayList<>(n);
2837 try {
2838 for (Callable<T> c : tasks) {
2839 if (c == null)
2840 throw new NullPointerException();
2841 InvokeAnyTask<T> f = new InvokeAnyTask<T>(root, c);
2842 fs.add(f);
2843 externalSubmit(f);
2844 if (root.isDone())
2845 break;
2846 }
2847 return root.get(nanos, TimeUnit.NANOSECONDS);
2848 } finally {
2849 for (InvokeAnyTask<T> f : fs)
2850 ForkJoinTask.cancelIgnoringExceptions(f);
2851 }
2852 }
2853
2854 /**
2855 * Returns the factory used for constructing new workers.
2856 *
2857 * @return the factory used for constructing new workers
2858 */
2859 public ForkJoinWorkerThreadFactory getFactory() {
2860 return factory;
2861 }
2862
2863 /**
2864 * Returns the handler for internal worker threads that terminate
2865 * due to unrecoverable errors encountered while executing tasks.
2866 *
2867 * @return the handler, or {@code null} if none
2868 */
2869 public UncaughtExceptionHandler getUncaughtExceptionHandler() {
2870 return ueh;
2871 }
2872
2873 /**
2874 * Returns the targeted parallelism level of this pool.
2875 *
2876 * @return the targeted parallelism level of this pool
2877 */
2878 public int getParallelism() {
2879 int par = mode & SMASK;
2880 return (par > 0) ? par : 1;
2881 }
2882
2883 /**
2884 * Returns the targeted parallelism level of the common pool.
2885 *
2886 * @return the targeted parallelism level of the common pool
2887 * @since 1.8
2888 */
2889 public static int getCommonPoolParallelism() {
2890 return COMMON_PARALLELISM;
2891 }
2892
2893 /**
2894 * Returns the number of worker threads that have started but not
2895 * yet terminated. The result returned by this method may differ
2896 * from {@link #getParallelism} when threads are created to
2897 * maintain parallelism when others are cooperatively blocked.
2898 *
2899 * @return the number of worker threads
2900 */
2901 public int getPoolSize() {
2902 return ((mode & SMASK) + (short)(ctl >>> TC_SHIFT));
2903 }
2904
2905 /**
2906 * Returns {@code true} if this pool uses local first-in-first-out
2907 * scheduling mode for forked tasks that are never joined.
2908 *
2909 * @return {@code true} if this pool uses async mode
2910 */
2911 public boolean getAsyncMode() {
2912 return (mode & FIFO) != 0;
2913 }
2914
2915 /**
2916 * Returns an estimate of the number of worker threads that are
2917 * not blocked waiting to join tasks or for other managed
2918 * synchronization. This method may overestimate the
2919 * number of running threads.
2920 *
2921 * @return the number of worker threads
2922 */
2923 public int getRunningThreadCount() {
2924 VarHandle.acquireFence();
2925 WorkQueue[] qs; WorkQueue q;
2926 int rc = 0;
2927 if ((qs = queues) != null) {
2928 for (int i = 1; i < qs.length; i += 2) {
2929 if ((q = qs[i]) != null && q.isApparentlyUnblocked())
2930 ++rc;
2931 }
2932 }
2933 return rc;
2934 }
2935
2936 /**
2937 * Returns an estimate of the number of threads that are currently
2938 * stealing or executing tasks. This method may overestimate the
2939 * number of active threads.
2940 *
2941 * @return the number of active threads
2942 */
2943 public int getActiveThreadCount() {
2944 int r = (mode & SMASK) + (int)(ctl >> RC_SHIFT);
2945 return (r <= 0) ? 0 : r; // suppress momentarily negative values
2946 }
2947
2948 /**
2949 * Returns {@code true} if all worker threads are currently idle.
2950 * An idle worker is one that cannot obtain a task to execute
2951 * because none are available to steal from other threads, and
2952 * there are no pending submissions to the pool. This method is
2953 * conservative; it might not return {@code true} immediately upon
2954 * idleness of all threads, but will eventually become true if
2955 * threads remain inactive.
2956 *
2957 * @return {@code true} if all threads are currently idle
2958 */
2959 public boolean isQuiescent() {
2960 return canStop();
2961 }
2962
2963 /**
2964 * Returns an estimate of the total number of completed tasks that
2965 * were executed by a thread other than their submitter. The
2966 * reported value underestimates the actual total number of steals
2967 * when the pool is not quiescent. This value may be useful for
2968 * monitoring and tuning fork/join programs: in general, steal
2969 * counts should be high enough to keep threads busy, but low
2970 * enough to avoid overhead and contention across threads.
2971 *
2972 * @return the number of steals
2973 */
2974 public long getStealCount() {
2975 long count = stealCount;
2976 WorkQueue[] qs; WorkQueue q;
2977 if ((qs = queues) != null) {
2978 for (int i = 1; i < qs.length; i += 2) {
2979 if ((q = qs[i]) != null)
2980 count += (long)q.nsteals & 0xffffffffL;
2981 }
2982 }
2983 return count;
2984 }
2985
2986 /**
2987 * Returns an estimate of the total number of tasks currently held
2988 * in queues by worker threads (but not including tasks submitted
2989 * to the pool that have not begun executing). This value is only
2990 * an approximation, obtained by iterating across all threads in
2991 * the pool. This method may be useful for tuning task
2992 * granularities.
2993 *
2994 * @return the number of queued tasks
2995 */
2996 public long getQueuedTaskCount() {
2997 VarHandle.acquireFence();
2998 WorkQueue[] qs; WorkQueue q;
2999 int count = 0;
3000 if ((qs = queues) != null) {
3001 for (int i = 1; i < qs.length; i += 2) {
3002 if ((q = qs[i]) != null)
3003 count += q.queueSize();
3004 }
3005 }
3006 return count;
3007 }
3008
3009 /**
3010 * Returns an estimate of the number of tasks submitted to this
3011 * pool that have not yet begun executing. This method may take
3012 * time proportional to the number of submissions.
3013 *
3014 * @return the number of queued submissions
3015 */
3016 public int getQueuedSubmissionCount() {
3017 VarHandle.acquireFence();
3018 WorkQueue[] qs; WorkQueue q;
3019 int count = 0;
3020 if ((qs = queues) != null) {
3021 for (int i = 0; i < qs.length; i += 2) {
3022 if ((q = qs[i]) != null)
3023 count += q.queueSize();
3024 }
3025 }
3026 return count;
3027 }
3028
3029 /**
3030 * Returns {@code true} if there are any tasks submitted to this
3031 * pool that have not yet begun executing.
3032 *
3033 * @return {@code true} if there are any queued submissions
3034 */
3035 public boolean hasQueuedSubmissions() {
3036 VarHandle.acquireFence();
3037 WorkQueue[] qs; WorkQueue q;
3038 if ((qs = queues) != null) {
3039 for (int i = 0; i < qs.length; i += 2) {
3040 if ((q = qs[i]) != null && !q.isEmpty())
3041 return true;
3042 }
3043 }
3044 return false;
3045 }
3046
3047 /**
3048 * Removes and returns the next unexecuted submission if one is
3049 * available. This method may be useful in extensions to this
3050 * class that re-assign work in systems with multiple pools.
3051 *
3052 * @return the next submission, or {@code null} if none
3053 */
3054 protected ForkJoinTask<?> pollSubmission() {
3055 return pollScan(true);
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 for (ForkJoinTask<?> t; (t = pollScan(false)) != null; ) {
3078 c.add(t);
3079 ++count;
3080 }
3081 return count;
3082 }
3083
3084 /**
3085 * Returns a string identifying this pool, as well as its state,
3086 * including indications of run state, parallelism level, and
3087 * worker and task counts.
3088 *
3089 * @return a string identifying this pool, as well as its state
3090 */
3091 public String toString() {
3092 // Use a single pass through queues to collect counts
3093 int md = mode; // read volatile fields first
3094 long c = ctl;
3095 long st = stealCount;
3096 long qt = 0L, ss = 0L; int rc = 0;
3097 WorkQueue[] qs; WorkQueue q;
3098 if ((qs = queues) != null) {
3099 for (int i = 0; i < qs.length; ++i) {
3100 if ((q = qs[i]) != null) {
3101 int size = q.queueSize();
3102 if ((i & 1) == 0)
3103 ss += size;
3104 else {
3105 qt += size;
3106 st += (long)q.nsteals & 0xffffffffL;
3107 if (q.isApparentlyUnblocked())
3108 ++rc;
3109 }
3110 }
3111 }
3112 }
3113
3114 int pc = (md & SMASK);
3115 int tc = pc + (short)(c >>> TC_SHIFT);
3116 int ac = pc + (int)(c >> RC_SHIFT);
3117 if (ac < 0) // ignore transient negative
3118 ac = 0;
3119 String level = ((md & TERMINATED) != 0 ? "Terminated" :
3120 (md & STOP) != 0 ? "Terminating" :
3121 (md & SHUTDOWN) != 0 ? "Shutting down" :
3122 "Running");
3123 return super.toString() +
3124 "[" + level +
3125 ", parallelism = " + pc +
3126 ", size = " + tc +
3127 ", active = " + ac +
3128 ", running = " + rc +
3129 ", steals = " + st +
3130 ", tasks = " + qt +
3131 ", submissions = " + ss +
3132 "]";
3133 }
3134
3135 /**
3136 * Possibly initiates an orderly shutdown in which previously
3137 * submitted tasks are executed, but no new tasks will be
3138 * accepted. Invocation has no effect on execution state if this
3139 * is the {@link #commonPool()}, and no additional effect if
3140 * already shut down. Tasks that are in the process of being
3141 * submitted concurrently during the course of this method may or
3142 * may not be rejected.
3143 *
3144 * @throws SecurityException if a security manager exists and
3145 * the caller is not permitted to modify threads
3146 * because it does not hold {@link
3147 * java.lang.RuntimePermission}{@code ("modifyThread")}
3148 */
3149 public void shutdown() {
3150 checkPermission();
3151 if (this != common)
3152 tryTerminate(false, true);
3153 }
3154
3155 /**
3156 * Possibly attempts to cancel and/or stop all tasks, and reject
3157 * all subsequently submitted tasks. Invocation has no effect on
3158 * execution state if this is the {@link #commonPool()}, and no
3159 * additional effect if already shut down. Otherwise, tasks that
3160 * are in the process of being submitted or executed concurrently
3161 * during the course of this method may or may not be
3162 * rejected. This method cancels both existing and unexecuted
3163 * tasks, in order to permit termination in the presence of task
3164 * dependencies. So the method always returns an empty list
3165 * (unlike the case for some other Executors).
3166 *
3167 * @return an empty list
3168 * @throws SecurityException if a security manager exists and
3169 * the caller is not permitted to modify threads
3170 * because it does not hold {@link
3171 * java.lang.RuntimePermission}{@code ("modifyThread")}
3172 */
3173 public List<Runnable> shutdownNow() {
3174 checkPermission();
3175 if (this != common)
3176 tryTerminate(true, true);
3177 return Collections.emptyList();
3178 }
3179
3180 /**
3181 * Returns {@code true} if all tasks have completed following shut down.
3182 *
3183 * @return {@code true} if all tasks have completed following shut down
3184 */
3185 public boolean isTerminated() {
3186 return (mode & TERMINATED) != 0;
3187 }
3188
3189 /**
3190 * Returns {@code true} if the process of termination has
3191 * commenced but not yet completed. This method may be useful for
3192 * debugging. A return of {@code true} reported a sufficient
3193 * period after shutdown may indicate that submitted tasks have
3194 * ignored or suppressed interruption, or are waiting for I/O,
3195 * causing this executor not to properly terminate. (See the
3196 * advisory notes for class {@link ForkJoinTask} stating that
3197 * tasks should not normally entail blocking operations. But if
3198 * they do, they must abort them on interrupt.)
3199 *
3200 * @return {@code true} if terminating but not yet terminated
3201 */
3202 public boolean isTerminating() {
3203 return (mode & (STOP | TERMINATED)) == STOP;
3204 }
3205
3206 /**
3207 * Returns {@code true} if this pool has been shut down.
3208 *
3209 * @return {@code true} if this pool has been shut down
3210 */
3211 public boolean isShutdown() {
3212 return (mode & SHUTDOWN) != 0;
3213 }
3214
3215 /**
3216 * Blocks until all tasks have completed execution after a
3217 * shutdown request, or the timeout occurs, or the current thread
3218 * is interrupted, whichever happens first. Because the {@link
3219 * #commonPool()} never terminates until program shutdown, when
3220 * applied to the common pool, this method is equivalent to {@link
3221 * #awaitQuiescence(long, TimeUnit)} but always returns {@code false}.
3222 *
3223 * @param timeout the maximum time to wait
3224 * @param unit the time unit of the timeout argument
3225 * @return {@code true} if this executor terminated and
3226 * {@code false} if the timeout elapsed before termination
3227 * @throws InterruptedException if interrupted while waiting
3228 */
3229 public boolean awaitTermination(long timeout, TimeUnit unit)
3230 throws InterruptedException {
3231 ReentrantLock lock; Condition cond;
3232 long nanos = unit.toNanos(timeout);
3233 boolean terminated = false;
3234 if (this == common) {
3235 Thread t; ForkJoinWorkerThread wt; int q;
3236 if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread &&
3237 (wt = (ForkJoinWorkerThread)t).pool == this)
3238 q = helpQuiescePool(wt.workQueue, nanos, true);
3239 else
3240 q = externalHelpQuiescePool(nanos, true);
3241 if (q < 0)
3242 throw new InterruptedException();
3243 }
3244 else if (!(terminated = ((mode & TERMINATED) != 0)) &&
3245 (lock = registrationLock) != null) {
3246 lock.lock();
3247 try {
3248 if ((cond = termination) == null)
3249 termination = cond = lock.newCondition();
3250 while (!(terminated = ((mode & TERMINATED) != 0)) && nanos > 0L)
3251 nanos = cond.awaitNanos(nanos);
3252 } finally {
3253 lock.unlock();
3254 }
3255 }
3256 return terminated;
3257 }
3258
3259 /**
3260 * If called by a ForkJoinTask operating in this pool, equivalent
3261 * in effect to {@link ForkJoinTask#helpQuiesce}. Otherwise,
3262 * waits and/or attempts to assist performing tasks until this
3263 * pool {@link #isQuiescent} or the indicated timeout elapses.
3264 *
3265 * @param timeout the maximum time to wait
3266 * @param unit the time unit of the timeout argument
3267 * @return {@code true} if quiescent; {@code false} if the
3268 * timeout elapsed.
3269 */
3270 public boolean awaitQuiescence(long timeout, TimeUnit unit) {
3271 Thread t; ForkJoinWorkerThread wt; int q;
3272 long nanos = unit.toNanos(timeout);
3273 if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread &&
3274 (wt = (ForkJoinWorkerThread)t).pool == this)
3275 q = helpQuiescePool(wt.workQueue, nanos, false);
3276 else
3277 q = externalHelpQuiescePool(nanos, false);
3278 return (q > 0);
3279 }
3280
3281 /**
3282 * Interface for extending managed parallelism for tasks running
3283 * in {@link ForkJoinPool}s.
3284 *
3285 * <p>A {@code ManagedBlocker} provides two methods. Method
3286 * {@link #isReleasable} must return {@code true} if blocking is
3287 * not necessary. Method {@link #block} blocks the current thread
3288 * if necessary (perhaps internally invoking {@code isReleasable}
3289 * before actually blocking). These actions are performed by any
3290 * thread invoking {@link
3291 * ForkJoinPool#managedBlock(ManagedBlocker)}. The unusual
3292 * methods in this API accommodate synchronizers that may, but
3293 * don't usually, block for long periods. Similarly, they allow
3294 * more efficient internal handling of cases in which additional
3295 * workers may be, but usually are not, needed to ensure
3296 * sufficient parallelism. Toward this end, implementations of
3297 * method {@code isReleasable} must be amenable to repeated
3298 * invocation. Neither method is invoked after a prior invocation
3299 * of {@code isReleasable} or {@code block} returns {@code true}.
3300 *
3301 * <p>For example, here is a ManagedBlocker based on a
3302 * ReentrantLock:
3303 * <pre> {@code
3304 * class ManagedLocker implements ManagedBlocker {
3305 * final ReentrantLock lock;
3306 * boolean hasLock = false;
3307 * ManagedLocker(ReentrantLock lock) { this.lock = lock; }
3308 * public boolean block() {
3309 * if (!hasLock)
3310 * lock.lock();
3311 * return true;
3312 * }
3313 * public boolean isReleasable() {
3314 * return hasLock || (hasLock = lock.tryLock());
3315 * }
3316 * }}</pre>
3317 *
3318 * <p>Here is a class that possibly blocks waiting for an
3319 * item on a given queue:
3320 * <pre> {@code
3321 * class QueueTaker<E> implements ManagedBlocker {
3322 * final BlockingQueue<E> queue;
3323 * volatile E item = null;
3324 * QueueTaker(BlockingQueue<E> q) { this.queue = q; }
3325 * public boolean block() throws InterruptedException {
3326 * if (item == null)
3327 * item = queue.take();
3328 * return true;
3329 * }
3330 * public boolean isReleasable() {
3331 * return item != null || (item = queue.poll()) != null;
3332 * }
3333 * public E getItem() { // call after pool.managedBlock completes
3334 * return item;
3335 * }
3336 * }}</pre>
3337 */
3338 public static interface ManagedBlocker {
3339 /**
3340 * Possibly blocks the current thread, for example waiting for
3341 * a lock or condition.
3342 *
3343 * @return {@code true} if no additional blocking is necessary
3344 * (i.e., if isReleasable would return true)
3345 * @throws InterruptedException if interrupted while waiting
3346 * (the method is not required to do so, but is allowed to)
3347 */
3348 boolean block() throws InterruptedException;
3349
3350 /**
3351 * Returns {@code true} if blocking is unnecessary.
3352 * @return {@code true} if blocking is unnecessary
3353 */
3354 boolean isReleasable();
3355 }
3356
3357 /**
3358 * Runs the given possibly blocking task. When {@linkplain
3359 * ForkJoinTask#inForkJoinPool() running in a ForkJoinPool}, this
3360 * method possibly arranges for a spare thread to be activated if
3361 * necessary to ensure sufficient parallelism while the current
3362 * thread is blocked in {@link ManagedBlocker#block blocker.block()}.
3363 *
3364 * <p>This method repeatedly calls {@code blocker.isReleasable()} and
3365 * {@code blocker.block()} until either method returns {@code true}.
3366 * Every call to {@code blocker.block()} is preceded by a call to
3367 * {@code blocker.isReleasable()} that returned {@code false}.
3368 *
3369 * <p>If not running in a ForkJoinPool, this method is
3370 * behaviorally equivalent to
3371 * <pre> {@code
3372 * while (!blocker.isReleasable())
3373 * if (blocker.block())
3374 * break;}</pre>
3375 *
3376 * If running in a ForkJoinPool, the pool may first be expanded to
3377 * ensure sufficient parallelism available during the call to
3378 * {@code blocker.block()}.
3379 *
3380 * @param blocker the blocker task
3381 * @throws InterruptedException if {@code blocker.block()} did so
3382 */
3383 public static void managedBlock(ManagedBlocker blocker)
3384 throws InterruptedException {
3385 Thread t; ForkJoinPool p;
3386 if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread &&
3387 (p = ((ForkJoinWorkerThread)t).pool) != null)
3388 p.compensatedBlock(blocker);
3389 else
3390 unmanagedBlock(blocker);
3391 }
3392
3393 /** ManagedBlock for ForkJoinWorkerThreads */
3394 private void compensatedBlock(ManagedBlocker blocker)
3395 throws InterruptedException {
3396 if (blocker == null) throw new NullPointerException();
3397 for (;;) {
3398 int comp; boolean done;
3399 long c = ctl;
3400 if (blocker.isReleasable())
3401 break;
3402 if ((comp = tryCompensate(c)) >= 0) {
3403 long post = (comp == 0) ? 0L : RC_UNIT;
3404 try {
3405 done = blocker.block();
3406 } finally {
3407 getAndAddCtl(post);
3408 }
3409 if (done)
3410 break;
3411 }
3412 }
3413 }
3414
3415 /** ManagedBlock for external threads */
3416 private static void unmanagedBlock(ManagedBlocker blocker)
3417 throws InterruptedException {
3418 if (blocker == null) throw new NullPointerException();
3419 do {} while (!blocker.isReleasable() && !blocker.block());
3420 }
3421
3422 // AbstractExecutorService.newTaskFor overrides rely on
3423 // undocumented fact that ForkJoinTask.adapt returns ForkJoinTasks
3424 // that also implement RunnableFuture.
3425
3426 @Override
3427 protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
3428 return new ForkJoinTask.AdaptedRunnable<T>(runnable, value);
3429 }
3430
3431 @Override
3432 protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
3433 return new ForkJoinTask.AdaptedCallable<T>(callable);
3434 }
3435
3436 static {
3437 try {
3438 MethodHandles.Lookup l = MethodHandles.lookup();
3439 CTL = l.findVarHandle(ForkJoinPool.class, "ctl", long.class);
3440 MODE = l.findVarHandle(ForkJoinPool.class, "mode", int.class);
3441 THREADIDS = l.findVarHandle(ForkJoinPool.class, "threadIds", int.class);
3442 POOLIDS = l.findStaticVarHandle(ForkJoinPool.class, "poolIds", int.class);
3443 } catch (ReflectiveOperationException e) {
3444 throw new ExceptionInInitializerError(e);
3445 }
3446
3447 // Reduce the risk of rare disastrous classloading in first call to
3448 // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
3449 Class<?> ensureLoaded = LockSupport.class;
3450
3451 int commonMaxSpares = DEFAULT_COMMON_MAX_SPARES;
3452 try {
3453 String p = System.getProperty
3454 ("java.util.concurrent.ForkJoinPool.common.maximumSpares");
3455 if (p != null)
3456 commonMaxSpares = Integer.parseInt(p);
3457 } catch (Exception ignore) {}
3458 COMMON_MAX_SPARES = commonMaxSpares;
3459
3460 defaultForkJoinWorkerThreadFactory =
3461 new DefaultForkJoinWorkerThreadFactory();
3462 modifyThreadPermission = new RuntimePermission("modifyThread");
3463 common = AccessController.doPrivileged(new PrivilegedAction<>() {
3464 public ForkJoinPool run() {
3465 return new ForkJoinPool((byte)0); }});
3466
3467 COMMON_PARALLELISM = Math.max(common.mode & SMASK, 1);
3468 }
3469 }