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.locks.LockSupport;
53
54 /**
55 * An {@link ExecutorService} for running {@link ForkJoinTask}s.
56 * A {@code ForkJoinPool} provides the entry point for submissions
57 * from non-{@code ForkJoinTask} clients, as well as management and
58 * monitoring operations.
59 *
60 * <p>A {@code ForkJoinPool} differs from other kinds of {@link
61 * ExecutorService} mainly by virtue of employing
62 * <em>work-stealing</em>: all threads in the pool attempt to find and
63 * execute tasks submitted to the pool and/or created by other active
64 * tasks (eventually blocking waiting for work if none exist). This
65 * enables efficient processing when most tasks spawn other subtasks
66 * (as do most {@code ForkJoinTask}s), as well as when many small
67 * tasks are submitted to the pool from external clients. Especially
68 * when setting <em>asyncMode</em> to true in constructors, {@code
69 * ForkJoinPool}s may also be appropriate for use with event-style
70 * tasks that are never joined. All worker threads are initialized
71 * with {@link Thread#isDaemon} set {@code true}.
72 *
73 * <p>A static {@link #commonPool()} is available and appropriate for
74 * most applications. The common pool is used by any ForkJoinTask that
75 * is not explicitly submitted to a specified pool. Using the common
76 * pool normally reduces resource usage (its threads are slowly
77 * reclaimed during periods of non-use, and reinstated upon subsequent
78 * use).
79 *
80 * <p>For applications that require separate or custom pools, a {@code
81 * ForkJoinPool} may be constructed with a given target parallelism
82 * level; by default, equal to the number of available processors.
83 * The pool attempts to maintain enough active (or available) threads
84 * by dynamically adding, suspending, or resuming internal worker
85 * threads, even if some tasks are stalled waiting to join others.
86 * However, no such adjustments are guaranteed in the face of blocked
87 * I/O or other unmanaged synchronization. The nested {@link
88 * ManagedBlocker} interface enables extension of the kinds of
89 * synchronization accommodated. The default policies may be
90 * overridden using a constructor with parameters corresponding to
91 * those documented in class {@link ThreadPoolExecutor}.
92 *
93 * <p>In addition to execution and lifecycle control methods, this
94 * class provides status check methods (for example
95 * {@link #getStealCount}) that are intended to aid in developing,
96 * tuning, and monitoring fork/join applications. Also, method
97 * {@link #toString} returns indications of pool state in a
98 * convenient form for informal monitoring.
99 *
100 * <p>As is the case with other ExecutorServices, there are three
101 * main task execution methods summarized in the following table.
102 * These are designed to be used primarily by clients not already
103 * engaged in fork/join computations in the current pool. The main
104 * forms of these methods accept instances of {@code ForkJoinTask},
105 * but overloaded forms also allow mixed execution of plain {@code
106 * Runnable}- or {@code Callable}- based activities as well. However,
107 * tasks that are already executing in a pool should normally instead
108 * use the within-computation forms listed in the table unless using
109 * async event-style tasks that are not usually joined, in which case
110 * there is little difference among choice of methods.
111 *
112 * <table class="plain">
113 * <caption>Summary of task execution methods</caption>
114 * <tr>
115 * <td></td>
116 * <td style="text-align:center"> <b>Call from non-fork/join clients</b></td>
117 * <td style="text-align:center"> <b>Call from within fork/join computations</b></td>
118 * </tr>
119 * <tr>
120 * <td> <b>Arrange async execution</b></td>
121 * <td> {@link #execute(ForkJoinTask)}</td>
122 * <td> {@link ForkJoinTask#fork}</td>
123 * </tr>
124 * <tr>
125 * <td> <b>Await and obtain result</b></td>
126 * <td> {@link #invoke(ForkJoinTask)}</td>
127 * <td> {@link ForkJoinTask#invoke}</td>
128 * </tr>
129 * <tr>
130 * <td> <b>Arrange exec and obtain Future</b></td>
131 * <td> {@link #submit(ForkJoinTask)}</td>
132 * <td> {@link ForkJoinTask#fork} (ForkJoinTasks <em>are</em> Futures)</td>
133 * </tr>
134 * </table>
135 *
136 * <p>The parameters used to construct the common pool may be controlled by
137 * setting the following {@linkplain System#getProperty system properties}:
138 * <ul>
139 * <li>{@code java.util.concurrent.ForkJoinPool.common.parallelism}
140 * - the parallelism level, a non-negative integer
141 * <li>{@code java.util.concurrent.ForkJoinPool.common.threadFactory}
142 * - the class name of a {@link ForkJoinWorkerThreadFactory}.
143 * The {@linkplain ClassLoader#getSystemClassLoader() system class loader}
144 * is used to load this class.
145 * <li>{@code java.util.concurrent.ForkJoinPool.common.exceptionHandler}
146 * - the class name of a {@link UncaughtExceptionHandler}.
147 * The {@linkplain ClassLoader#getSystemClassLoader() system class loader}
148 * is used to load this class.
149 * <li>{@code java.util.concurrent.ForkJoinPool.common.maximumSpares}
150 * - the maximum number of allowed extra threads to maintain target
151 * parallelism (default 256).
152 * </ul>
153 * If no thread factory is supplied via a system property, then the
154 * common pool uses a factory that uses the system class loader as the
155 * {@linkplain Thread#getContextClassLoader() thread context class loader}.
156 * In addition, if a {@link SecurityManager} is present, then
157 * the common pool uses a factory supplying threads that have no
158 * {@link Permissions} enabled.
159 *
160 * Upon any error in establishing these settings, default parameters
161 * are used. It is possible to disable or limit the use of threads in
162 * the common pool by setting the parallelism property to zero, and/or
163 * using a factory that may return {@code null}. However doing so may
164 * cause unjoined tasks to never be executed.
165 *
166 * <p><b>Implementation notes</b>: This implementation restricts the
167 * maximum number of running threads to 32767. Attempts to create
168 * pools with greater than the maximum number result in
169 * {@code IllegalArgumentException}.
170 *
171 * <p>This implementation rejects submitted tasks (that is, by throwing
172 * {@link RejectedExecutionException}) only when the pool is shut down
173 * or internal resources have been exhausted.
174 *
175 * @since 1.7
176 * @author Doug Lea
177 */
178 public class ForkJoinPool extends AbstractExecutorService {
179
180 /*
181 * Implementation Overview
182 *
183 * This class and its nested classes provide the main
184 * functionality and control for a set of worker threads:
185 * Submissions from non-FJ threads enter into submission queues.
186 * Workers take these tasks and typically split them into subtasks
187 * that may be stolen by other workers. Preference rules give
188 * first priority to processing tasks from their own queues (LIFO
189 * or FIFO, depending on mode), then to randomized FIFO steals of
190 * tasks in other queues. This framework began as vehicle for
191 * supporting tree-structured parallelism using work-stealing.
192 * Over time, its scalability advantages led to extensions and
193 * changes to better support more diverse usage contexts. Because
194 * most internal methods and nested classes are interrelated,
195 * their main rationale and descriptions are presented here;
196 * individual methods and nested classes contain only brief
197 * comments about details.
198 *
199 * WorkQueues
200 * ==========
201 *
202 * Most operations occur within work-stealing queues (in nested
203 * class WorkQueue). These are special forms of Deques that
204 * support only three of the four possible end-operations -- push,
205 * pop, and poll (aka steal), under the further constraints that
206 * push and pop are called only from the owning thread (or, as
207 * extended here, under a lock), while poll may be called from
208 * other threads. (If you are unfamiliar with them, you probably
209 * want to read Herlihy and Shavit's book "The Art of
210 * Multiprocessor programming", chapter 16 describing these in
211 * more detail before proceeding.) The main work-stealing queue
212 * design is roughly similar to those in the papers "Dynamic
213 * Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005
214 * (http://research.sun.com/scalable/pubs/index.html) and
215 * "Idempotent work stealing" by Michael, Saraswat, and Vechev,
216 * PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186).
217 * The main differences ultimately stem from GC requirements that
218 * we null out taken slots as soon as we can, to maintain as small
219 * a footprint as possible even in programs generating huge
220 * numbers of tasks. To accomplish this, we shift the CAS
221 * arbitrating pop vs poll (steal) from being on the indices
222 * ("base" and "top") to the slots themselves.
223 *
224 * Adding tasks then takes the form of a classic array push(task)
225 * in a circular buffer:
226 * q.array[q.top++ % length] = task;
227 *
228 * (The actual code needs to null-check and size-check the array,
229 * uses masking, not mod, for indexing a power-of-two-sized array,
230 * properly fences accesses, and possibly signals waiting workers
231 * to start scanning -- see below.) Both a successful pop and
232 * poll mainly entail a CAS of a slot from non-null to null.
233 *
234 * The pop operation (always performed by owner) is:
235 * if ((the task at top slot is not null) and
236 * (CAS slot to null))
237 * decrement top and return task;
238 *
239 * And the poll operation (usually by a stealer) is
240 * if ((the task at base slot is not null) and
241 * (CAS slot to null))
242 * increment base and return task;
243 *
244 * There are several variants of each of these. In particular,
245 * almost all uses of poll occur within scan operations that also
246 * interleave contention tracking (with associated code sprawl.)
247 *
248 * Memory ordering. See "Correct and Efficient Work-Stealing for
249 * Weak Memory Models" by Le, Pop, Cohen, and Nardelli, PPoPP 2013
250 * (http://www.di.ens.fr/~zappa/readings/ppopp13.pdf) for an
251 * analysis of memory ordering requirements in work-stealing
252 * algorithms similar to (but different than) the one used here.
253 * Extracting tasks in array slots via (fully fenced) CAS provides
254 * primary synchronization. The base and top indices imprecisely
255 * guide where to extract from. We do not always require strict
256 * orderings of array and index updates, so sometimes let them be
257 * subject to compiler and processor reorderings. However, the
258 * volatile "base" index also serves as a basis for memory
259 * ordering: Slot accesses are preceded by a read of base,
260 * ensuring happens-before ordering with respect to stealers (so
261 * the slots themselves can be read via plain array reads.) The
262 * only other memory orderings relied on are maintained in the
263 * course of signalling and activation (see below). A check that
264 * base == top indicates (momentary) emptiness, but otherwise may
265 * err on the side of possibly making the queue appear nonempty
266 * when a push, pop, or poll have not fully committed, or making
267 * it appear empty when an update of top has not yet been visibly
268 * written. (Method isEmpty() checks the case of a partially
269 * completed removal of the last element.) Because of this, the
270 * poll operation, considered individually, is not wait-free. One
271 * thief cannot successfully continue until another in-progress
272 * one (or, if previously empty, a push) visibly completes.
273 * However, in the aggregate, we ensure at least probabilistic
274 * non-blockingness. If an attempted steal fails, a scanning
275 * thief chooses a different random victim target to try next. So,
276 * in order for one thief to progress, it suffices for any
277 * in-progress poll or new push on any empty queue to
278 * complete.
279 *
280 * This approach also enables support of a user mode in which
281 * local task processing is in FIFO, not LIFO order, simply by
282 * using poll rather than pop. This can be useful in
283 * message-passing frameworks in which tasks are never joined.
284 *
285 * WorkQueues are also used in a similar way for tasks submitted
286 * to the pool. We cannot mix these tasks in the same queues used
287 * by workers. Instead, we randomly associate submission queues
288 * with submitting threads, using a form of hashing. The
289 * ThreadLocalRandom probe value serves as a hash code for
290 * choosing existing queues, and may be randomly repositioned upon
291 * contention with other submitters. In essence, submitters act
292 * like workers except that they are restricted to executing local
293 * tasks that they submitted. Insertion of tasks in shared mode
294 * requires a lock but we use only a simple spinlock (using field
295 * phase), because submitters encountering a busy queue move to a
296 * different position to use or create other queues -- they block
297 * only when creating and registering new queues. Because it is
298 * used only as a spinlock, unlocking requires only a "releasing"
299 * store (using setRelease).
300 *
301 * Management
302 * ==========
303 *
304 * The main throughput advantages of work-stealing stem from
305 * decentralized control -- workers mostly take tasks from
306 * themselves or each other, at rates that can exceed a billion
307 * per second. The pool itself creates, activates (enables
308 * scanning for and running tasks), deactivates, blocks, and
309 * terminates threads, all with minimal central information.
310 * There are only a few properties that we can globally track or
311 * maintain, so we pack them into a small number of variables,
312 * often maintaining atomicity without blocking or locking.
313 * Nearly all essentially atomic control state is held in a few
314 * volatile variables that are by far most often read (not
315 * written) as status and consistency checks. We pack as much
316 * information into them as we can.
317 *
318 * Field "ctl" contains 64 bits holding information needed to
319 * atomically decide to add, enqueue (on an event queue), and
320 * dequeue (and release)-activate workers. To enable this
321 * packing, we restrict maximum parallelism to (1<<15)-1 (which is
322 * far in excess of normal operating range) to allow ids, counts,
323 * and their negations (used for thresholding) to fit into 16bit
324 * subfields.
325 *
326 * Field "mode" holds configuration parameters as well as lifetime
327 * status, atomically and monotonically setting SHUTDOWN, STOP,
328 * and finally TERMINATED bits.
329 *
330 * Field "workQueues" holds references to WorkQueues. It is
331 * updated (only during worker creation and termination) under
332 * lock (using field workerNamePrefix as lock), but is otherwise
333 * concurrently readable, and accessed directly. We also ensure
334 * that uses of the array reference itself never become too stale
335 * in case of resizing. To simplify index-based operations, the
336 * array size is always a power of two, and all readers must
337 * tolerate null slots. Worker queues are at odd indices. Shared
338 * (submission) queues are at even indices, up to a maximum of 64
339 * slots, to limit growth even if array needs to expand to add
340 * more workers. Grouping them together in this way simplifies and
341 * speeds up task scanning.
342 *
343 * All worker thread creation is on-demand, triggered by task
344 * submissions, replacement of terminated workers, and/or
345 * compensation for blocked workers. However, all other support
346 * code is set up to work with other policies. To ensure that we
347 * do not hold on to worker references that would prevent GC, all
348 * accesses to workQueues are via indices into the workQueues
349 * array (which is one source of some of the messy code
350 * constructions here). In essence, the workQueues array serves as
351 * a weak reference mechanism. Thus for example the stack top
352 * subfield of ctl stores indices, not references.
353 *
354 * Queuing Idle Workers. Unlike HPC work-stealing frameworks, we
355 * cannot let workers spin indefinitely scanning for tasks when
356 * none can be found immediately, and we cannot start/resume
357 * workers unless there appear to be tasks available. On the
358 * other hand, we must quickly prod them into action when new
359 * tasks are submitted or generated. In many usages, ramp-up time
360 * is the main limiting factor in overall performance, which is
361 * compounded at program start-up by JIT compilation and
362 * allocation. So we streamline this as much as possible.
363 *
364 * The "ctl" field atomically maintains total worker and
365 * "released" worker counts, plus the head of the available worker
366 * queue (actually stack, represented by the lower 32bit subfield
367 * of ctl). Released workers are those known to be scanning for
368 * and/or running tasks. Unreleased ("available") workers are
369 * recorded in the ctl stack. These workers are made available for
370 * signalling by enqueuing in ctl (see method runWorker). The
371 * "queue" is a form of Treiber stack. This is ideal for
372 * activating threads in most-recently used order, and improves
373 * performance and locality, outweighing the disadvantages of
374 * being prone to contention and inability to release a worker
375 * unless it is topmost on stack. To avoid missed signal problems
376 * inherent in any wait/signal design, available workers rescan
377 * for (and if found run) tasks after enqueuing. Normally their
378 * release status will be updated while doing so, but the released
379 * worker ctl count may underestimate the number of active
380 * threads. (However, it is still possible to determine quiescence
381 * via a validation traversal -- see isQuiescent). After an
382 * unsuccessful rescan, available workers are blocked until
383 * signalled (see signalWork). The top stack state holds the
384 * value of the "phase" field of the worker: its index and status,
385 * plus a version counter that, in addition to the count subfields
386 * (also serving as version stamps) provide protection against
387 * Treiber stack ABA effects.
388 *
389 * Creating workers. To create a worker, we pre-increment counts
390 * (serving as a reservation), and attempt to construct a
391 * ForkJoinWorkerThread via its factory. Upon construction, the
392 * new thread invokes registerWorker, where it constructs a
393 * WorkQueue and is assigned an index in the workQueues array
394 * (expanding the array if necessary). The thread is then started.
395 * Upon any exception across these steps, or null return from
396 * factory, deregisterWorker adjusts counts and records
397 * accordingly. If a null return, the pool continues running with
398 * fewer than the target number workers. If exceptional, the
399 * exception is propagated, generally to some external caller.
400 * Worker index assignment avoids the bias in scanning that would
401 * occur if entries were sequentially packed starting at the front
402 * of the workQueues array. We treat the array as a simple
403 * power-of-two hash table, expanding as needed. The seedIndex
404 * increment ensures no collisions until a resize is needed or a
405 * worker is deregistered and replaced, and thereafter keeps
406 * probability of collision low. We cannot use
407 * ThreadLocalRandom.getProbe() for similar purposes here because
408 * the thread has not started yet, but do so for creating
409 * submission queues for existing external threads (see
410 * externalPush).
411 *
412 * WorkQueue field "phase" is used by both workers and the pool to
413 * manage and track whether a worker is UNSIGNALLED (possibly
414 * blocked waiting for a signal). When a worker is enqueued its
415 * phase field is set. Note that phase field updates lag queue CAS
416 * releases so usage requires care -- seeing a negative phase does
417 * not guarantee that the worker is available. When queued, the
418 * lower 16 bits of scanState must hold its pool index. So we
419 * place the index there upon initialization (see registerWorker)
420 * and otherwise keep it there or restore it when necessary.
421 *
422 * The ctl field also serves as the basis for memory
423 * synchronization surrounding activation. This uses a more
424 * efficient version of a Dekker-like rule that task producers and
425 * consumers sync with each other by both writing/CASing ctl (even
426 * if to its current value). This would be extremely costly. So
427 * we relax it in several ways: (1) Producers only signal when
428 * their queue is empty. Other workers propagate this signal (in
429 * method scan) when they find tasks; to further reduce flailing,
430 * each worker signals only one other per activation. (2) Workers
431 * only enqueue after scanning (see below) and not finding any
432 * tasks. (3) Rather than CASing ctl to its current value in the
433 * common case where no action is required, we reduce write
434 * contention by equivalently prefacing signalWork when called by
435 * an external task producer using a memory access with
436 * full-volatile semantics or a "fullFence".
437 *
438 * Almost always, too many signals are issued. A task producer
439 * cannot in general tell if some existing worker is in the midst
440 * of finishing one task (or already scanning) and ready to take
441 * another without being signalled. So the producer might instead
442 * activate a different worker that does not find any work, and
443 * then inactivates. This scarcely matters in steady-state
444 * computations involving all workers, but can create contention
445 * and bookkeeping bottlenecks during ramp-up, ramp-down, and small
446 * computations involving only a few workers.
447 *
448 * Scanning. Method runWorker performs top-level scanning for
449 * tasks. Each scan traverses and tries to poll from each queue
450 * starting at a random index and circularly stepping. Scans are
451 * not performed in ideal random permutation order, to reduce
452 * cacheline contention. The pseudorandom generator need not have
453 * high-quality statistical properties in the long term, but just
454 * within computations; We use Marsaglia XorShifts (often via
455 * ThreadLocalRandom.nextSecondarySeed), which are cheap and
456 * suffice. Scanning also employs contention reduction: When
457 * scanning workers fail to extract an apparently existing task,
458 * they soon restart at a different pseudorandom index. This
459 * improves throughput when many threads are trying to take tasks
460 * from few queues, which can be common in some usages. Scans do
461 * not otherwise explicitly take into account core affinities,
462 * loads, cache localities, etc, However, they do exploit temporal
463 * locality (which usually approximates these) by preferring to
464 * re-poll (at most #workers times) from the same queue after a
465 * successful poll before trying others.
466 *
467 * Trimming workers. To release resources after periods of lack of
468 * use, a worker starting to wait when the pool is quiescent will
469 * time out and terminate (see method scan) if the pool has
470 * remained quiescent for period given by field keepAlive.
471 *
472 * Shutdown and Termination. A call to shutdownNow invokes
473 * tryTerminate to atomically set a runState bit. The calling
474 * thread, as well as every other worker thereafter terminating,
475 * helps terminate others by cancelling their unprocessed tasks,
476 * and waking them up, doing so repeatedly until stable. Calls to
477 * non-abrupt shutdown() preface this by checking whether
478 * termination should commence by sweeping through queues (until
479 * stable) to ensure lack of in-flight submissions and workers
480 * about to process them before triggering the "STOP" phase of
481 * termination.
482 *
483 * Joining Tasks
484 * =============
485 *
486 * Any of several actions may be taken when one worker is waiting
487 * to join a task stolen (or always held) by another. Because we
488 * are multiplexing many tasks on to a pool of workers, we can't
489 * always just let them block (as in Thread.join). We also cannot
490 * just reassign the joiner's run-time stack with another and
491 * replace it later, which would be a form of "continuation", that
492 * even if possible is not necessarily a good idea since we may
493 * need both an unblocked task and its continuation to progress.
494 * Instead we combine two tactics:
495 *
496 * Helping: Arranging for the joiner to execute some task that it
497 * would be running if the steal had not occurred.
498 *
499 * Compensating: Unless there are already enough live threads,
500 * method tryCompensate() may create or re-activate a spare
501 * thread to compensate for blocked joiners until they unblock.
502 *
503 * A third form (implemented in tryRemoveAndExec) amounts to
504 * helping a hypothetical compensator: If we can readily tell that
505 * a possible action of a compensator is to steal and execute the
506 * task being joined, the joining thread can do so directly,
507 * without the need for a compensation thread.
508 *
509 * The ManagedBlocker extension API can't use helping so relies
510 * only on compensation in method awaitBlocker.
511 *
512 * The algorithm in awaitJoin entails a form of "linear helping".
513 * Each worker records (in field source) the id of the queue from
514 * which it last stole a task. The scan in method awaitJoin uses
515 * these markers to try to find a worker to help (i.e., steal back
516 * a task from and execute it) that could hasten completion of the
517 * actively joined task. Thus, the joiner executes a task that
518 * would be on its own local deque if the to-be-joined task had
519 * not been stolen. This is a conservative variant of the approach
520 * described in Wagner & Calder "Leapfrogging: a portable
521 * technique for implementing efficient futures" SIGPLAN Notices,
522 * 1993 (http://portal.acm.org/citation.cfm?id=155354). It differs
523 * mainly in that we only record queue ids, not full dependency
524 * links. This requires a linear scan of the workQueues array to
525 * locate stealers, but isolates cost to when it is needed, rather
526 * than adding to per-task overhead. Searches can fail to locate
527 * stealers GC stalls and the like delay recording sources.
528 * Further, even when accurately identified, stealers might not
529 * ever produce a task that the joiner can in turn help with. So,
530 * compensation is tried upon failure to find tasks to run.
531 *
532 * Compensation does not by default aim to keep exactly the target
533 * parallelism number of unblocked threads running at any given
534 * time. Some previous versions of this class employed immediate
535 * compensations for any blocked join. However, in practice, the
536 * vast majority of blockages are transient byproducts of GC and
537 * other JVM or OS activities that are made worse by replacement.
538 * Rather than impose arbitrary policies, we allow users to
539 * override the default of only adding threads upon apparent
540 * starvation. The compensation mechanism may also be bounded.
541 * Bounds for the commonPool (see COMMON_MAX_SPARES) better enable
542 * JVMs to cope with programming errors and abuse before running
543 * out of resources to do so.
544 *
545 * Common Pool
546 * ===========
547 *
548 * The static common pool always exists after static
549 * initialization. Since it (or any other created pool) need
550 * never be used, we minimize initial construction overhead and
551 * footprint to the setup of about a dozen fields.
552 *
553 * When external threads submit to the common pool, they can
554 * perform subtask processing (see externalHelpComplete and
555 * related methods) upon joins. This caller-helps policy makes it
556 * sensible to set common pool parallelism level to one (or more)
557 * less than the total number of available cores, or even zero for
558 * pure caller-runs. We do not need to record whether external
559 * submissions are to the common pool -- if not, external help
560 * methods return quickly. These submitters would otherwise be
561 * blocked waiting for completion, so the extra effort (with
562 * liberally sprinkled task status checks) in inapplicable cases
563 * amounts to an odd form of limited spin-wait before blocking in
564 * ForkJoinTask.join.
565 *
566 * As a more appropriate default in managed environments, unless
567 * overridden by system properties, we use workers of subclass
568 * InnocuousForkJoinWorkerThread when there is a SecurityManager
569 * present. These workers have no permissions set, do not belong
570 * to any user-defined ThreadGroup, and erase all ThreadLocals
571 * after executing any top-level task (see
572 * WorkQueue.afterTopLevelExec). The associated mechanics (mainly
573 * in ForkJoinWorkerThread) may be JVM-dependent and must access
574 * particular Thread class fields to achieve this effect.
575 *
576 * Style notes
577 * ===========
578 *
579 * Memory ordering relies mainly on VarHandles. This can be
580 * awkward and ugly, but also reflects the need to control
581 * outcomes across the unusual cases that arise in very racy code
582 * with very few invariants. All fields are read into locals
583 * before use, and null-checked if they are references. This is
584 * usually done in a "C"-like style of listing declarations at the
585 * heads of methods or blocks, and using inline assignments on
586 * first encounter. Nearly all explicit checks lead to
587 * bypass/return, not exception throws, because they may
588 * legitimately arise due to cancellation/revocation during
589 * shutdown.
590 *
591 * There is a lot of representation-level coupling among classes
592 * ForkJoinPool, ForkJoinWorkerThread, and ForkJoinTask. The
593 * fields of WorkQueue maintain data structures managed by
594 * ForkJoinPool, so are directly accessed. There is little point
595 * trying to reduce this, since any associated future changes in
596 * representations will need to be accompanied by algorithmic
597 * changes anyway. Several methods intrinsically sprawl because
598 * they must accumulate sets of consistent reads of fields held in
599 * local variables. There are also other coding oddities
600 * (including several unnecessary-looking hoisted null checks)
601 * that help some methods perform reasonably even when interpreted
602 * (not compiled).
603 *
604 * The order of declarations in this file is (with a few exceptions):
605 * (1) Static utility functions
606 * (2) Nested (static) classes
607 * (3) Static fields
608 * (4) Fields, along with constants used when unpacking some of them
609 * (5) Internal control methods
610 * (6) Callbacks and other support for ForkJoinTask methods
611 * (7) Exported methods
612 * (8) Static block initializing statics in minimally dependent order
613 */
614
615 // Static utilities
616
617 /**
618 * If there is a security manager, makes sure caller has
619 * permission to modify threads.
620 */
621 private static void checkPermission() {
622 SecurityManager security = System.getSecurityManager();
623 if (security != null)
624 security.checkPermission(modifyThreadPermission);
625 }
626
627 // Nested classes
628
629 /**
630 * Factory for creating new {@link ForkJoinWorkerThread}s.
631 * A {@code ForkJoinWorkerThreadFactory} must be defined and used
632 * for {@code ForkJoinWorkerThread} subclasses that extend base
633 * functionality or initialize threads with different contexts.
634 */
635 public static interface ForkJoinWorkerThreadFactory {
636 /**
637 * Returns a new worker thread operating in the given pool.
638 * Returning null or throwing an exception may result in tasks
639 * never being executed. If this method throws an exception,
640 * it is relayed to the caller of the method (for example
641 * {@code execute}) causing attempted thread creation. If this
642 * method returns null or throws an exception, it is not
643 * retried until the next attempted creation (for example
644 * another call to {@code execute}).
645 *
646 * @param pool the pool this thread works in
647 * @return the new worker thread, or {@code null} if the request
648 * to create a thread is rejected
649 * @throws NullPointerException if the pool is null
650 */
651 public ForkJoinWorkerThread newThread(ForkJoinPool pool);
652 }
653
654 static AccessControlContext contextWithPermissions(Permission ... perms) {
655 Permissions permissions = new Permissions();
656 for (Permission perm : perms)
657 permissions.add(perm);
658 return new AccessControlContext(
659 new ProtectionDomain[] { new ProtectionDomain(null, permissions) });
660 }
661
662 /**
663 * Default ForkJoinWorkerThreadFactory implementation; creates a
664 * new ForkJoinWorkerThread using the system class loader as the
665 * thread context class loader.
666 */
667 private static final class DefaultForkJoinWorkerThreadFactory
668 implements ForkJoinWorkerThreadFactory {
669 private static final AccessControlContext ACC = contextWithPermissions(
670 new RuntimePermission("getClassLoader"),
671 new RuntimePermission("setContextClassLoader"));
672
673 public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
674 return AccessController.doPrivileged(
675 new PrivilegedAction<>() {
676 public ForkJoinWorkerThread run() {
677 return new ForkJoinWorkerThread(
678 pool, ClassLoader.getSystemClassLoader()); }},
679 ACC);
680 }
681 }
682
683 // Constants shared across ForkJoinPool and WorkQueue
684
685 // Bounds
686 static final int SWIDTH = 16; // width of short
687 static final int SMASK = 0xffff; // short bits == max index
688 static final int MAX_CAP = 0x7fff; // max #workers - 1
689 static final int SQMASK = 0x007e; // max 64 (even) slots
690
691 // Masks and units for WorkQueue.phase and ctl sp subfield
692 static final int UNSIGNALLED = 1 << 31; // must be negative
693 static final int SS_SEQ = 1 << 16; // version count
694 static final int QLOCK = 1; // must be 1
695
696 // Mode bits and sentinels, some also used in WorkQueue id and.source fields
697 static final int OWNED = 1; // queue has owner thread
698 static final int FIFO = 1 << 16; // fifo queue or access mode
699 static final int SHUTDOWN = 1 << 18;
700 static final int TERMINATED = 1 << 19;
701 static final int STOP = 1 << 31; // must be negative
702 static final int QUIET = 1 << 30; // not scanning or working
703 static final int DORMANT = QUIET | UNSIGNALLED;
704
705 /**
706 * The maximum number of local polls from the same queue before
707 * checking others. This is a safeguard against infinitely unfair
708 * looping under unbounded user task recursion, and must be larger
709 * than plausible cases of intentional bounded task recursion.
710 */
711 static final int POLL_LIMIT = 1 << 10;
712
713 /**
714 * Queues supporting work-stealing as well as external task
715 * submission. See above for descriptions and algorithms.
716 * Performance on most platforms is very sensitive to placement of
717 * instances of both WorkQueues and their arrays -- we absolutely
718 * do not want multiple WorkQueue instances or multiple queue
719 * arrays sharing cache lines. The @Contended annotation alerts
720 * JVMs to try to keep instances apart.
721 */
722 @jdk.internal.vm.annotation.Contended
723 static final class WorkQueue {
724
725 /**
726 * Capacity of work-stealing queue array upon initialization.
727 * Must be a power of two; at least 4, but should be larger to
728 * reduce or eliminate cacheline sharing among queues.
729 * Currently, it is much larger, as a partial workaround for
730 * the fact that JVMs often place arrays in locations that
731 * share GC bookkeeping (especially cardmarks) such that
732 * per-write accesses encounter serious memory contention.
733 */
734 static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
735
736 /**
737 * Maximum size for queue arrays. Must be a power of two less
738 * than or equal to 1 << (31 - width of array entry) to ensure
739 * lack of wraparound of index calculations, but defined to a
740 * value a bit less than this to help users trap runaway
741 * programs before saturating systems.
742 */
743 static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M
744
745 // Instance fields
746 volatile int phase; // versioned, negative: queued, 1: locked
747 int stackPred; // pool stack (ctl) predecessor link
748 int nsteals; // number of steals
749 int id; // index, mode, tag
750 volatile int source; // source queue id, or sentinel
751 volatile int base; // index of next slot for poll
752 int top; // index of next slot for push
753 ForkJoinTask<?>[] array; // the elements (initially unallocated)
754 final ForkJoinPool pool; // the containing pool (may be null)
755 final ForkJoinWorkerThread owner; // owning thread or null if shared
756
757 WorkQueue(ForkJoinPool pool, ForkJoinWorkerThread owner) {
758 this.pool = pool;
759 this.owner = owner;
760 // Place indices in the center of array (that is not yet allocated)
761 base = top = INITIAL_QUEUE_CAPACITY >>> 1;
762 }
763
764 /**
765 * Returns an exportable index (used by ForkJoinWorkerThread).
766 */
767 final int getPoolIndex() {
768 return (id & 0xffff) >>> 1; // ignore odd/even tag bit
769 }
770
771 /**
772 * Returns the approximate number of tasks in the queue.
773 */
774 final int queueSize() {
775 int n = base - top; // read base first
776 return (n >= 0) ? 0 : -n; // ignore transient negative
777 }
778
779 /**
780 * Provides a more accurate estimate of whether this queue has
781 * any tasks than does queueSize, by checking whether a
782 * near-empty queue has at least one unclaimed task.
783 */
784 final boolean isEmpty() {
785 ForkJoinTask<?>[] a; int n, al, b;
786 return ((n = (b = base) - top) >= 0 || // possibly one task
787 (n == -1 && ((a = array) == null ||
788 (al = a.length) == 0 ||
789 a[(al - 1) & b] == null)));
790 }
791
792
793 /**
794 * Pushes a task. Call only by owner in unshared queues.
795 *
796 * @param task the task. Caller must ensure non-null.
797 * @throws RejectedExecutionException if array cannot be resized
798 */
799 final void push(ForkJoinTask<?> task) {
800 int s = top; ForkJoinTask<?>[] a; int al, d;
801 if ((a = array) != null && (al = a.length) > 0) {
802 int index = (al - 1) & s;
803 ForkJoinPool p = pool;
804 top = s + 1;
805 QA.setRelease(a, index, task);
806 if ((d = base - s) == 0 && p != null) {
807 VarHandle.fullFence();
808 p.signalWork();
809 }
810 else if (d + al == 1)
811 growArray();
812 }
813 }
814
815 /**
816 * Initializes or doubles the capacity of array. Call either
817 * by owner or with lock held -- it is OK for base, but not
818 * top, to move while resizings are in progress.
819 */
820 final ForkJoinTask<?>[] growArray() {
821 ForkJoinTask<?>[] oldA = array;
822 int oldSize = oldA != null ? oldA.length : 0;
823 int size = oldSize > 0 ? oldSize << 1 : INITIAL_QUEUE_CAPACITY;
824 if (size < INITIAL_QUEUE_CAPACITY || size > MAXIMUM_QUEUE_CAPACITY)
825 throw new RejectedExecutionException("Queue capacity exceeded");
826 int oldMask, t, b;
827 ForkJoinTask<?>[] a = array = new ForkJoinTask<?>[size];
828 if (oldA != null && (oldMask = oldSize - 1) > 0 &&
829 (t = top) - (b = base) > 0) {
830 int mask = size - 1;
831 do { // emulate poll from old array, push to new array
832 int index = b & oldMask;
833 ForkJoinTask<?> x = (ForkJoinTask<?>)
834 QA.getAcquire(oldA, index);
835 if (x != null &&
836 QA.compareAndSet(oldA, index, x, null))
837 a[b & mask] = x;
838 } while (++b != t);
839 VarHandle.releaseFence();
840 }
841 return a;
842 }
843
844 /**
845 * Takes next task, if one exists, in LIFO order. Call only
846 * by owner in unshared queues.
847 */
848 final ForkJoinTask<?> pop() {
849 int b = base, s = top, al, i; ForkJoinTask<?>[] a;
850 if ((a = array) != null && b != s && (al = a.length) > 0) {
851 int index = (al - 1) & --s;
852 ForkJoinTask<?> t = (ForkJoinTask<?>)
853 QA.get(a, index);
854 if (t != null &&
855 QA.compareAndSet(a, index, t, null)) {
856 top = s;
857 VarHandle.releaseFence();
858 return t;
859 }
860 }
861 return null;
862 }
863
864 /**
865 * Takes next task, if one exists, in FIFO order.
866 */
867 final ForkJoinTask<?> poll() {
868 for (;;) {
869 int b = base, s = top, d, al; ForkJoinTask<?>[] a;
870 if ((a = array) != null && (d = b - s) < 0 &&
871 (al = a.length) > 0) {
872 int index = (al - 1) & b;
873 ForkJoinTask<?> t = (ForkJoinTask<?>)
874 QA.getAcquire(a, index);
875 if (b++ == base) {
876 if (t != null) {
877 if (QA.compareAndSet(a, index, t, null)) {
878 base = b;
879 return t;
880 }
881 }
882 else if (d == -1)
883 break; // now empty
884 }
885 }
886 else
887 break;
888 }
889 return null;
890 }
891
892 /**
893 * Takes next task, if one exists, in order specified by mode.
894 */
895 final ForkJoinTask<?> nextLocalTask() {
896 return ((id & FIFO) != 0) ? poll() : pop();
897 }
898
899 /**
900 * Returns next task, if one exists, in order specified by mode.
901 */
902 final ForkJoinTask<?> peek() {
903 int al; ForkJoinTask<?>[] a;
904 return ((a = array) != null && (al = a.length) > 0) ?
905 a[(al - 1) &
906 ((id & FIFO) != 0 ? base : top - 1)] : null;
907 }
908
909 /**
910 * Pops the given task only if it is at the current top.
911 */
912 final boolean tryUnpush(ForkJoinTask<?> task) {
913 int b = base, s = top, al; ForkJoinTask<?>[] a;
914 if ((a = array) != null && b != s && (al = a.length) > 0) {
915 int index = (al - 1) & --s;
916 if (QA.compareAndSet(a, index, task, null)) {
917 top = s;
918 VarHandle.releaseFence();
919 return true;
920 }
921 }
922 return false;
923 }
924
925 /**
926 * Removes and cancels all known tasks, ignoring any exceptions.
927 */
928 final void cancelAll() {
929 for (ForkJoinTask<?> t; (t = poll()) != null; )
930 ForkJoinTask.cancelIgnoringExceptions(t);
931 }
932
933 // Specialized execution methods
934
935 /**
936 * Pops and executes up to limit consecutive tasks or until empty.
937 *
938 * @param limit max runs, or zero for no limit
939 */
940 final void localPopAndExec(int limit) {
941 for (;;) {
942 int b = base, s = top, al; ForkJoinTask<?>[] a;
943 if ((a = array) != null && b != s && (al = a.length) > 0) {
944 int index = (al - 1) & --s;
945 ForkJoinTask<?> t = (ForkJoinTask<?>)
946 QA.getAndSet(a, index, null);
947 if (t != null) {
948 top = s;
949 VarHandle.releaseFence();
950 t.doExec();
951 if (limit != 0 && --limit == 0)
952 break;
953 }
954 else
955 break;
956 }
957 else
958 break;
959 }
960 }
961
962 /**
963 * Polls and executes up to limit consecutive tasks or until empty.
964 *
965 * @param limit, or zero for no limit
966 */
967 final void localPollAndExec(int limit) {
968 for (int polls = 0;;) {
969 int b = base, s = top, d, al; ForkJoinTask<?>[] a;
970 if ((a = array) != null && (d = b - s) < 0 &&
971 (al = a.length) > 0) {
972 int index = (al - 1) & b++;
973 ForkJoinTask<?> t = (ForkJoinTask<?>)
974 QA.getAndSet(a, index, null);
975 if (t != null) {
976 base = b;
977 t.doExec();
978 if (limit != 0 && ++polls == limit)
979 break;
980 }
981 else if (d == -1)
982 break; // now empty
983 else
984 polls = 0; // stolen; reset
985 }
986 else
987 break;
988 }
989 }
990
991 /**
992 * If present, removes task from queue and executes it.
993 */
994 final void tryRemoveAndExec(ForkJoinTask<?> task) {
995 ForkJoinTask<?>[] wa; int s, wal;
996 if (base - (s = top) < 0 && // traverse from top
997 (wa = array) != null && (wal = wa.length) > 0) {
998 for (int m = wal - 1, ns = s - 1, i = ns; ; --i) {
999 int index = i & m;
1000 ForkJoinTask<?> t = (ForkJoinTask<?>)
1001 QA.get(wa, index);
1002 if (t == null)
1003 break;
1004 else if (t == task) {
1005 if (QA.compareAndSet(wa, index, t, null)) {
1006 top = ns; // safely shift down
1007 for (int j = i; j != ns; ++j) {
1008 ForkJoinTask<?> f;
1009 int pindex = (j + 1) & m;
1010 f = (ForkJoinTask<?>)QA.get(wa, pindex);
1011 QA.setVolatile(wa, pindex, null);
1012 int jindex = j & m;
1013 QA.setRelease(wa, jindex, f);
1014 }
1015 VarHandle.releaseFence();
1016 t.doExec();
1017 }
1018 break;
1019 }
1020 }
1021 }
1022 }
1023
1024 /**
1025 * Tries to steal and run tasks within the target's
1026 * computation until done, not found, or limit exceeded.
1027 *
1028 * @param task root of CountedCompleter computation
1029 * @param limit max runs, or zero for no limit
1030 * @return task status on exit
1031 */
1032 final int localHelpCC(CountedCompleter<?> task, int limit) {
1033 int status = 0;
1034 if (task != null && (status = task.status) >= 0) {
1035 for (;;) {
1036 boolean help = false;
1037 int b = base, s = top, al; ForkJoinTask<?>[] a;
1038 if ((a = array) != null && b != s && (al = a.length) > 0) {
1039 int index = (al - 1) & (s - 1);
1040 ForkJoinTask<?> o = (ForkJoinTask<?>)
1041 QA.get(a, index);
1042 if (o instanceof CountedCompleter) {
1043 CountedCompleter<?> t = (CountedCompleter<?>)o;
1044 for (CountedCompleter<?> f = t;;) {
1045 if (f != task) {
1046 if ((f = f.completer) == null) // try parent
1047 break;
1048 }
1049 else {
1050 if (QA.compareAndSet(a, index, t, null)) {
1051 top = s - 1;
1052 VarHandle.releaseFence();
1053 t.doExec();
1054 help = true;
1055 }
1056 break;
1057 }
1058 }
1059 }
1060 }
1061 if ((status = task.status) < 0 || !help ||
1062 (limit != 0 && --limit == 0))
1063 break;
1064 }
1065 }
1066 return status;
1067 }
1068
1069 // Operations on shared queues
1070
1071 /**
1072 * Tries to lock shared queue by CASing phase field.
1073 */
1074 final boolean tryLockSharedQueue() {
1075 return PHASE.compareAndSet(this, 0, QLOCK);
1076 }
1077
1078 /**
1079 * Shared version of tryUnpush.
1080 */
1081 final boolean trySharedUnpush(ForkJoinTask<?> task) {
1082 boolean popped = false;
1083 int s = top - 1, al; ForkJoinTask<?>[] a;
1084 if ((a = array) != null && (al = a.length) > 0) {
1085 int index = (al - 1) & s;
1086 ForkJoinTask<?> t = (ForkJoinTask<?>) QA.get(a, index);
1087 if (t == task &&
1088 PHASE.compareAndSet(this, 0, QLOCK)) {
1089 if (top == s + 1 && array == a &&
1090 QA.compareAndSet(a, index, task, null)) {
1091 popped = true;
1092 top = s;
1093 }
1094 PHASE.setRelease(this, 0);
1095 }
1096 }
1097 return popped;
1098 }
1099
1100 /**
1101 * Shared version of localHelpCC.
1102 */
1103 final int sharedHelpCC(CountedCompleter<?> task, int limit) {
1104 int status = 0;
1105 if (task != null && (status = task.status) >= 0) {
1106 for (;;) {
1107 boolean help = false;
1108 int b = base, s = top, al; ForkJoinTask<?>[] a;
1109 if ((a = array) != null && b != s && (al = a.length) > 0) {
1110 int index = (al - 1) & (s - 1);
1111 ForkJoinTask<?> o = (ForkJoinTask<?>)
1112 QA.get(a, index);
1113 if (o instanceof CountedCompleter) {
1114 CountedCompleter<?> t = (CountedCompleter<?>)o;
1115 for (CountedCompleter<?> f = t;;) {
1116 if (f != task) {
1117 if ((f = f.completer) == null)
1118 break;
1119 }
1120 else {
1121 if (PHASE.compareAndSet(this, 0, QLOCK)) {
1122 if (top == s && array == a &&
1123 QA.compareAndSet(a, index, t, null)) {
1124 help = true;
1125 top = s - 1;
1126 }
1127 PHASE.setRelease(this, 0);
1128 if (help)
1129 t.doExec();
1130 }
1131 break;
1132 }
1133 }
1134 }
1135 }
1136 if ((status = task.status) < 0 || !help ||
1137 (limit != 0 && --limit == 0))
1138 break;
1139 }
1140 }
1141 return status;
1142 }
1143
1144 /**
1145 * Returns true if owned and not known to be blocked.
1146 */
1147 final boolean isApparentlyUnblocked() {
1148 Thread wt; Thread.State s;
1149 return ((wt = owner) != null &&
1150 (s = wt.getState()) != Thread.State.BLOCKED &&
1151 s != Thread.State.WAITING &&
1152 s != Thread.State.TIMED_WAITING);
1153 }
1154
1155 // VarHandle mechanics.
1156 private static final VarHandle PHASE;
1157 static {
1158 try {
1159 MethodHandles.Lookup l = MethodHandles.lookup();
1160 PHASE = l.findVarHandle(WorkQueue.class, "phase", int.class);
1161 } catch (ReflectiveOperationException e) {
1162 throw new Error(e);
1163 }
1164 }
1165 }
1166
1167 // static fields (initialized in static initializer below)
1168
1169 /**
1170 * Creates a new ForkJoinWorkerThread. This factory is used unless
1171 * overridden in ForkJoinPool constructors.
1172 */
1173 public static final ForkJoinWorkerThreadFactory
1174 defaultForkJoinWorkerThreadFactory;
1175
1176 /**
1177 * Permission required for callers of methods that may start or
1178 * kill threads.
1179 */
1180 static final RuntimePermission modifyThreadPermission;
1181
1182 /**
1183 * Common (static) pool. Non-null for public use unless a static
1184 * construction exception, but internal usages null-check on use
1185 * to paranoically avoid potential initialization circularities
1186 * as well as to simplify generated code.
1187 */
1188 static final ForkJoinPool common;
1189
1190 /**
1191 * Common pool parallelism. To allow simpler use and management
1192 * when common pool threads are disabled, we allow the underlying
1193 * common.parallelism field to be zero, but in that case still report
1194 * parallelism as 1 to reflect resulting caller-runs mechanics.
1195 */
1196 static final int COMMON_PARALLELISM;
1197
1198 /**
1199 * Limit on spare thread construction in tryCompensate.
1200 */
1201 private static final int COMMON_MAX_SPARES;
1202
1203 /**
1204 * Sequence number for creating workerNamePrefix.
1205 */
1206 private static int poolNumberSequence;
1207
1208 /**
1209 * Returns the next sequence number. We don't expect this to
1210 * ever contend, so use simple builtin sync.
1211 */
1212 private static final synchronized int nextPoolId() {
1213 return ++poolNumberSequence;
1214 }
1215
1216 // static configuration constants
1217
1218 /**
1219 * Default idle timeout value (in milliseconds) for the thread
1220 * triggering quiescence to park waiting for new work
1221 */
1222 private static final long DEFAULT_KEEPALIVE = 60_000L;
1223
1224 /**
1225 * Undershoot tolerance for idle timeouts
1226 */
1227 private static final long TIMEOUT_SLOP = 20L;
1228
1229 /**
1230 * The default value for COMMON_MAX_SPARES. Overridable using the
1231 * "java.util.concurrent.ForkJoinPool.common.maximumSpares" system
1232 * property. The default value is far in excess of normal
1233 * requirements, but also far short of MAX_CAP and typical OS
1234 * thread limits, so allows JVMs to catch misuse/abuse before
1235 * running out of resources needed to do so.
1236 */
1237 private static final int DEFAULT_COMMON_MAX_SPARES = 256;
1238
1239 /**
1240 * Increment for seed generators. See class ThreadLocal for
1241 * explanation.
1242 */
1243 private static final int SEED_INCREMENT = 0x9e3779b9;
1244
1245 /*
1246 * Bits and masks for field ctl, packed with 4 16 bit subfields:
1247 * RC: Number of released (unqueued) workers minus target parallelism
1248 * TC: Number of total workers minus target parallelism
1249 * SS: version count and status of top waiting thread
1250 * ID: poolIndex of top of Treiber stack of waiters
1251 *
1252 * When convenient, we can extract the lower 32 stack top bits
1253 * (including version bits) as sp=(int)ctl. The offsets of counts
1254 * by the target parallelism and the positionings of fields makes
1255 * it possible to perform the most common checks via sign tests of
1256 * fields: When ac is negative, there are not enough unqueued
1257 * workers, when tc is negative, there are not enough total
1258 * workers. When sp is non-zero, there are waiting workers. To
1259 * deal with possibly negative fields, we use casts in and out of
1260 * "short" and/or signed shifts to maintain signedness.
1261 *
1262 * Because it occupies uppermost bits, we can add one release count
1263 * using getAndAddLong of RC_UNIT, rather than CAS, when returning
1264 * from a blocked join. Other updates entail multiple subfields
1265 * and masking, requiring CAS.
1266 *
1267 * The limits packed in field "bounds" are also offset by the
1268 * parallelism level to make them comparable to the ctl rc and tc
1269 * fields.
1270 */
1271
1272 // Lower and upper word masks
1273 private static final long SP_MASK = 0xffffffffL;
1274 private static final long UC_MASK = ~SP_MASK;
1275
1276 // Release counts
1277 private static final int RC_SHIFT = 48;
1278 private static final long RC_UNIT = 0x0001L << RC_SHIFT;
1279 private static final long RC_MASK = 0xffffL << RC_SHIFT;
1280
1281 // Total counts
1282 private static final int TC_SHIFT = 32;
1283 private static final long TC_UNIT = 0x0001L << TC_SHIFT;
1284 private static final long TC_MASK = 0xffffL << TC_SHIFT;
1285 private static final long ADD_WORKER = 0x0001L << (TC_SHIFT + 15); // sign
1286
1287 // Instance fields
1288
1289 volatile long stealCount; // collects worker nsteals
1290 final long keepAlive; // milliseconds before dropping if idle
1291 int indexSeed; // next worker index
1292 final int bounds; // min, max threads packed as shorts
1293 volatile int mode; // parallelism, runstate, queue mode
1294 WorkQueue[] workQueues; // main registry
1295 final String workerNamePrefix; // for worker thread string; sync lock
1296 final ForkJoinWorkerThreadFactory factory;
1297 final UncaughtExceptionHandler ueh; // per-worker UEH
1298 final Predicate<? super ForkJoinPool> saturate;
1299
1300 @jdk.internal.vm.annotation.Contended("fjpctl") // segregate
1301 volatile long ctl; // main pool control
1302
1303 // Creating, registering and deregistering workers
1304
1305 /**
1306 * Tries to construct and start one worker. Assumes that total
1307 * count has already been incremented as a reservation. Invokes
1308 * deregisterWorker on any failure.
1309 *
1310 * @return true if successful
1311 */
1312 private boolean createWorker() {
1313 ForkJoinWorkerThreadFactory fac = factory;
1314 Throwable ex = null;
1315 ForkJoinWorkerThread wt = null;
1316 try {
1317 if (fac != null && (wt = fac.newThread(this)) != null) {
1318 wt.start();
1319 return true;
1320 }
1321 } catch (Throwable rex) {
1322 ex = rex;
1323 }
1324 deregisterWorker(wt, ex);
1325 return false;
1326 }
1327
1328 /**
1329 * Tries to add one worker, incrementing ctl counts before doing
1330 * so, relying on createWorker to back out on failure.
1331 *
1332 * @param c incoming ctl value, with total count negative and no
1333 * idle workers. On CAS failure, c is refreshed and retried if
1334 * this holds (otherwise, a new worker is not needed).
1335 */
1336 private void tryAddWorker(long c) {
1337 do {
1338 long nc = ((RC_MASK & (c + RC_UNIT)) |
1339 (TC_MASK & (c + TC_UNIT)));
1340 if (ctl == c && CTL.compareAndSet(this, c, nc)) {
1341 createWorker();
1342 break;
1343 }
1344 } while (((c = ctl) & ADD_WORKER) != 0L && (int)c == 0);
1345 }
1346
1347 /**
1348 * Callback from ForkJoinWorkerThread constructor to establish and
1349 * record its WorkQueue.
1350 *
1351 * @param wt the worker thread
1352 * @return the worker's queue
1353 */
1354 final WorkQueue registerWorker(ForkJoinWorkerThread wt) {
1355 UncaughtExceptionHandler handler;
1356 wt.setDaemon(true); // configure thread
1357 if ((handler = ueh) != null)
1358 wt.setUncaughtExceptionHandler(handler);
1359 WorkQueue w = new WorkQueue(this, wt);
1360 int tid = 0; // for thread name
1361 int fifo = mode & FIFO;
1362 String prefix = workerNamePrefix;
1363 if (prefix != null) {
1364 synchronized (prefix) {
1365 WorkQueue[] ws = workQueues; int n;
1366 int s = indexSeed += SEED_INCREMENT;
1367 if (ws != null && (n = ws.length) > 1) {
1368 int m = n - 1;
1369 tid = s & m;
1370 int i = m & ((s << 1) | 1); // odd-numbered indices
1371 for (int probes = n >>> 1;;) { // find empty slot
1372 WorkQueue q;
1373 if ((q = ws[i]) == null || q.phase == QUIET)
1374 break;
1375 else if (--probes == 0) {
1376 i = n | 1; // resize below
1377 break;
1378 }
1379 else
1380 i = (i + 2) & m;
1381 }
1382
1383 int id = i | fifo | (s & ~(SMASK | FIFO | DORMANT));
1384 w.phase = w.id = id; // now publishable
1385
1386 if (i < n)
1387 ws[i] = w;
1388 else { // expand array
1389 int an = n << 1;
1390 WorkQueue[] as = new WorkQueue[an];
1391 as[i] = w;
1392 int am = an - 1;
1393 for (int j = 0; j < n; ++j) {
1394 WorkQueue v; // copy external queue
1395 if ((v = ws[j]) != null) // position may change
1396 as[v.id & am & SQMASK] = v;
1397 if (++j >= n)
1398 break;
1399 as[j] = ws[j]; // copy worker
1400 }
1401 workQueues = as;
1402 }
1403 }
1404 }
1405 wt.setName(prefix.concat(Integer.toString(tid)));
1406 }
1407 return w;
1408 }
1409
1410 /**
1411 * Final callback from terminating worker, as well as upon failure
1412 * to construct or start a worker. Removes record of worker from
1413 * array, and adjusts counts. If pool is shutting down, tries to
1414 * complete termination.
1415 *
1416 * @param wt the worker thread, or null if construction failed
1417 * @param ex the exception causing failure, or null if none
1418 */
1419 final void deregisterWorker(ForkJoinWorkerThread wt, Throwable ex) {
1420 WorkQueue w = null;
1421 int phase = 0;
1422 if (wt != null && (w = wt.workQueue) != null) {
1423 Object lock = workerNamePrefix;
1424 long ns = (long)w.nsteals & 0xffffffffL;
1425 int idx = w.id & SMASK;
1426 if (lock != null) {
1427 WorkQueue[] ws; // remove index from array
1428 synchronized (lock) {
1429 if ((ws = workQueues) != null && ws.length > idx &&
1430 ws[idx] == w)
1431 ws[idx] = null;
1432 stealCount += ns;
1433 }
1434 }
1435 phase = w.phase;
1436 }
1437 if (phase != QUIET) { // else pre-adjusted
1438 long c; // decrement counts
1439 do {} while (!CTL.weakCompareAndSet
1440 (this, c = ctl, ((RC_MASK & (c - RC_UNIT)) |
1441 (TC_MASK & (c - TC_UNIT)) |
1442 (SP_MASK & c))));
1443 }
1444 if (w != null)
1445 w.cancelAll(); // cancel remaining tasks
1446
1447 if (!tryTerminate(false, false) && // possibly replace worker
1448 w != null && w.array != null) // avoid repeated failures
1449 signalWork();
1450
1451 if (ex == null) // help clean on way out
1452 ForkJoinTask.helpExpungeStaleExceptions();
1453 else // rethrow
1454 ForkJoinTask.rethrow(ex);
1455 }
1456
1457 /**
1458 * Tries to create or release a worker if too few are running.
1459 */
1460 final void signalWork() {
1461 for (;;) {
1462 long c; int sp; WorkQueue[] ws; int i; WorkQueue v;
1463 if ((c = ctl) >= 0L) // enough workers
1464 break;
1465 else if ((sp = (int)c) == 0) { // no idle workers
1466 if ((c & ADD_WORKER) != 0L) // too few workers
1467 tryAddWorker(c);
1468 break;
1469 }
1470 else if ((ws = workQueues) == null)
1471 break; // unstarted/terminated
1472 else if (ws.length <= (i = sp & SMASK))
1473 break; // terminated
1474 else if ((v = ws[i]) == null)
1475 break; // terminating
1476 else {
1477 int np = sp & ~UNSIGNALLED;
1478 int vp = v.phase;
1479 long nc = (v.stackPred & SP_MASK) | (UC_MASK & (c + RC_UNIT));
1480 Thread vt = v.owner;
1481 if (sp == vp && CTL.compareAndSet(this, c, nc)) {
1482 v.phase = np;
1483 if (v.source < 0)
1484 LockSupport.unpark(vt);
1485 break;
1486 }
1487 }
1488 }
1489 }
1490
1491 /**
1492 * Tries to decrement counts (sometimes implicitly) and possibly
1493 * arrange for a compensating worker in preparation for blocking:
1494 * If not all core workers yet exist, creates one, else if any are
1495 * unreleased (possibly including caller) releases one, else if
1496 * fewer than the minimum allowed number of workers running,
1497 * checks to see that they are all active, and if so creates an
1498 * extra worker unless over maximum limit and policy is to
1499 * saturate. Most of these steps can fail due to interference, in
1500 * which case 0 is returned so caller will retry. A negative
1501 * return value indicates that the caller doesn't need to
1502 * re-adjust counts when later unblocked.
1503 *
1504 * @return 1: block then adjust, -1: block without adjust, 0 : retry
1505 */
1506 private int tryCompensate(WorkQueue w) {
1507 int t, n, sp;
1508 long c = ctl;
1509 WorkQueue[] ws = workQueues;
1510 if ((t = (short)(c >>> TC_SHIFT)) >= 0) {
1511 if (ws == null || (n = ws.length) <= 0 || w == null)
1512 return 0; // disabled
1513 else if ((sp = (int)c) != 0) { // replace or release
1514 WorkQueue v = ws[sp & (n - 1)];
1515 int wp = w.phase;
1516 long uc = UC_MASK & ((wp < 0) ? c + RC_UNIT : c);
1517 int np = sp & ~UNSIGNALLED;
1518 if (v != null) {
1519 int vp = v.phase;
1520 Thread vt = v.owner;
1521 long nc = ((long)v.stackPred & SP_MASK) | uc;
1522 if (vp == sp && CTL.compareAndSet(this, c, nc)) {
1523 v.phase = np;
1524 if (v.source < 0)
1525 LockSupport.unpark(vt);
1526 return (wp < 0) ? -1 : 1;
1527 }
1528 }
1529 return 0;
1530 }
1531 else if ((int)(c >> RC_SHIFT) - // reduce parallelism
1532 (short)(bounds & SMASK) > 0) {
1533 long nc = ((RC_MASK & (c - RC_UNIT)) | (~RC_MASK & c));
1534 return CTL.compareAndSet(this, c, nc) ? 1 : 0;
1535 }
1536 else { // validate
1537 int md = mode, pc = md & SMASK, tc = pc + t, bc = 0;
1538 boolean unstable = false;
1539 for (int i = 1; i < n; i += 2) {
1540 WorkQueue q; Thread wt; Thread.State ts;
1541 if ((q = ws[i]) != null) {
1542 if (q.source == 0) {
1543 unstable = true;
1544 break;
1545 }
1546 else {
1547 --tc;
1548 if ((wt = q.owner) != null &&
1549 ((ts = wt.getState()) == Thread.State.BLOCKED ||
1550 ts == Thread.State.WAITING))
1551 ++bc; // worker is blocking
1552 }
1553 }
1554 }
1555 if (unstable || tc != 0 || ctl != c)
1556 return 0; // inconsistent
1557 else if (t + pc >= MAX_CAP || t >= (bounds >>> SWIDTH)) {
1558 Predicate<? super ForkJoinPool> sat;
1559 if ((sat = saturate) != null && sat.test(this))
1560 return -1;
1561 else if (bc < pc) { // lagging
1562 Thread.yield(); // for retry spins
1563 return 0;
1564 }
1565 else
1566 throw new RejectedExecutionException(
1567 "Thread limit exceeded replacing blocked worker");
1568 }
1569 }
1570 }
1571
1572 long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK); // expand pool
1573 return CTL.compareAndSet(this, c, nc) && createWorker() ? 1 : 0;
1574 }
1575
1576 /**
1577 * Top-level runloop for workers, called by ForkJoinWorkerThread.run.
1578 * See above for explanation.
1579 */
1580 final void runWorker(WorkQueue w) {
1581 WorkQueue[] ws;
1582 w.growArray(); // allocate queue
1583 int r = w.id ^ ThreadLocalRandom.nextSecondarySeed();
1584 if (r == 0) // initial nonzero seed
1585 r = 1;
1586 int lastSignalId = 0; // avoid unneeded signals
1587 while ((ws = workQueues) != null) {
1588 boolean nonempty = false; // scan
1589 for (int n = ws.length, j = n, m = n - 1; j > 0; --j) {
1590 WorkQueue q; int i, b, al; ForkJoinTask<?>[] a;
1591 if ((i = r & m) >= 0 && i < n && // always true
1592 (q = ws[i]) != null && (b = q.base) - q.top < 0 &&
1593 (a = q.array) != null && (al = a.length) > 0) {
1594 int qid = q.id; // (never zero)
1595 int index = (al - 1) & b;
1596 ForkJoinTask<?> t = (ForkJoinTask<?>)
1597 QA.getAcquire(a, index);
1598 if (t != null && b++ == q.base &&
1599 QA.compareAndSet(a, index, t, null)) {
1600 if ((q.base = b) - q.top < 0 && qid != lastSignalId)
1601 signalWork(); // propagate signal
1602 w.source = lastSignalId = qid;
1603 t.doExec();
1604 if ((w.id & FIFO) != 0) // run remaining locals
1605 w.localPollAndExec(POLL_LIMIT);
1606 else
1607 w.localPopAndExec(POLL_LIMIT);
1608 ForkJoinWorkerThread thread = w.owner;
1609 ++w.nsteals;
1610 w.source = 0; // now idle
1611 if (thread != null)
1612 thread.afterTopLevelExec();
1613 }
1614 nonempty = true;
1615 }
1616 else if (nonempty)
1617 break;
1618 else
1619 ++r;
1620 }
1621
1622 if (nonempty) { // move (xorshift)
1623 r ^= r << 13; r ^= r >>> 17; r ^= r << 5;
1624 }
1625 else {
1626 int phase;
1627 lastSignalId = 0; // clear for next scan
1628 if ((phase = w.phase) >= 0) { // enqueue
1629 int np = w.phase = (phase + SS_SEQ) | UNSIGNALLED;
1630 long c, nc;
1631 do {
1632 w.stackPred = (int)(c = ctl);
1633 nc = ((c - RC_UNIT) & UC_MASK) | (SP_MASK & np);
1634 } while (!CTL.weakCompareAndSet(this, c, nc));
1635 }
1636 else { // already queued
1637 int pred = w.stackPred;
1638 w.source = DORMANT; // enable signal
1639 for (int steps = 0;;) {
1640 int md, rc; long c;
1641 if (w.phase >= 0) {
1642 w.source = 0;
1643 break;
1644 }
1645 else if ((md = mode) < 0) // shutting down
1646 return;
1647 else if ((rc = ((md & SMASK) + // possibly quiescent
1648 (int)((c = ctl) >> RC_SHIFT))) <= 0 &&
1649 (md & SHUTDOWN) != 0 &&
1650 tryTerminate(false, false))
1651 return; // help terminate
1652 else if ((++steps & 1) == 0)
1653 Thread.interrupted(); // clear between parks
1654 else if (rc <= 0 && pred != 0 && phase == (int)c) {
1655 long d = keepAlive + System.currentTimeMillis();
1656 LockSupport.parkUntil(this, d);
1657 if (ctl == c &&
1658 d - System.currentTimeMillis() <= TIMEOUT_SLOP) {
1659 long nc = ((UC_MASK & (c - TC_UNIT)) |
1660 (SP_MASK & pred));
1661 if (CTL.compareAndSet(this, c, nc)) {
1662 w.phase = QUIET;
1663 return; // drop on timeout
1664 }
1665 }
1666 }
1667 else
1668 LockSupport.park(this);
1669 }
1670 }
1671 }
1672 }
1673 }
1674
1675 /**
1676 * Helps and/or blocks until the given task is done or timeout.
1677 * First tries locally helping, then scans other queues for a task
1678 * produced by one of w's stealers; compensating and blocking if
1679 * none are found (rescanning if tryCompensate fails).
1680 *
1681 * @param w caller
1682 * @param task the task
1683 * @param deadline for timed waits, if nonzero
1684 * @return task status on exit
1685 */
1686 final int awaitJoin(WorkQueue w, ForkJoinTask<?> task, long deadline) {
1687 int s = 0;
1688 if (w != null && task != null &&
1689 (!(task instanceof CountedCompleter) ||
1690 (s = w.localHelpCC((CountedCompleter<?>)task, 0)) >= 0)) {
1691 w.tryRemoveAndExec(task);
1692 int src = w.source, id = w.id;
1693 s = task.status;
1694 while (s >= 0) {
1695 WorkQueue[] ws;
1696 boolean nonempty = false;
1697 int r = ThreadLocalRandom.nextSecondarySeed() | 1; // odd indices
1698 if ((ws = workQueues) != null) { // scan for matching id
1699 for (int n = ws.length, m = n - 1, j = -n; j < n; j += 2) {
1700 WorkQueue q; int i, b, al; ForkJoinTask<?>[] a;
1701 if ((i = (r + j) & m) >= 0 && i < n &&
1702 (q = ws[i]) != null && q.source == id &&
1703 (b = q.base) - q.top < 0 &&
1704 (a = q.array) != null && (al = a.length) > 0) {
1705 int qid = q.id;
1706 int index = (al - 1) & b;
1707 ForkJoinTask<?> t = (ForkJoinTask<?>)
1708 QA.getAcquire(a, index);
1709 if (t != null && b++ == q.base && id == q.source &&
1710 QA.compareAndSet(a, index, t, null)) {
1711 q.base = b;
1712 w.source = qid;
1713 t.doExec();
1714 w.source = src;
1715 }
1716 nonempty = true;
1717 break;
1718 }
1719 }
1720 }
1721 if ((s = task.status) < 0)
1722 break;
1723 else if (!nonempty) {
1724 long ms, ns; int block;
1725 if (deadline == 0L)
1726 ms = 0L; // untimed
1727 else if ((ns = deadline - System.nanoTime()) <= 0L)
1728 break; // timeout
1729 else if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) <= 0L)
1730 ms = 1L; // avoid 0 for timed wait
1731 if ((block = tryCompensate(w)) != 0) {
1732 task.internalWait(ms);
1733 CTL.getAndAdd(this, (block > 0) ? RC_UNIT : 0L);
1734 }
1735 s = task.status;
1736 }
1737 }
1738 }
1739 return s;
1740 }
1741
1742 /**
1743 * Runs tasks until {@code isQuiescent()}. Rather than blocking
1744 * when tasks cannot be found, rescans until all others cannot
1745 * find tasks either.
1746 */
1747 final void helpQuiescePool(WorkQueue w) {
1748 int prevSrc = w.source, fifo = w.id & FIFO;
1749 for (int source = prevSrc, released = -1;;) { // -1 until known
1750 WorkQueue[] ws;
1751 if (fifo != 0)
1752 w.localPollAndExec(0);
1753 else
1754 w.localPopAndExec(0);
1755 if (released == -1 && w.phase >= 0)
1756 released = 1;
1757 boolean quiet = true, empty = true;
1758 int r = ThreadLocalRandom.nextSecondarySeed();
1759 if ((ws = workQueues) != null) {
1760 for (int n = ws.length, j = n, m = n - 1; j > 0; --j) {
1761 WorkQueue q; int i, b, al; ForkJoinTask<?>[] a;
1762 if ((i = (r - j) & m) >= 0 && i < n && (q = ws[i]) != null) {
1763 if ((b = q.base) - q.top < 0 &&
1764 (a = q.array) != null && (al = a.length) > 0) {
1765 int qid = q.id;
1766 if (released == 0) { // increment
1767 released = 1;
1768 CTL.getAndAdd(this, RC_UNIT);
1769 }
1770 int index = (al - 1) & b;
1771 ForkJoinTask<?> t = (ForkJoinTask<?>)
1772 QA.getAcquire(a, index);
1773 if (t != null && b++ == q.base &&
1774 QA.compareAndSet(a, index, t, null)) {
1775 q.base = b;
1776 w.source = source = q.id;
1777 t.doExec();
1778 w.source = source = prevSrc;
1779 }
1780 quiet = empty = false;
1781 break;
1782 }
1783 else if ((q.source & QUIET) == 0)
1784 quiet = false;
1785 }
1786 }
1787 }
1788 if (quiet) {
1789 if (released == 0)
1790 CTL.getAndAdd(this, RC_UNIT);
1791 w.source = prevSrc;
1792 break;
1793 }
1794 else if (empty) {
1795 if (source != QUIET)
1796 w.source = source = QUIET;
1797 if (released == 1) { // decrement
1798 released = 0;
1799 CTL.getAndAdd(this, RC_MASK & -RC_UNIT);
1800 }
1801 }
1802 }
1803 }
1804
1805 /**
1806 * Scans for and returns a polled task, if available.
1807 * Used only for untracked polls.
1808 *
1809 * @param submissionsOnly if true, only scan submission queues
1810 */
1811 private ForkJoinTask<?> pollScan(boolean submissionsOnly) {
1812 WorkQueue[] ws; int n;
1813 rescan: while ((mode & STOP) == 0 && (ws = workQueues) != null &&
1814 (n = ws.length) > 0) {
1815 int m = n - 1;
1816 int r = ThreadLocalRandom.nextSecondarySeed();
1817 int h = r >>> 16;
1818 int origin, step;
1819 if (submissionsOnly) {
1820 origin = (r & ~1) & m; // even indices and steps
1821 step = (h & ~1) | 2;
1822 }
1823 else {
1824 origin = r & m;
1825 step = h | 1;
1826 }
1827 for (int k = origin, oldSum = 0, checkSum = 0;;) {
1828 WorkQueue q; int b, al; ForkJoinTask<?>[] a;
1829 if ((q = ws[k]) != null) {
1830 checkSum += b = q.base;
1831 if (b - q.top < 0 &&
1832 (a = q.array) != null && (al = a.length) > 0) {
1833 int index = (al - 1) & b;
1834 ForkJoinTask<?> t = (ForkJoinTask<?>)
1835 QA.getAcquire(a, index);
1836 if (t != null && b++ == q.base &&
1837 QA.compareAndSet(a, index, t, null)) {
1838 q.base = b;
1839 return t;
1840 }
1841 else
1842 break; // restart
1843 }
1844 }
1845 if ((k = (k + step) & m) == origin) {
1846 if (oldSum == (oldSum = checkSum))
1847 break rescan;
1848 checkSum = 0;
1849 }
1850 }
1851 }
1852 return null;
1853 }
1854
1855 /**
1856 * Gets and removes a local or stolen task for the given worker.
1857 *
1858 * @return a task, if available
1859 */
1860 final ForkJoinTask<?> nextTaskFor(WorkQueue w) {
1861 ForkJoinTask<?> t;
1862 if (w != null &&
1863 (t = (w.id & FIFO) != 0 ? w.poll() : w.pop()) != null)
1864 return t;
1865 else
1866 return pollScan(false);
1867 }
1868
1869 // External operations
1870
1871 /**
1872 * Adds the given task to a submission queue at submitter's
1873 * current queue, creating one if null or contended.
1874 *
1875 * @param task the task. Caller must ensure non-null.
1876 */
1877 final void externalPush(ForkJoinTask<?> task) {
1878 int r; // initialize caller's probe
1879 if ((r = ThreadLocalRandom.getProbe()) == 0) {
1880 ThreadLocalRandom.localInit();
1881 r = ThreadLocalRandom.getProbe();
1882 }
1883 for (;;) {
1884 int md = mode, n;
1885 WorkQueue[] ws = workQueues;
1886 if ((md & SHUTDOWN) != 0 || ws == null || (n = ws.length) <= 0)
1887 throw new RejectedExecutionException();
1888 else {
1889 WorkQueue q;
1890 boolean push = false, grow = false;
1891 if ((q = ws[(n - 1) & r & SQMASK]) == null) {
1892 Object lock = workerNamePrefix;
1893 int qid = (r | QUIET) & ~(FIFO | OWNED);
1894 q = new WorkQueue(this, null);
1895 q.id = qid;
1896 q.source = QUIET;
1897 q.phase = QLOCK; // lock queue
1898 if (lock != null) {
1899 synchronized (lock) { // lock pool to install
1900 int i;
1901 if ((ws = workQueues) != null &&
1902 (n = ws.length) > 0 &&
1903 ws[i = qid & (n - 1) & SQMASK] == null) {
1904 ws[i] = q;
1905 push = grow = true;
1906 }
1907 }
1908 }
1909 }
1910 else if (q.tryLockSharedQueue()) {
1911 int b = q.base, s = q.top, al, d; ForkJoinTask<?>[] a;
1912 if ((a = q.array) != null && (al = a.length) > 0 &&
1913 al - 1 + (d = b - s) > 0) {
1914 a[(al - 1) & s] = task;
1915 q.top = s + 1; // relaxed writes OK here
1916 q.phase = 0;
1917 if (d < 0 && q.base - s < -1)
1918 break; // no signal needed
1919 }
1920 else
1921 grow = true;
1922 push = true;
1923 }
1924 if (push) {
1925 if (grow) {
1926 try {
1927 q.growArray();
1928 int s = q.top, al; ForkJoinTask<?>[] a;
1929 if ((a = q.array) != null && (al = a.length) > 0) {
1930 a[(al - 1) & s] = task;
1931 q.top = s + 1;
1932 }
1933 } finally {
1934 q.phase = 0;
1935 }
1936 }
1937 signalWork();
1938 break;
1939 }
1940 else // move if busy
1941 r = ThreadLocalRandom.advanceProbe(r);
1942 }
1943 }
1944 }
1945
1946 /**
1947 * Pushes a possibly-external submission.
1948 */
1949 private <T> ForkJoinTask<T> externalSubmit(ForkJoinTask<T> task) {
1950 Thread t; ForkJoinWorkerThread w; WorkQueue q;
1951 if (task == null)
1952 throw new NullPointerException();
1953 if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) &&
1954 (w = (ForkJoinWorkerThread)t).pool == this &&
1955 (q = w.workQueue) != null)
1956 q.push(task);
1957 else
1958 externalPush(task);
1959 return task;
1960 }
1961
1962 /**
1963 * Returns common pool queue for an external thread.
1964 */
1965 static WorkQueue commonSubmitterQueue() {
1966 ForkJoinPool p = common;
1967 int r = ThreadLocalRandom.getProbe();
1968 WorkQueue[] ws; int n;
1969 return (p != null && (ws = p.workQueues) != null &&
1970 (n = ws.length) > 0) ?
1971 ws[(n - 1) & r & SQMASK] : null;
1972 }
1973
1974 /**
1975 * Performs tryUnpush for an external submitter.
1976 */
1977 final boolean tryExternalUnpush(ForkJoinTask<?> task) {
1978 int r = ThreadLocalRandom.getProbe();
1979 WorkQueue[] ws; WorkQueue w; int n;
1980 return ((ws = workQueues) != null &&
1981 (n = ws.length) > 0 &&
1982 (w = ws[(n - 1) & r & SQMASK]) != null &&
1983 w.trySharedUnpush(task));
1984 }
1985
1986 /**
1987 * Performs helpComplete for an external submitter.
1988 */
1989 final int externalHelpComplete(CountedCompleter<?> task, int maxTasks) {
1990 int r = ThreadLocalRandom.getProbe();
1991 WorkQueue[] ws; WorkQueue w; int n;
1992 return ((ws = workQueues) != null && (n = ws.length) > 0 &&
1993 (w = ws[(n - 1) & r & SQMASK]) != null) ?
1994 w.sharedHelpCC(task, maxTasks) : 0;
1995 }
1996
1997 /**
1998 * Tries to steal and run tasks within the target's computation.
1999 * The maxTasks argument supports external usages; internal calls
2000 * use zero, allowing unbounded steps (external calls trap
2001 * non-positive values).
2002 *
2003 * @param w caller
2004 * @param maxTasks if non-zero, the maximum number of other tasks to run
2005 * @return task status on exit
2006 */
2007 final int helpComplete(WorkQueue w, CountedCompleter<?> task,
2008 int maxTasks) {
2009 return (w == null) ? 0 : w.localHelpCC(task, maxTasks);
2010 }
2011
2012 /**
2013 * Returns a cheap heuristic guide for task partitioning when
2014 * programmers, frameworks, tools, or languages have little or no
2015 * idea about task granularity. In essence, by offering this
2016 * method, we ask users only about tradeoffs in overhead vs
2017 * expected throughput and its variance, rather than how finely to
2018 * partition tasks.
2019 *
2020 * In a steady state strict (tree-structured) computation, each
2021 * thread makes available for stealing enough tasks for other
2022 * threads to remain active. Inductively, if all threads play by
2023 * the same rules, each thread should make available only a
2024 * constant number of tasks.
2025 *
2026 * The minimum useful constant is just 1. But using a value of 1
2027 * would require immediate replenishment upon each steal to
2028 * maintain enough tasks, which is infeasible. Further,
2029 * partitionings/granularities of offered tasks should minimize
2030 * steal rates, which in general means that threads nearer the top
2031 * of computation tree should generate more than those nearer the
2032 * bottom. In perfect steady state, each thread is at
2033 * approximately the same level of computation tree. However,
2034 * producing extra tasks amortizes the uncertainty of progress and
2035 * diffusion assumptions.
2036 *
2037 * So, users will want to use values larger (but not much larger)
2038 * than 1 to both smooth over transient shortages and hedge
2039 * against uneven progress; as traded off against the cost of
2040 * extra task overhead. We leave the user to pick a threshold
2041 * value to compare with the results of this call to guide
2042 * decisions, but recommend values such as 3.
2043 *
2044 * When all threads are active, it is on average OK to estimate
2045 * surplus strictly locally. In steady-state, if one thread is
2046 * maintaining say 2 surplus tasks, then so are others. So we can
2047 * just use estimated queue length. However, this strategy alone
2048 * leads to serious mis-estimates in some non-steady-state
2049 * conditions (ramp-up, ramp-down, other stalls). We can detect
2050 * many of these by further considering the number of "idle"
2051 * threads, that are known to have zero queued tasks, so
2052 * compensate by a factor of (#idle/#active) threads.
2053 */
2054 static int getSurplusQueuedTaskCount() {
2055 Thread t; ForkJoinWorkerThread wt; ForkJoinPool pool; WorkQueue q;
2056 if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) &&
2057 (pool = (wt = (ForkJoinWorkerThread)t).pool) != null &&
2058 (q = wt.workQueue) != null) {
2059 int p = pool.mode & SMASK;
2060 int a = p + (int)(pool.ctl >> RC_SHIFT);
2061 int n = q.top - q.base;
2062 return n - (a > (p >>>= 1) ? 0 :
2063 a > (p >>>= 1) ? 1 :
2064 a > (p >>>= 1) ? 2 :
2065 a > (p >>>= 1) ? 4 :
2066 8);
2067 }
2068 return 0;
2069 }
2070
2071 // Termination
2072
2073 /**
2074 * Possibly initiates and/or completes termination.
2075 *
2076 * @param now if true, unconditionally terminate, else only
2077 * if no work and no active workers
2078 * @param enable if true, terminate when next possible
2079 * @return true if terminating or terminated
2080 */
2081 private boolean tryTerminate(boolean now, boolean enable) {
2082 int md; // 3 phases: try to set SHUTDOWN, then STOP, then TERMINATED
2083
2084 while (((md = mode) & SHUTDOWN) == 0) {
2085 if (!enable || this == common) // cannot shutdown
2086 return false;
2087 else
2088 MODE.compareAndSet(this, md, md | SHUTDOWN);
2089 }
2090
2091 while (((md = mode) & STOP) == 0) { // try to initiate termination
2092 if (!now) { // check if quiescent & empty
2093 for (long oldSum = 0L;;) { // repeat until stable
2094 boolean running = false;
2095 long checkSum = ctl;
2096 WorkQueue[] ws = workQueues;
2097 if ((md & SMASK) + (int)(checkSum >> RC_SHIFT) > 0)
2098 running = true;
2099 else if (ws != null) {
2100 WorkQueue w; int b;
2101 for (int i = 0; i < ws.length; ++i) {
2102 if ((w = ws[i]) != null) {
2103 checkSum += (b = w.base) + w.id;
2104 if (b != w.top ||
2105 ((i & 1) == 1 && w.source >= 0)) {
2106 running = true;
2107 break;
2108 }
2109 }
2110 }
2111 }
2112 if (((md = mode) & STOP) != 0)
2113 break; // already triggered
2114 else if (running)
2115 return false;
2116 else if (workQueues == ws && oldSum == (oldSum = checkSum))
2117 break;
2118 }
2119 }
2120 if ((md & STOP) == 0)
2121 MODE.compareAndSet(this, md, md | STOP);
2122 }
2123
2124 while (((md = mode) & TERMINATED) == 0) { // help terminate others
2125 for (long oldSum = 0L;;) { // repeat until stable
2126 WorkQueue[] ws; WorkQueue w;
2127 long checkSum = ctl;
2128 if ((ws = workQueues) != null) {
2129 for (int i = 0; i < ws.length; ++i) {
2130 if ((w = ws[i]) != null) {
2131 ForkJoinWorkerThread wt = w.owner;
2132 w.cancelAll(); // clear queues
2133 if (wt != null) {
2134 try { // unblock join or park
2135 wt.interrupt();
2136 } catch (Throwable ignore) {
2137 }
2138 }
2139 checkSum += w.base + w.id;
2140 }
2141 }
2142 }
2143 if (((md = mode) & TERMINATED) != 0 ||
2144 (workQueues == ws && oldSum == (oldSum = checkSum)))
2145 break;
2146 }
2147 if ((md & TERMINATED) != 0)
2148 break;
2149 else if ((md & SMASK) + (short)(ctl >>> TC_SHIFT) > 0)
2150 break;
2151 else if (MODE.compareAndSet(this, md, md | TERMINATED)) {
2152 synchronized (this) {
2153 notifyAll(); // for awaitTermination
2154 }
2155 break;
2156 }
2157 }
2158 return true;
2159 }
2160
2161 // Exported methods
2162
2163 // Constructors
2164
2165 /**
2166 * Creates a {@code ForkJoinPool} with parallelism equal to {@link
2167 * java.lang.Runtime#availableProcessors}, using defaults for all
2168 * other parameters (see {@link #ForkJoinPool(int,
2169 * ForkJoinWorkerThreadFactory, UncaughtExceptionHandler, boolean,
2170 * int, int, int, Predicate, long, TimeUnit)}).
2171 *
2172 * @throws SecurityException if a security manager exists and
2173 * the caller is not permitted to modify threads
2174 * because it does not hold {@link
2175 * java.lang.RuntimePermission}{@code ("modifyThread")}
2176 */
2177 public ForkJoinPool() {
2178 this(Math.min(MAX_CAP, Runtime.getRuntime().availableProcessors()),
2179 defaultForkJoinWorkerThreadFactory, null, false,
2180 0, MAX_CAP, 1, null, DEFAULT_KEEPALIVE, TimeUnit.MILLISECONDS);
2181 }
2182
2183 /**
2184 * Creates a {@code ForkJoinPool} with the indicated parallelism
2185 * level, using defaults for all other parameters (see {@link
2186 * #ForkJoinPool(int, ForkJoinWorkerThreadFactory,
2187 * UncaughtExceptionHandler, boolean, int, int, int, Predicate,
2188 * long, TimeUnit)}).
2189 *
2190 * @param parallelism the parallelism level
2191 * @throws IllegalArgumentException if parallelism less than or
2192 * equal to zero, or greater than implementation limit
2193 * @throws SecurityException if a security manager exists and
2194 * the caller is not permitted to modify threads
2195 * because it does not hold {@link
2196 * java.lang.RuntimePermission}{@code ("modifyThread")}
2197 */
2198 public ForkJoinPool(int parallelism) {
2199 this(parallelism, defaultForkJoinWorkerThreadFactory, null, false,
2200 0, MAX_CAP, 1, null, DEFAULT_KEEPALIVE, TimeUnit.MILLISECONDS);
2201 }
2202
2203 /**
2204 * Creates a {@code ForkJoinPool} with the given parameters (using
2205 * defaults for others -- see {@link #ForkJoinPool(int,
2206 * ForkJoinWorkerThreadFactory, UncaughtExceptionHandler, boolean,
2207 * int, int, int, Predicate, long, TimeUnit)}).
2208 *
2209 * @param parallelism the parallelism level. For default value,
2210 * use {@link java.lang.Runtime#availableProcessors}.
2211 * @param factory the factory for creating new threads. For default value,
2212 * use {@link #defaultForkJoinWorkerThreadFactory}.
2213 * @param handler the handler for internal worker threads that
2214 * terminate due to unrecoverable errors encountered while executing
2215 * tasks. For default value, use {@code null}.
2216 * @param asyncMode if true,
2217 * establishes local first-in-first-out scheduling mode for forked
2218 * tasks that are never joined. This mode may be more appropriate
2219 * than default locally stack-based mode in applications in which
2220 * worker threads only process event-style asynchronous tasks.
2221 * For default value, use {@code false}.
2222 * @throws IllegalArgumentException if parallelism less than or
2223 * equal to zero, or greater than implementation limit
2224 * @throws NullPointerException if the factory is null
2225 * @throws SecurityException if a security manager exists and
2226 * the caller is not permitted to modify threads
2227 * because it does not hold {@link
2228 * java.lang.RuntimePermission}{@code ("modifyThread")}
2229 */
2230 public ForkJoinPool(int parallelism,
2231 ForkJoinWorkerThreadFactory factory,
2232 UncaughtExceptionHandler handler,
2233 boolean asyncMode) {
2234 this(parallelism, factory, handler, asyncMode,
2235 0, MAX_CAP, 1, null, DEFAULT_KEEPALIVE, TimeUnit.MILLISECONDS);
2236 }
2237
2238 /**
2239 * Creates a {@code ForkJoinPool} with the given parameters.
2240 *
2241 * @param parallelism the parallelism level. For default value,
2242 * use {@link java.lang.Runtime#availableProcessors}.
2243 *
2244 * @param factory the factory for creating new threads. For
2245 * default value, use {@link #defaultForkJoinWorkerThreadFactory}.
2246 *
2247 * @param handler the handler for internal worker threads that
2248 * terminate due to unrecoverable errors encountered while
2249 * executing tasks. For default value, use {@code null}.
2250 *
2251 * @param asyncMode if true, establishes local first-in-first-out
2252 * scheduling mode for forked tasks that are never joined. This
2253 * mode may be more appropriate than default locally stack-based
2254 * mode in applications in which worker threads only process
2255 * event-style asynchronous tasks. For default value, use {@code
2256 * false}.
2257 *
2258 * @param corePoolSize the number of threads to keep in the pool
2259 * (unless timed out after an elapsed keep-alive). Normally (and
2260 * by default) this is the same value as the parallelism level,
2261 * but may be set to a larger value to reduce dynamic overhead if
2262 * tasks regularly block. Using a smaller value (for example
2263 * {@code 0}) has the same effect as the default.
2264 *
2265 * @param maximumPoolSize the maximum number of threads allowed.
2266 * When the maximum is reached, attempts to replace blocked
2267 * threads fail. (However, because creation and termination of
2268 * different threads may overlap, and may be managed by the given
2269 * thread factory, this value may be transiently exceeded.) To
2270 * arrange the same value as is used by default for the common
2271 * pool, use {@code 256} plus the {@code parallelism} level. (By
2272 * default, the common pool allows a maximum of 256 spare
2273 * threads.) Using a value (for example {@code
2274 * Integer.MAX_VALUE}) larger than the implementation's total
2275 * thread limit has the same effect as using this limit (which is
2276 * the default).
2277 *
2278 * @param minimumRunnable the minimum allowed number of core
2279 * threads not blocked by a join or {@link ManagedBlocker}. To
2280 * ensure progress, when too few unblocked threads exist and
2281 * unexecuted tasks may exist, new threads are constructed, up to
2282 * the given maximumPoolSize. For the default value, use {@code
2283 * 1}, that ensures liveness. A larger value might improve
2284 * throughput in the presence of blocked activities, but might
2285 * not, due to increased overhead. A value of zero may be
2286 * acceptable when submitted tasks cannot have dependencies
2287 * requiring additional threads.
2288 *
2289 * @param saturate if non-null, a predicate invoked upon attempts
2290 * to create more than the maximum total allowed threads. By
2291 * default, when a thread is about to block on a join or {@link
2292 * ManagedBlocker}, but cannot be replaced because the
2293 * maximumPoolSize would be exceeded, a {@link
2294 * RejectedExecutionException} is thrown. But if this predicate
2295 * returns {@code true}, then no exception is thrown, so the pool
2296 * continues to operate with fewer than the target number of
2297 * runnable threads, which might not ensure progress.
2298 *
2299 * @param keepAliveTime the elapsed time since last use before
2300 * a thread is terminated (and then later replaced if needed).
2301 * For the default value, use {@code 60, TimeUnit.SECONDS}.
2302 *
2303 * @param unit the time unit for the {@code keepAliveTime} argument
2304 *
2305 * @throws IllegalArgumentException if parallelism is less than or
2306 * equal to zero, or is greater than implementation limit,
2307 * or if maximumPoolSize is less than parallelism,
2308 * of if the keepAliveTime is less than or equal to zero.
2309 * @throws NullPointerException if the factory is null
2310 * @throws SecurityException if a security manager exists and
2311 * the caller is not permitted to modify threads
2312 * because it does not hold {@link
2313 * java.lang.RuntimePermission}{@code ("modifyThread")}
2314 * @since 9
2315 */
2316 public ForkJoinPool(int parallelism,
2317 ForkJoinWorkerThreadFactory factory,
2318 UncaughtExceptionHandler handler,
2319 boolean asyncMode,
2320 int corePoolSize,
2321 int maximumPoolSize,
2322 int minimumRunnable,
2323 Predicate<? super ForkJoinPool> saturate,
2324 long keepAliveTime,
2325 TimeUnit unit) {
2326 // check, encode, pack parameters
2327 if (parallelism <= 0 || parallelism > MAX_CAP ||
2328 maximumPoolSize < parallelism || keepAliveTime <= 0L)
2329 throw new IllegalArgumentException();
2330 if (factory == null)
2331 throw new NullPointerException();
2332 long ms = Math.max(unit.toMillis(keepAliveTime), TIMEOUT_SLOP);
2333
2334 int corep = Math.min(Math.max(corePoolSize, parallelism), MAX_CAP);
2335 long c = ((((long)(-corep) << TC_SHIFT) & TC_MASK) |
2336 (((long)(-parallelism) << RC_SHIFT) & RC_MASK));
2337 int m = parallelism | (asyncMode ? FIFO : 0);
2338 int maxSpares = Math.min(maximumPoolSize, MAX_CAP) - parallelism;
2339 int minAvail = Math.min(Math.max(minimumRunnable, 0), MAX_CAP);
2340 int b = ((minAvail - parallelism) & SMASK) | (maxSpares << SWIDTH);
2341 int n = (parallelism > 1) ? parallelism - 1 : 1; // at least 2 slots
2342 n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8; n |= n >>> 16;
2343 n = (n + 1) << 1; // power of two, including space for submission queues
2344
2345 this.workerNamePrefix = "ForkJoinPool-" + nextPoolId() + "-worker-";
2346 this.workQueues = new WorkQueue[n];
2347 this.factory = factory;
2348 this.ueh = handler;
2349 this.saturate = saturate;
2350 this.keepAlive = ms;
2351 this.bounds = b;
2352 this.mode = m;
2353 this.ctl = c;
2354 checkPermission();
2355 }
2356
2357 private static Object newInstanceFromSystemProperty(String property)
2358 throws ReflectiveOperationException {
2359 String className = System.getProperty(property);
2360 return (className == null)
2361 ? null
2362 : ClassLoader.getSystemClassLoader().loadClass(className)
2363 .getConstructor().newInstance();
2364 }
2365
2366 /**
2367 * Constructor for common pool using parameters possibly
2368 * overridden by system properties
2369 */
2370 private ForkJoinPool(byte forCommonPoolOnly) {
2371 int parallelism = -1;
2372 ForkJoinWorkerThreadFactory fac = null;
2373 UncaughtExceptionHandler handler = null;
2374 try { // ignore exceptions in accessing/parsing properties
2375 String pp = System.getProperty
2376 ("java.util.concurrent.ForkJoinPool.common.parallelism");
2377 if (pp != null)
2378 parallelism = Integer.parseInt(pp);
2379 fac = (ForkJoinWorkerThreadFactory) newInstanceFromSystemProperty(
2380 "java.util.concurrent.ForkJoinPool.common.threadFactory");
2381 handler = (UncaughtExceptionHandler) newInstanceFromSystemProperty(
2382 "java.util.concurrent.ForkJoinPool.common.exceptionHandler");
2383 } catch (Exception ignore) {
2384 }
2385
2386 if (fac == null) {
2387 if (System.getSecurityManager() == null)
2388 fac = defaultForkJoinWorkerThreadFactory;
2389 else // use security-managed default
2390 fac = new InnocuousForkJoinWorkerThreadFactory();
2391 }
2392 if (parallelism < 0 && // default 1 less than #cores
2393 (parallelism = Runtime.getRuntime().availableProcessors() - 1) <= 0)
2394 parallelism = 1;
2395 if (parallelism > MAX_CAP)
2396 parallelism = MAX_CAP;
2397
2398 long c = ((((long)(-parallelism) << TC_SHIFT) & TC_MASK) |
2399 (((long)(-parallelism) << RC_SHIFT) & RC_MASK));
2400 int b = ((1 - parallelism) & SMASK) | (COMMON_MAX_SPARES << SWIDTH);
2401 int n = (parallelism > 1) ? parallelism - 1 : 1;
2402 n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8; n |= n >>> 16;
2403 n = (n + 1) << 1;
2404
2405 this.workerNamePrefix = "ForkJoinPool.commonPool-worker-";
2406 this.workQueues = new WorkQueue[n];
2407 this.factory = fac;
2408 this.ueh = handler;
2409 this.saturate = null;
2410 this.keepAlive = DEFAULT_KEEPALIVE;
2411 this.bounds = b;
2412 this.mode = parallelism;
2413 this.ctl = c;
2414 }
2415
2416 /**
2417 * Returns the common pool instance. This pool is statically
2418 * constructed; its run state is unaffected by attempts to {@link
2419 * #shutdown} or {@link #shutdownNow}. However this pool and any
2420 * ongoing processing are automatically terminated upon program
2421 * {@link System#exit}. Any program that relies on asynchronous
2422 * task processing to complete before program termination should
2423 * invoke {@code commonPool().}{@link #awaitQuiescence awaitQuiescence},
2424 * before exit.
2425 *
2426 * @return the common pool instance
2427 * @since 1.8
2428 */
2429 public static ForkJoinPool commonPool() {
2430 // assert common != null : "static init error";
2431 return common;
2432 }
2433
2434 // Execution methods
2435
2436 /**
2437 * Performs the given task, returning its result upon completion.
2438 * If the computation encounters an unchecked Exception or Error,
2439 * it is rethrown as the outcome of this invocation. Rethrown
2440 * exceptions behave in the same way as regular exceptions, but,
2441 * when possible, contain stack traces (as displayed for example
2442 * using {@code ex.printStackTrace()}) of both the current thread
2443 * as well as the thread actually encountering the exception;
2444 * minimally only the latter.
2445 *
2446 * @param task the task
2447 * @param <T> the type of the task's result
2448 * @return the task's result
2449 * @throws NullPointerException if the task is null
2450 * @throws RejectedExecutionException if the task cannot be
2451 * scheduled for execution
2452 */
2453 public <T> T invoke(ForkJoinTask<T> task) {
2454 if (task == null)
2455 throw new NullPointerException();
2456 externalSubmit(task);
2457 return task.join();
2458 }
2459
2460 /**
2461 * Arranges for (asynchronous) execution of the given task.
2462 *
2463 * @param task the task
2464 * @throws NullPointerException if the task is null
2465 * @throws RejectedExecutionException if the task cannot be
2466 * scheduled for execution
2467 */
2468 public void execute(ForkJoinTask<?> task) {
2469 externalSubmit(task);
2470 }
2471
2472 // AbstractExecutorService methods
2473
2474 /**
2475 * @throws NullPointerException if the task is null
2476 * @throws RejectedExecutionException if the task cannot be
2477 * scheduled for execution
2478 */
2479 public void execute(Runnable task) {
2480 if (task == null)
2481 throw new NullPointerException();
2482 ForkJoinTask<?> job;
2483 if (task instanceof ForkJoinTask<?>) // avoid re-wrap
2484 job = (ForkJoinTask<?>) task;
2485 else
2486 job = new ForkJoinTask.RunnableExecuteAction(task);
2487 externalSubmit(job);
2488 }
2489
2490 /**
2491 * Submits a ForkJoinTask for execution.
2492 *
2493 * @param task the task to submit
2494 * @param <T> the type of the task's result
2495 * @return the task
2496 * @throws NullPointerException if the task is null
2497 * @throws RejectedExecutionException if the task cannot be
2498 * scheduled for execution
2499 */
2500 public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
2501 return externalSubmit(task);
2502 }
2503
2504 /**
2505 * @throws NullPointerException if the task is null
2506 * @throws RejectedExecutionException if the task cannot be
2507 * scheduled for execution
2508 */
2509 public <T> ForkJoinTask<T> submit(Callable<T> task) {
2510 return externalSubmit(new ForkJoinTask.AdaptedCallable<T>(task));
2511 }
2512
2513 /**
2514 * @throws NullPointerException if the task is null
2515 * @throws RejectedExecutionException if the task cannot be
2516 * scheduled for execution
2517 */
2518 public <T> ForkJoinTask<T> submit(Runnable task, T result) {
2519 return externalSubmit(new ForkJoinTask.AdaptedRunnable<T>(task, result));
2520 }
2521
2522 /**
2523 * @throws NullPointerException if the task is null
2524 * @throws RejectedExecutionException if the task cannot be
2525 * scheduled for execution
2526 */
2527 @SuppressWarnings("unchecked")
2528 public ForkJoinTask<?> submit(Runnable task) {
2529 if (task == null)
2530 throw new NullPointerException();
2531 return externalSubmit((task instanceof ForkJoinTask<?>)
2532 ? (ForkJoinTask<Void>) task // avoid re-wrap
2533 : new ForkJoinTask.AdaptedRunnableAction(task));
2534 }
2535
2536 /**
2537 * @throws NullPointerException {@inheritDoc}
2538 * @throws RejectedExecutionException {@inheritDoc}
2539 */
2540 public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
2541 // In previous versions of this class, this method constructed
2542 // a task to run ForkJoinTask.invokeAll, but now external
2543 // invocation of multiple tasks is at least as efficient.
2544 ArrayList<Future<T>> futures = new ArrayList<>(tasks.size());
2545
2546 try {
2547 for (Callable<T> t : tasks) {
2548 ForkJoinTask<T> f = new ForkJoinTask.AdaptedCallable<T>(t);
2549 futures.add(f);
2550 externalSubmit(f);
2551 }
2552 for (int i = 0, size = futures.size(); i < size; i++)
2553 ((ForkJoinTask<?>)futures.get(i)).quietlyJoin();
2554 return futures;
2555 } catch (Throwable t) {
2556 for (int i = 0, size = futures.size(); i < size; i++)
2557 futures.get(i).cancel(false);
2558 throw t;
2559 }
2560 }
2561
2562 /**
2563 * Returns the factory used for constructing new workers.
2564 *
2565 * @return the factory used for constructing new workers
2566 */
2567 public ForkJoinWorkerThreadFactory getFactory() {
2568 return factory;
2569 }
2570
2571 /**
2572 * Returns the handler for internal worker threads that terminate
2573 * due to unrecoverable errors encountered while executing tasks.
2574 *
2575 * @return the handler, or {@code null} if none
2576 */
2577 public UncaughtExceptionHandler getUncaughtExceptionHandler() {
2578 return ueh;
2579 }
2580
2581 /**
2582 * Returns the targeted parallelism level of this pool.
2583 *
2584 * @return the targeted parallelism level of this pool
2585 */
2586 public int getParallelism() {
2587 int par = mode & SMASK;
2588 return (par > 0) ? par : 1;
2589 }
2590
2591 /**
2592 * Returns the targeted parallelism level of the common pool.
2593 *
2594 * @return the targeted parallelism level of the common pool
2595 * @since 1.8
2596 */
2597 public static int getCommonPoolParallelism() {
2598 return COMMON_PARALLELISM;
2599 }
2600
2601 /**
2602 * Returns the number of worker threads that have started but not
2603 * yet terminated. The result returned by this method may differ
2604 * from {@link #getParallelism} when threads are created to
2605 * maintain parallelism when others are cooperatively blocked.
2606 *
2607 * @return the number of worker threads
2608 */
2609 public int getPoolSize() {
2610 return ((mode & SMASK) + (short)(ctl >>> TC_SHIFT));
2611 }
2612
2613 /**
2614 * Returns {@code true} if this pool uses local first-in-first-out
2615 * scheduling mode for forked tasks that are never joined.
2616 *
2617 * @return {@code true} if this pool uses async mode
2618 */
2619 public boolean getAsyncMode() {
2620 return (mode & FIFO) != 0;
2621 }
2622
2623 /**
2624 * Returns an estimate of the number of worker threads that are
2625 * not blocked waiting to join tasks or for other managed
2626 * synchronization. This method may overestimate the
2627 * number of running threads.
2628 *
2629 * @return the number of worker threads
2630 */
2631 public int getRunningThreadCount() {
2632 int rc = 0;
2633 WorkQueue[] ws; WorkQueue w;
2634 if ((ws = workQueues) != null) {
2635 for (int i = 1; i < ws.length; i += 2) {
2636 if ((w = ws[i]) != null && w.isApparentlyUnblocked())
2637 ++rc;
2638 }
2639 }
2640 return rc;
2641 }
2642
2643 /**
2644 * Returns an estimate of the number of threads that are currently
2645 * stealing or executing tasks. This method may overestimate the
2646 * number of active threads.
2647 *
2648 * @return the number of active threads
2649 */
2650 public int getActiveThreadCount() {
2651 int r = (mode & SMASK) + (int)(ctl >> RC_SHIFT);
2652 return (r <= 0) ? 0 : r; // suppress momentarily negative values
2653 }
2654
2655 /**
2656 * Returns {@code true} if all worker threads are currently idle.
2657 * An idle worker is one that cannot obtain a task to execute
2658 * because none are available to steal from other threads, and
2659 * there are no pending submissions to the pool. This method is
2660 * conservative; it might not return {@code true} immediately upon
2661 * idleness of all threads, but will eventually become true if
2662 * threads remain inactive.
2663 *
2664 * @return {@code true} if all threads are currently idle
2665 */
2666 public boolean isQuiescent() {
2667 for (;;) {
2668 long c = ctl;
2669 int md = mode, pc = md & SMASK;
2670 int tc = pc + (short)(c >>> TC_SHIFT);
2671 int rc = pc + (int)(c >> RC_SHIFT);
2672 if ((md & (STOP | TERMINATED)) != 0)
2673 return true;
2674 else if (rc > 0)
2675 return false;
2676 else {
2677 WorkQueue[] ws; WorkQueue v;
2678 if ((ws = workQueues) != null) {
2679 for (int i = 1; i < ws.length; i += 2) {
2680 if ((v = ws[i]) != null) {
2681 if ((v.source & QUIET) == 0)
2682 return false;
2683 --tc;
2684 }
2685 }
2686 }
2687 if (tc == 0 && ctl == c)
2688 return true;
2689 }
2690 }
2691 }
2692
2693 /**
2694 * Returns an estimate of the total number of tasks stolen from
2695 * one thread's work queue by another. The reported value
2696 * underestimates the actual total number of steals when the pool
2697 * is not quiescent. This value may be useful for monitoring and
2698 * tuning fork/join programs: in general, steal counts should be
2699 * high enough to keep threads busy, but low enough to avoid
2700 * overhead and contention across threads.
2701 *
2702 * @return the number of steals
2703 */
2704 public long getStealCount() {
2705 long count = stealCount;
2706 WorkQueue[] ws; WorkQueue w;
2707 if ((ws = workQueues) != null) {
2708 for (int i = 1; i < ws.length; i += 2) {
2709 if ((w = ws[i]) != null)
2710 count += (long)w.nsteals & 0xffffffffL;
2711 }
2712 }
2713 return count;
2714 }
2715
2716 /**
2717 * Returns an estimate of the total number of tasks currently held
2718 * in queues by worker threads (but not including tasks submitted
2719 * to the pool that have not begun executing). This value is only
2720 * an approximation, obtained by iterating across all threads in
2721 * the pool. This method may be useful for tuning task
2722 * granularities.
2723 *
2724 * @return the number of queued tasks
2725 */
2726 public long getQueuedTaskCount() {
2727 long count = 0;
2728 WorkQueue[] ws; WorkQueue w;
2729 if ((ws = workQueues) != null) {
2730 for (int i = 1; i < ws.length; i += 2) {
2731 if ((w = ws[i]) != null)
2732 count += w.queueSize();
2733 }
2734 }
2735 return count;
2736 }
2737
2738 /**
2739 * Returns an estimate of the number of tasks submitted to this
2740 * pool that have not yet begun executing. This method may take
2741 * time proportional to the number of submissions.
2742 *
2743 * @return the number of queued submissions
2744 */
2745 public int getQueuedSubmissionCount() {
2746 int count = 0;
2747 WorkQueue[] ws; WorkQueue w;
2748 if ((ws = workQueues) != null) {
2749 for (int i = 0; i < ws.length; i += 2) {
2750 if ((w = ws[i]) != null)
2751 count += w.queueSize();
2752 }
2753 }
2754 return count;
2755 }
2756
2757 /**
2758 * Returns {@code true} if there are any tasks submitted to this
2759 * pool that have not yet begun executing.
2760 *
2761 * @return {@code true} if there are any queued submissions
2762 */
2763 public boolean hasQueuedSubmissions() {
2764 WorkQueue[] ws; WorkQueue w;
2765 if ((ws = workQueues) != null) {
2766 for (int i = 0; i < ws.length; i += 2) {
2767 if ((w = ws[i]) != null && !w.isEmpty())
2768 return true;
2769 }
2770 }
2771 return false;
2772 }
2773
2774 /**
2775 * Removes and returns the next unexecuted submission if one is
2776 * available. This method may be useful in extensions to this
2777 * class that re-assign work in systems with multiple pools.
2778 *
2779 * @return the next submission, or {@code null} if none
2780 */
2781 protected ForkJoinTask<?> pollSubmission() {
2782 return pollScan(true);
2783 }
2784
2785 /**
2786 * Removes all available unexecuted submitted and forked tasks
2787 * from scheduling queues and adds them to the given collection,
2788 * without altering their execution status. These may include
2789 * artificially generated or wrapped tasks. This method is
2790 * designed to be invoked only when the pool is known to be
2791 * quiescent. Invocations at other times may not remove all
2792 * tasks. A failure encountered while attempting to add elements
2793 * to collection {@code c} may result in elements being in
2794 * neither, either or both collections when the associated
2795 * exception is thrown. The behavior of this operation is
2796 * undefined if the specified collection is modified while the
2797 * operation is in progress.
2798 *
2799 * @param c the collection to transfer elements into
2800 * @return the number of elements transferred
2801 */
2802 protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
2803 int count = 0;
2804 WorkQueue[] ws; WorkQueue w; ForkJoinTask<?> t;
2805 if ((ws = workQueues) != null) {
2806 for (int i = 0; i < ws.length; ++i) {
2807 if ((w = ws[i]) != null) {
2808 while ((t = w.poll()) != null) {
2809 c.add(t);
2810 ++count;
2811 }
2812 }
2813 }
2814 }
2815 return count;
2816 }
2817
2818 /**
2819 * Returns a string identifying this pool, as well as its state,
2820 * including indications of run state, parallelism level, and
2821 * worker and task counts.
2822 *
2823 * @return a string identifying this pool, as well as its state
2824 */
2825 public String toString() {
2826 // Use a single pass through workQueues to collect counts
2827 long qt = 0L, qs = 0L; int rc = 0;
2828 long st = stealCount;
2829 WorkQueue[] ws; WorkQueue w;
2830 if ((ws = workQueues) != null) {
2831 for (int i = 0; i < ws.length; ++i) {
2832 if ((w = ws[i]) != null) {
2833 int size = w.queueSize();
2834 if ((i & 1) == 0)
2835 qs += size;
2836 else {
2837 qt += size;
2838 st += (long)w.nsteals & 0xffffffffL;
2839 if (w.isApparentlyUnblocked())
2840 ++rc;
2841 }
2842 }
2843 }
2844 }
2845
2846 int md = mode;
2847 int pc = (md & SMASK);
2848 long c = ctl;
2849 int tc = pc + (short)(c >>> TC_SHIFT);
2850 int ac = pc + (int)(c >> RC_SHIFT);
2851 if (ac < 0) // ignore transient negative
2852 ac = 0;
2853 String level = ((md & TERMINATED) != 0 ? "Terminated" :
2854 (md & STOP) != 0 ? "Terminating" :
2855 (md & SHUTDOWN) != 0 ? "Shutting down" :
2856 "Running");
2857 return super.toString() +
2858 "[" + level +
2859 ", parallelism = " + pc +
2860 ", size = " + tc +
2861 ", active = " + ac +
2862 ", running = " + rc +
2863 ", steals = " + st +
2864 ", tasks = " + qt +
2865 ", submissions = " + qs +
2866 "]";
2867 }
2868
2869 /**
2870 * Possibly initiates an orderly shutdown in which previously
2871 * submitted tasks are executed, but no new tasks will be
2872 * accepted. Invocation has no effect on execution state if this
2873 * is the {@link #commonPool()}, and no additional effect if
2874 * already shut down. Tasks that are in the process of being
2875 * submitted concurrently during the course of this method may or
2876 * may not be rejected.
2877 *
2878 * @throws SecurityException if a security manager exists and
2879 * the caller is not permitted to modify threads
2880 * because it does not hold {@link
2881 * java.lang.RuntimePermission}{@code ("modifyThread")}
2882 */
2883 public void shutdown() {
2884 checkPermission();
2885 tryTerminate(false, true);
2886 }
2887
2888 /**
2889 * Possibly attempts to cancel and/or stop all tasks, and reject
2890 * all subsequently submitted tasks. Invocation has no effect on
2891 * execution state if this is the {@link #commonPool()}, and no
2892 * additional effect if already shut down. Otherwise, tasks that
2893 * are in the process of being submitted or executed concurrently
2894 * during the course of this method may or may not be
2895 * rejected. This method cancels both existing and unexecuted
2896 * tasks, in order to permit termination in the presence of task
2897 * dependencies. So the method always returns an empty list
2898 * (unlike the case for some other Executors).
2899 *
2900 * @return an empty list
2901 * @throws SecurityException if a security manager exists and
2902 * the caller is not permitted to modify threads
2903 * because it does not hold {@link
2904 * java.lang.RuntimePermission}{@code ("modifyThread")}
2905 */
2906 public List<Runnable> shutdownNow() {
2907 checkPermission();
2908 tryTerminate(true, true);
2909 return Collections.emptyList();
2910 }
2911
2912 /**
2913 * Returns {@code true} if all tasks have completed following shut down.
2914 *
2915 * @return {@code true} if all tasks have completed following shut down
2916 */
2917 public boolean isTerminated() {
2918 return (mode & TERMINATED) != 0;
2919 }
2920
2921 /**
2922 * Returns {@code true} if the process of termination has
2923 * commenced but not yet completed. This method may be useful for
2924 * debugging. A return of {@code true} reported a sufficient
2925 * period after shutdown may indicate that submitted tasks have
2926 * ignored or suppressed interruption, or are waiting for I/O,
2927 * causing this executor not to properly terminate. (See the
2928 * advisory notes for class {@link ForkJoinTask} stating that
2929 * tasks should not normally entail blocking operations. But if
2930 * they do, they must abort them on interrupt.)
2931 *
2932 * @return {@code true} if terminating but not yet terminated
2933 */
2934 public boolean isTerminating() {
2935 int md = mode;
2936 return (md & STOP) != 0 && (md & TERMINATED) == 0;
2937 }
2938
2939 /**
2940 * Returns {@code true} if this pool has been shut down.
2941 *
2942 * @return {@code true} if this pool has been shut down
2943 */
2944 public boolean isShutdown() {
2945 return (mode & SHUTDOWN) != 0;
2946 }
2947
2948 /**
2949 * Blocks until all tasks have completed execution after a
2950 * shutdown request, or the timeout occurs, or the current thread
2951 * is interrupted, whichever happens first. Because the {@link
2952 * #commonPool()} never terminates until program shutdown, when
2953 * applied to the common pool, this method is equivalent to {@link
2954 * #awaitQuiescence(long, TimeUnit)} but always returns {@code false}.
2955 *
2956 * @param timeout the maximum time to wait
2957 * @param unit the time unit of the timeout argument
2958 * @return {@code true} if this executor terminated and
2959 * {@code false} if the timeout elapsed before termination
2960 * @throws InterruptedException if interrupted while waiting
2961 */
2962 public boolean awaitTermination(long timeout, TimeUnit unit)
2963 throws InterruptedException {
2964 if (Thread.interrupted())
2965 throw new InterruptedException();
2966 if (this == common) {
2967 awaitQuiescence(timeout, unit);
2968 return false;
2969 }
2970 long nanos = unit.toNanos(timeout);
2971 if (isTerminated())
2972 return true;
2973 if (nanos <= 0L)
2974 return false;
2975 long deadline = System.nanoTime() + nanos;
2976 synchronized (this) {
2977 for (;;) {
2978 if (isTerminated())
2979 return true;
2980 if (nanos <= 0L)
2981 return false;
2982 long millis = TimeUnit.NANOSECONDS.toMillis(nanos);
2983 wait(millis > 0L ? millis : 1L);
2984 nanos = deadline - System.nanoTime();
2985 }
2986 }
2987 }
2988
2989 /**
2990 * If called by a ForkJoinTask operating in this pool, equivalent
2991 * in effect to {@link ForkJoinTask#helpQuiesce}. Otherwise,
2992 * waits and/or attempts to assist performing tasks until this
2993 * pool {@link #isQuiescent} or the indicated timeout elapses.
2994 *
2995 * @param timeout the maximum time to wait
2996 * @param unit the time unit of the timeout argument
2997 * @return {@code true} if quiescent; {@code false} if the
2998 * timeout elapsed.
2999 */
3000 public boolean awaitQuiescence(long timeout, TimeUnit unit) {
3001 long nanos = unit.toNanos(timeout);
3002 ForkJoinWorkerThread wt;
3003 Thread thread = Thread.currentThread();
3004 if ((thread instanceof ForkJoinWorkerThread) &&
3005 (wt = (ForkJoinWorkerThread)thread).pool == this) {
3006 helpQuiescePool(wt.workQueue);
3007 return true;
3008 }
3009 else {
3010 for (long startTime = System.nanoTime();;) {
3011 ForkJoinTask<?> t;
3012 if ((t = pollScan(false)) != null)
3013 t.doExec();
3014 else if (isQuiescent())
3015 return true;
3016 else if ((System.nanoTime() - startTime) > nanos)
3017 return false;
3018 else
3019 Thread.yield(); // cannot block
3020 }
3021 }
3022 }
3023
3024 /**
3025 * Waits and/or attempts to assist performing tasks indefinitely
3026 * until the {@link #commonPool()} {@link #isQuiescent}.
3027 */
3028 static void quiesceCommonPool() {
3029 common.awaitQuiescence(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
3030 }
3031
3032 /**
3033 * Interface for extending managed parallelism for tasks running
3034 * in {@link ForkJoinPool}s.
3035 *
3036 * <p>A {@code ManagedBlocker} provides two methods. Method
3037 * {@link #isReleasable} must return {@code true} if blocking is
3038 * not necessary. Method {@link #block} blocks the current thread
3039 * if necessary (perhaps internally invoking {@code isReleasable}
3040 * before actually blocking). These actions are performed by any
3041 * thread invoking {@link ForkJoinPool#managedBlock(ManagedBlocker)}.
3042 * The unusual methods in this API accommodate synchronizers that
3043 * may, but don't usually, block for long periods. Similarly, they
3044 * allow more efficient internal handling of cases in which
3045 * additional workers may be, but usually are not, needed to
3046 * ensure sufficient parallelism. Toward this end,
3047 * implementations of method {@code isReleasable} must be amenable
3048 * to repeated invocation.
3049 *
3050 * <p>For example, here is a ManagedBlocker based on a
3051 * ReentrantLock:
3052 * <pre> {@code
3053 * class ManagedLocker implements ManagedBlocker {
3054 * final ReentrantLock lock;
3055 * boolean hasLock = false;
3056 * ManagedLocker(ReentrantLock lock) { this.lock = lock; }
3057 * public boolean block() {
3058 * if (!hasLock)
3059 * lock.lock();
3060 * return true;
3061 * }
3062 * public boolean isReleasable() {
3063 * return hasLock || (hasLock = lock.tryLock());
3064 * }
3065 * }}</pre>
3066 *
3067 * <p>Here is a class that possibly blocks waiting for an
3068 * item on a given queue:
3069 * <pre> {@code
3070 * class QueueTaker<E> implements ManagedBlocker {
3071 * final BlockingQueue<E> queue;
3072 * volatile E item = null;
3073 * QueueTaker(BlockingQueue<E> q) { this.queue = q; }
3074 * public boolean block() throws InterruptedException {
3075 * if (item == null)
3076 * item = queue.take();
3077 * return true;
3078 * }
3079 * public boolean isReleasable() {
3080 * return item != null || (item = queue.poll()) != null;
3081 * }
3082 * public E getItem() { // call after pool.managedBlock completes
3083 * return item;
3084 * }
3085 * }}</pre>
3086 */
3087 public static interface ManagedBlocker {
3088 /**
3089 * Possibly blocks the current thread, for example waiting for
3090 * a lock or condition.
3091 *
3092 * @return {@code true} if no additional blocking is necessary
3093 * (i.e., if isReleasable would return true)
3094 * @throws InterruptedException if interrupted while waiting
3095 * (the method is not required to do so, but is allowed to)
3096 */
3097 boolean block() throws InterruptedException;
3098
3099 /**
3100 * Returns {@code true} if blocking is unnecessary.
3101 * @return {@code true} if blocking is unnecessary
3102 */
3103 boolean isReleasable();
3104 }
3105
3106 /**
3107 * Runs the given possibly blocking task. When {@linkplain
3108 * ForkJoinTask#inForkJoinPool() running in a ForkJoinPool}, this
3109 * method possibly arranges for a spare thread to be activated if
3110 * necessary to ensure sufficient parallelism while the current
3111 * thread is blocked in {@link ManagedBlocker#block blocker.block()}.
3112 *
3113 * <p>This method repeatedly calls {@code blocker.isReleasable()} and
3114 * {@code blocker.block()} until either method returns {@code true}.
3115 * Every call to {@code blocker.block()} is preceded by a call to
3116 * {@code blocker.isReleasable()} that returned {@code false}.
3117 *
3118 * <p>If not running in a ForkJoinPool, this method is
3119 * behaviorally equivalent to
3120 * <pre> {@code
3121 * while (!blocker.isReleasable())
3122 * if (blocker.block())
3123 * break;}</pre>
3124 *
3125 * If running in a ForkJoinPool, the pool may first be expanded to
3126 * ensure sufficient parallelism available during the call to
3127 * {@code blocker.block()}.
3128 *
3129 * @param blocker the blocker task
3130 * @throws InterruptedException if {@code blocker.block()} did so
3131 */
3132 public static void managedBlock(ManagedBlocker blocker)
3133 throws InterruptedException {
3134 ForkJoinPool p;
3135 ForkJoinWorkerThread wt;
3136 WorkQueue w;
3137 Thread t = Thread.currentThread();
3138 if ((t instanceof ForkJoinWorkerThread) &&
3139 (p = (wt = (ForkJoinWorkerThread)t).pool) != null &&
3140 (w = wt.workQueue) != null) {
3141 int block;
3142 while (!blocker.isReleasable()) {
3143 if ((block = p.tryCompensate(w)) != 0) {
3144 try {
3145 do {} while (!blocker.isReleasable() &&
3146 !blocker.block());
3147 } finally {
3148 CTL.getAndAdd(p, (block > 0) ? RC_UNIT : 0L);
3149 }
3150 break;
3151 }
3152 }
3153 }
3154 else {
3155 do {} while (!blocker.isReleasable() &&
3156 !blocker.block());
3157 }
3158 }
3159
3160 /**
3161 * If the given executor is a ForkJoinPool, poll and execute
3162 * AsynchronousCompletionTasks from worker's queue until none are
3163 * available or blocker is released.
3164 */
3165 static void helpAsyncBlocker(Executor e, ManagedBlocker blocker) {
3166 if (blocker != null && (e instanceof ForkJoinPool)) {
3167 WorkQueue w; ForkJoinWorkerThread wt; WorkQueue[] ws; int r, n;
3168 ForkJoinPool p = (ForkJoinPool)e;
3169 Thread thread = Thread.currentThread();
3170 if (thread instanceof ForkJoinWorkerThread &&
3171 (wt = (ForkJoinWorkerThread)thread).pool == p)
3172 w = wt.workQueue;
3173 else if ((r = ThreadLocalRandom.getProbe()) != 0 &&
3174 (ws = p.workQueues) != null && (n = ws.length) > 0)
3175 w = ws[(n - 1) & r & SQMASK];
3176 else
3177 w = null;
3178 if (w != null) {
3179 for (;;) {
3180 int b = w.base, s = w.top, d, al; ForkJoinTask<?>[] a;
3181 if ((a = w.array) != null && (d = b - s) < 0 &&
3182 (al = a.length) > 0) {
3183 int index = (al - 1) & b;
3184 ForkJoinTask<?> t = (ForkJoinTask<?>)
3185 QA.getAcquire(a, index);
3186 if (blocker.isReleasable())
3187 break;
3188 else if (b++ == w.base) {
3189 if (t == null) {
3190 if (d == -1)
3191 break;
3192 }
3193 else if (!(t instanceof CompletableFuture.
3194 AsynchronousCompletionTask))
3195 break;
3196 else if (QA.compareAndSet(a, index, t, null)) {
3197 w.base = b;
3198 t.doExec();
3199 }
3200 }
3201 }
3202 else
3203 break;
3204 }
3205 }
3206 }
3207 }
3208
3209 // AbstractExecutorService overrides. These rely on undocumented
3210 // fact that ForkJoinTask.adapt returns ForkJoinTasks that also
3211 // implement RunnableFuture.
3212
3213 protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
3214 return new ForkJoinTask.AdaptedRunnable<T>(runnable, value);
3215 }
3216
3217 protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
3218 return new ForkJoinTask.AdaptedCallable<T>(callable);
3219 }
3220
3221 // VarHandle mechanics
3222 private static final VarHandle CTL;
3223 private static final VarHandle MODE;
3224 private static final VarHandle QA;
3225
3226 static {
3227 try {
3228 MethodHandles.Lookup l = MethodHandles.lookup();
3229 CTL = l.findVarHandle(ForkJoinPool.class, "ctl", long.class);
3230 MODE = l.findVarHandle(ForkJoinPool.class, "mode", int.class);
3231 QA = MethodHandles.arrayElementVarHandle(ForkJoinTask[].class);
3232 } catch (ReflectiveOperationException e) {
3233 throw new Error(e);
3234 }
3235
3236 // Reduce the risk of rare disastrous classloading in first call to
3237 // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
3238 Class<?> ensureLoaded = LockSupport.class;
3239
3240 int commonMaxSpares = DEFAULT_COMMON_MAX_SPARES;
3241 try {
3242 String p = System.getProperty
3243 ("java.util.concurrent.ForkJoinPool.common.maximumSpares");
3244 if (p != null)
3245 commonMaxSpares = Integer.parseInt(p);
3246 } catch (Exception ignore) {}
3247 COMMON_MAX_SPARES = commonMaxSpares;
3248
3249 defaultForkJoinWorkerThreadFactory =
3250 new DefaultForkJoinWorkerThreadFactory();
3251 modifyThreadPermission = new RuntimePermission("modifyThread");
3252
3253 common = AccessController.doPrivileged(new PrivilegedAction<>() {
3254 public ForkJoinPool run() {
3255 return new ForkJoinPool((byte)0); }});
3256
3257 COMMON_PARALLELISM = Math.max(common.mode & SMASK, 1);
3258 }
3259
3260 /**
3261 * Factory for innocuous worker threads.
3262 */
3263 private static final class InnocuousForkJoinWorkerThreadFactory
3264 implements ForkJoinWorkerThreadFactory {
3265
3266 /**
3267 * An ACC to restrict permissions for the factory itself.
3268 * The constructed workers have no permissions set.
3269 */
3270 private static final AccessControlContext ACC = contextWithPermissions(
3271 modifyThreadPermission,
3272 new RuntimePermission("enableContextClassLoaderOverride"),
3273 new RuntimePermission("modifyThreadGroup"),
3274 new RuntimePermission("getClassLoader"),
3275 new RuntimePermission("setContextClassLoader"));
3276
3277 public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
3278 return AccessController.doPrivileged(
3279 new PrivilegedAction<>() {
3280 public ForkJoinWorkerThread run() {
3281 return new ForkJoinWorkerThread.
3282 InnocuousForkJoinWorkerThread(pool); }},
3283 ACC);
3284 }
3285 }
3286 }