/* * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ /* * This file is available under and governed by the GNU General Public * License version 2 only, as published by the Free Software Foundation. * However, the following notice accompanied the original version of this * file: * * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/1.0/ */ package java.util.concurrent; import java.lang.Thread.UncaughtExceptionHandler; import java.util.ArrayList; import java.util.Arrays; import java.util.Collection; import java.util.Collections; import java.util.List; import java.util.concurrent.AbstractExecutorService; import java.util.concurrent.Callable; import java.util.concurrent.ExecutorService; import java.util.concurrent.Future; import java.util.concurrent.RejectedExecutionException; import java.util.concurrent.RunnableFuture; import java.util.concurrent.ThreadLocalRandom; import java.util.concurrent.TimeUnit; import java.util.concurrent.atomic.AtomicLong; import java.security.AccessControlContext; import java.security.ProtectionDomain; import java.security.Permissions; /** * An {@link ExecutorService} for running {@link ForkJoinTask}s. * A {@code ForkJoinPool} provides the entry point for submissions * from non-{@code ForkJoinTask} clients, as well as management and * monitoring operations. * *

A {@code ForkJoinPool} differs from other kinds of {@link * ExecutorService} mainly by virtue of employing * work-stealing: all threads in the pool attempt to find and * execute tasks submitted to the pool and/or created by other active * tasks (eventually blocking waiting for work if none exist). This * enables efficient processing when most tasks spawn other subtasks * (as do most {@code ForkJoinTask}s), as well as when many small * tasks are submitted to the pool from external clients. Especially * when setting asyncMode to true in constructors, {@code * ForkJoinPool}s may also be appropriate for use with event-style * tasks that are never joined. * *

A static {@link #commonPool()} is available and appropriate for * most applications. The common pool is used by any ForkJoinTask that * is not explicitly submitted to a specified pool. Using the common * pool normally reduces resource usage (its threads are slowly * reclaimed during periods of non-use, and reinstated upon subsequent * use). * *

For applications that require separate or custom pools, a {@code * ForkJoinPool} may be constructed with a given target parallelism * level; by default, equal to the number of available processors. * The pool attempts to maintain enough active (or available) threads * by dynamically adding, suspending, or resuming internal worker * threads, even if some tasks are stalled waiting to join others. * However, no such adjustments are guaranteed in the face of blocked * I/O or other unmanaged synchronization. The nested {@link * ManagedBlocker} interface enables extension of the kinds of * synchronization accommodated. * *

In addition to execution and lifecycle control methods, this * class provides status check methods (for example * {@link #getStealCount}) that are intended to aid in developing, * tuning, and monitoring fork/join applications. Also, method * {@link #toString} returns indications of pool state in a * convenient form for informal monitoring. * *

As is the case with other ExecutorServices, there are three * main task execution methods summarized in the following table. * These are designed to be used primarily by clients not already * engaged in fork/join computations in the current pool. The main * forms of these methods accept instances of {@code ForkJoinTask}, * but overloaded forms also allow mixed execution of plain {@code * Runnable}- or {@code Callable}- based activities as well. However, * tasks that are already executing in a pool should normally instead * use the within-computation forms listed in the table unless using * async event-style tasks that are not usually joined, in which case * there is little difference among choice of methods. * * * * * * * * * * * * * * * * * * * * * * * *
Summary of task execution methods
Call from non-fork/join clients Call from within fork/join computations
Arrange async execution {@link #execute(ForkJoinTask)} {@link ForkJoinTask#fork}
Await and obtain result {@link #invoke(ForkJoinTask)} {@link ForkJoinTask#invoke}
Arrange exec and obtain Future {@link #submit(ForkJoinTask)} {@link ForkJoinTask#fork} (ForkJoinTasks are Futures)
* *

The common pool is by default constructed with default * parameters, but these may be controlled by setting three * {@linkplain System#getProperty system properties}: *

* If a {@link SecurityManager} is present and no factory is * specified, then the default pool uses a factory supplying * threads that have no {@link Permissions} enabled. * The system class loader is used to load these classes. * Upon any error in establishing these settings, default parameters * are used. It is possible to disable or limit the use of threads in * the common pool by setting the parallelism property to zero, and/or * using a factory that may return {@code null}. However doing so may * cause unjoined tasks to never be executed. * *

Implementation notes: This implementation restricts the * maximum number of running threads to 32767. Attempts to create * pools with greater than the maximum number result in * {@code IllegalArgumentException}. * *

This implementation rejects submitted tasks (that is, by throwing * {@link RejectedExecutionException}) only when the pool is shut down * or internal resources have been exhausted. * * @since 1.7 * @author Doug Lea */ @sun.misc.Contended public class ForkJoinPool extends AbstractExecutorService { /* * Implementation Overview * * This class and its nested classes provide the main * functionality and control for a set of worker threads: * Submissions from non-FJ threads enter into submission queues. * Workers take these tasks and typically split them into subtasks * that may be stolen by other workers. Preference rules give * first priority to processing tasks from their own queues (LIFO * or FIFO, depending on mode), then to randomized FIFO steals of * tasks in other queues. This framework began as vehicle for * supporting tree-structured parallelism using work-stealing. * Over time, its scalability advantages led to extensions and * changes to better support more diverse usage contexts. Because * most internal methods and nested classes are interrelated, * their main rationale and descriptions are presented here; * individual methods and nested classes contain only brief * comments about details. * * WorkQueues * ========== * * Most operations occur within work-stealing queues (in nested * class WorkQueue). These are special forms of Deques that * support only three of the four possible end-operations -- push, * pop, and poll (aka steal), under the further constraints that * push and pop are called only from the owning thread (or, as * extended here, under a lock), while poll may be called from * other threads. (If you are unfamiliar with them, you probably * want to read Herlihy and Shavit's book "The Art of * Multiprocessor programming", chapter 16 describing these in * more detail before proceeding.) The main work-stealing queue * design is roughly similar to those in the papers "Dynamic * Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005 * (http://research.sun.com/scalable/pubs/index.html) and * "Idempotent work stealing" by Michael, Saraswat, and Vechev, * PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186). * The main differences ultimately stem from GC requirements that * we null out taken slots as soon as we can, to maintain as small * a footprint as possible even in programs generating huge * numbers of tasks. To accomplish this, we shift the CAS * arbitrating pop vs poll (steal) from being on the indices * ("base" and "top") to the slots themselves. * * Adding tasks then takes the form of a classic array push(task): * q.array[q.top] = task; ++q.top; * * (The actual code needs to null-check and size-check the array, * properly fence the accesses, and possibly signal waiting * workers to start scanning -- see below.) Both a successful pop * and poll mainly entail a CAS of a slot from non-null to null. * * The pop operation (always performed by owner) is: * if ((base != top) and * (the task at top slot is not null) and * (CAS slot to null)) * decrement top and return task; * * And the poll operation (usually by a stealer) is * if ((base != top) and * (the task at base slot is not null) and * (base has not changed) and * (CAS slot to null)) * increment base and return task; * * Because we rely on CASes of references, we do not need tag bits * on base or top. They are simple ints as used in any circular * array-based queue (see for example ArrayDeque). Updates to the * indices guarantee that top == base means the queue is empty, * but otherwise may err on the side of possibly making the queue * appear nonempty when a push, pop, or poll have not fully * committed. (Method isEmpty() checks the case of a partially * completed removal of the last element.) Because of this, the * poll operation, considered individually, is not wait-free. One * thief cannot successfully continue until another in-progress * one (or, if previously empty, a push) completes. However, in * the aggregate, we ensure at least probabilistic * non-blockingness. If an attempted steal fails, a thief always * chooses a different random victim target to try next. So, in * order for one thief to progress, it suffices for any * in-progress poll or new push on any empty queue to * complete. (This is why we normally use method pollAt and its * variants that try once at the apparent base index, else * consider alternative actions, rather than method poll, which * retries.) * * This approach also enables support of a user mode in which * local task processing is in FIFO, not LIFO order, simply by * using poll rather than pop. This can be useful in * message-passing frameworks in which tasks are never joined. * However neither mode considers affinities, loads, cache * localities, etc, so rarely provide the best possible * performance on a given machine, but portably provide good * throughput by averaging over these factors. Further, even if * we did try to use such information, we do not usually have a * basis for exploiting it. For example, some sets of tasks * profit from cache affinities, but others are harmed by cache * pollution effects. Additionally, even though it requires * scanning, long-term throughput is often best using random * selection rather than directed selection policies, so cheap * randomization of sufficient quality is used whenever * applicable. Various Marsaglia XorShifts (some with different * shift constants) are inlined at use points. * * WorkQueues are also used in a similar way for tasks submitted * to the pool. We cannot mix these tasks in the same queues used * by workers. Instead, we randomly associate submission queues * with submitting threads, using a form of hashing. The * ThreadLocalRandom probe value serves as a hash code for * choosing existing queues, and may be randomly repositioned upon * contention with other submitters. In essence, submitters act * like workers except that they are restricted to executing local * tasks that they submitted (or in the case of CountedCompleters, * others with the same root task). Insertion of tasks in shared * mode requires a lock (mainly to protect in the case of * resizing) but we use only a simple spinlock (using field * qlock), because submitters encountering a busy queue move on to * try or create other queues -- they block only when creating and * registering new queues. Additionally, "qlock" saturates to an * unlockable value (-1) at shutdown. Unlocking still can be and * is performed by cheaper ordered writes of "qlock" in successful * cases, but uses CAS in unsuccessful cases. * * Management * ========== * * The main throughput advantages of work-stealing stem from * decentralized control -- workers mostly take tasks from * themselves or each other, at rates that can exceed a billion * per second. The pool itself creates, activates (enables * scanning for and running tasks), deactivates, blocks, and * terminates threads, all with minimal central information. * There are only a few properties that we can globally track or * maintain, so we pack them into a small number of variables, * often maintaining atomicity without blocking or locking. * Nearly all essentially atomic control state is held in two * volatile variables that are by far most often read (not * written) as status and consistency checks. (Also, field * "config" holds unchanging configuration state.) * * Field "ctl" contains 64 bits holding information needed to * atomically decide to add, inactivate, enqueue (on an event * queue), dequeue, and/or re-activate workers. To enable this * packing, we restrict maximum parallelism to (1<<15)-1 (which is * far in excess of normal operating range) to allow ids, counts, * and their negations (used for thresholding) to fit into 16bit * subfields. * * Field "runState" holds lockable state bits (STARTED, STOP, etc) * also protecting updates to the workQueues array. When used as * a lock, it is normally held only for a few instructions (the * only exceptions are one-time array initialization and uncommon * resizing), so is nearly always available after at most a brief * spin. But to be extra-cautious, after spinning, method * awaitRunStateLock (called only if an initial CAS fails), uses a * wait/notify mechanics on a builtin monitor to block when * (rarely) needed. This would be a terrible idea for a highly * contended lock, but most pools run without the lock ever * contending after the spin limit, so this works fine as a more * conservative alternative. Because we don't otherwise have an * internal Object to use as a monitor, the "stealCounter" (an * AtomicLong) is used when available (it too must be lazily * initialized; see externalSubmit). * * Usages of "runState" vs "ctl" interact in only one case: * deciding to add a worker thread (see tryAddWorker), in which * case the ctl CAS is performed while the lock is held. * * Recording WorkQueues. WorkQueues are recorded in the * "workQueues" array. The array is created upon first use (see * externalSubmit) and expanded if necessary. Updates to the * array while recording new workers and unrecording terminated * ones are protected from each other by the runState lock, but * the array is otherwise concurrently readable, and accessed * directly. We also ensure that reads of the array reference * itself never become too stale. To simplify index-based * operations, the array size is always a power of two, and all * readers must tolerate null slots. Worker queues are at odd * indices. Shared (submission) queues are at even indices, up to * a maximum of 64 slots, to limit growth even if array needs to * expand to add more workers. Grouping them together in this way * simplifies and speeds up task scanning. * * All worker thread creation is on-demand, triggered by task * submissions, replacement of terminated workers, and/or * compensation for blocked workers. However, all other support * code is set up to work with other policies. To ensure that we * do not hold on to worker references that would prevent GC, All * accesses to workQueues are via indices into the workQueues * array (which is one source of some of the messy code * constructions here). In essence, the workQueues array serves as * a weak reference mechanism. Thus for example the stack top * subfield of ctl stores indices, not references. * * Queuing Idle Workers. Unlike HPC work-stealing frameworks, we * cannot let workers spin indefinitely scanning for tasks when * none can be found immediately, and we cannot start/resume * workers unless there appear to be tasks available. On the * other hand, we must quickly prod them into action when new * tasks are submitted or generated. In many usages, ramp-up time * to activate workers is the main limiting factor in overall * performance, which is compounded at program start-up by JIT * compilation and allocation. So we streamline this as much as * possible. * * The "ctl" field atomically maintains active and total worker * counts as well as a queue to place waiting threads so they can * be located for signalling. Active counts also play the role of * quiescence indicators, so are decremented when workers believe * that there are no more tasks to execute. The "queue" is * actually a form of Treiber stack. A stack is ideal for * activating threads in most-recently used order. This improves * performance and locality, outweighing the disadvantages of * being prone to contention and inability to release a worker * unless it is topmost on stack. We park/unpark workers after * pushing on the idle worker stack (represented by the lower * 32bit subfield of ctl) when they cannot find work. The top * stack state holds the value of the "scanState" field of the * worker: its index and status, plus a version counter that, in * addition to the count subfields (also serving as version * stamps) provide protection against Treiber stack ABA effects. * * Field scanState is used by both workers and the pool to manage * and track whether a worker is INACTIVE (possibly blocked * waiting for a signal), or SCANNING for tasks (when neither hold * it is busy running tasks). When a worker is inactivated, its * scanState field is set, and is prevented from executing tasks, * even though it must scan once for them to avoid queuing * races. Note that scanState updates lag queue CAS releases so * usage requires care. When queued, the lower 16 bits of * scanState must hold its pool index. So we place the index there * upon initialization (see registerWorker) and otherwise keep it * there or restore it when necessary. * * Memory ordering. See "Correct and Efficient Work-Stealing for * Weak Memory Models" by Le, Pop, Cohen, and Nardelli, PPoPP 2013 * (http://www.di.ens.fr/~zappa/readings/ppopp13.pdf) for an * analysis of memory ordering requirements in work-stealing * algorithms similar to the one used here. We usually need * stronger than minimal ordering because we must sometimes signal * workers, requiring Dekker-like full-fences to avoid lost * signals. Arranging for enough ordering without expensive * over-fencing requires tradeoffs among the supported means of * expressing access constraints. The most central operations, * taking from queues and updating ctl state, require full-fence * CAS. Array slots are read using the emulation of volatiles * provided by Unsafe. Access from other threads to WorkQueue * base, top, and array requires a volatile load of the first of * any of these read. We use the convention of declaring the * "base" index volatile, and always read it before other fields. * The owner thread must ensure ordered updates, so writes use * ordered intrinsics unless they can piggyback on those for other * writes. Similar conventions and rationales hold for other * WorkQueue fields (such as "currentSteal") that are only written * by owners but observed by others. * * Creating workers. To create a worker, we pre-increment total * count (serving as a reservation), and attempt to construct a * ForkJoinWorkerThread via its factory. Upon construction, the * new thread invokes registerWorker, where it constructs a * WorkQueue and is assigned an index in the workQueues array * (expanding the array if necessary). The thread is then * started. Upon any exception across these steps, or null return * from factory, deregisterWorker adjusts counts and records * accordingly. If a null return, the pool continues running with * fewer than the target number workers. If exceptional, the * exception is propagated, generally to some external caller. * Worker index assignment avoids the bias in scanning that would * occur if entries were sequentially packed starting at the front * of the workQueues array. We treat the array as a simple * power-of-two hash table, expanding as needed. The seedIndex * increment ensures no collisions until a resize is needed or a * worker is deregistered and replaced, and thereafter keeps * probability of collision low. We cannot use * ThreadLocalRandom.getProbe() for similar purposes here because * the thread has not started yet, but do so for creating * submission queues for existing external threads. * * Deactivation and waiting. Queuing encounters several intrinsic * races; most notably that a task-producing thread can miss * seeing (and signalling) another thread that gave up looking for * work but has not yet entered the wait queue. When a worker * cannot find a task to steal, it deactivates and enqueues. Very * often, the lack of tasks is transient due to GC or OS * scheduling. To reduce false-alarm deactivation, scanners * compute checksums of queue states during sweeps. (The * stability checks used here and elsewhere are probabilistic * variants of snapshot techniques -- see Herlihy & Shavit.) * Workers give up and try to deactivate only after the sum is * stable across scans. Further, to avoid missed signals, they * repeat this scanning process after successful enqueuing until * again stable. In this state, the worker cannot take/run a task * it sees until it is released from the queue, so the worker * itself eventually tries to release itself or any successor (see * tryRelease). Otherwise, upon an empty scan, a deactivated * worker uses an adaptive local spin construction (see awaitWork) * before blocking (via park). Note the unusual conventions about * Thread.interrupts surrounding parking and other blocking: * Because interrupts are used solely to alert threads to check * termination, which is checked anyway upon blocking, we clear * status (using Thread.interrupted) before any call to park, so * that park does not immediately return due to status being set * via some other unrelated call to interrupt in user code. * * Signalling and activation. Workers are created or activated * only when there appears to be at least one task they might be * able to find and execute. Upon push (either by a worker or an * external submission) to a previously (possibly) empty queue, * workers are signalled if idle, or created if fewer exist than * the given parallelism level. These primary signals are * buttressed by others whenever other threads remove a task from * a queue and notice that there are other tasks there as well. * On most platforms, signalling (unpark) overhead time is * noticeably long, and the time between signalling a thread and * it actually making progress can be very noticeably long, so it * is worth offloading these delays from critical paths as much as * possible. Also, because inactive workers are often rescanning * or spinning rather than blocking, we set and clear the "parker" * field of WorkQueues to reduce unnecessary calls to unpark. * (This requires a secondary recheck to avoid missed signals.) * * Trimming workers. To release resources after periods of lack of * use, a worker starting to wait when the pool is quiescent will * time out and terminate (see awaitWork) if the pool has remained * quiescent for period IDLE_TIMEOUT, increasing the period as the * number of threads decreases, eventually removing all workers. * Also, when more than two spare threads exist, excess threads * are immediately terminated at the next quiescent point. * (Padding by two avoids hysteresis.) * * Shutdown and Termination. A call to shutdownNow invokes * tryTerminate to atomically set a runState bit. The calling * thread, as well as every other worker thereafter terminating, * helps terminate others by setting their (qlock) status, * cancelling their unprocessed tasks, and waking them up, doing * so repeatedly until stable (but with a loop bounded by the * number of workers). Calls to non-abrupt shutdown() preface * this by checking whether termination should commence. This * relies primarily on the active count bits of "ctl" maintaining * consensus -- tryTerminate is called from awaitWork whenever * quiescent. However, external submitters do not take part in * this consensus. So, tryTerminate sweeps through queues (until * stable) to ensure lack of in-flight submissions and workers * about to process them before triggering the "STOP" phase of * termination. (Note: there is an intrinsic conflict if * helpQuiescePool is called when shutdown is enabled. Both wait * for quiescence, but tryTerminate is biased to not trigger until * helpQuiescePool completes.) * * * Joining Tasks * ============= * * Any of several actions may be taken when one worker is waiting * to join a task stolen (or always held) by another. Because we * are multiplexing many tasks on to a pool of workers, we can't * just let them block (as in Thread.join). We also cannot just * reassign the joiner's run-time stack with another and replace * it later, which would be a form of "continuation", that even if * possible is not necessarily a good idea since we may need both * an unblocked task and its continuation to progress. Instead we * combine two tactics: * * Helping: Arranging for the joiner to execute some task that it * would be running if the steal had not occurred. * * Compensating: Unless there are already enough live threads, * method tryCompensate() may create or re-activate a spare * thread to compensate for blocked joiners until they unblock. * * A third form (implemented in tryRemoveAndExec) amounts to * helping a hypothetical compensator: If we can readily tell that * a possible action of a compensator is to steal and execute the * task being joined, the joining thread can do so directly, * without the need for a compensation thread (although at the * expense of larger run-time stacks, but the tradeoff is * typically worthwhile). * * The ManagedBlocker extension API can't use helping so relies * only on compensation in method awaitBlocker. * * The algorithm in helpStealer entails a form of "linear * helping". Each worker records (in field currentSteal) the most * recent task it stole from some other worker (or a submission). * It also records (in field currentJoin) the task it is currently * actively joining. Method helpStealer uses these markers to try * to find a worker to help (i.e., steal back a task from and * execute it) that could hasten completion of the actively joined * task. Thus, the joiner executes a task that would be on its * own local deque had the to-be-joined task not been stolen. This * is a conservative variant of the approach described in Wagner & * Calder "Leapfrogging: a portable technique for implementing * efficient futures" SIGPLAN Notices, 1993 * (http://portal.acm.org/citation.cfm?id=155354). It differs in * that: (1) We only maintain dependency links across workers upon * steals, rather than use per-task bookkeeping. This sometimes * requires a linear scan of workQueues array to locate stealers, * but often doesn't because stealers leave hints (that may become * stale/wrong) of where to locate them. It is only a hint * because a worker might have had multiple steals and the hint * records only one of them (usually the most current). Hinting * isolates cost to when it is needed, rather than adding to * per-task overhead. (2) It is "shallow", ignoring nesting and * potentially cyclic mutual steals. (3) It is intentionally * racy: field currentJoin is updated only while actively joining, * which means that we miss links in the chain during long-lived * tasks, GC stalls etc (which is OK since blocking in such cases * is usually a good idea). (4) We bound the number of attempts * to find work using checksums and fall back to suspending the * worker and if necessary replacing it with another. * * Helping actions for CountedCompleters do not require tracking * currentJoins: Method helpComplete takes and executes any task * with the same root as the task being waited on (preferring * local pops to non-local polls). However, this still entails * some traversal of completer chains, so is less efficient than * using CountedCompleters without explicit joins. * * Compensation does not aim to keep exactly the target * parallelism number of unblocked threads running at any given * time. Some previous versions of this class employed immediate * compensations for any blocked join. However, in practice, the * vast majority of blockages are transient byproducts of GC and * other JVM or OS activities that are made worse by replacement. * Currently, compensation is attempted only after validating that * all purportedly active threads are processing tasks by checking * field WorkQueue.scanState, which eliminates most false * positives. Also, compensation is bypassed (tolerating fewer * threads) in the most common case in which it is rarely * beneficial: when a worker with an empty queue (thus no * continuation tasks) blocks on a join and there still remain * enough threads to ensure liveness. * * The compensation mechanism may be bounded. Bounds for the * commonPool (see commonMaxSpares) better enable JVMs to cope * with programming errors and abuse before running out of * resources to do so. In other cases, users may supply factories * that limit thread construction. The effects of bounding in this * pool (like all others) is imprecise. Total worker counts are * decremented when threads deregister, not when they exit and * resources are reclaimed by the JVM and OS. So the number of * simultaneously live threads may transiently exceed bounds. * * Common Pool * =========== * * The static common pool always exists after static * initialization. Since it (or any other created pool) need * never be used, we minimize initial construction overhead and * footprint to the setup of about a dozen fields, with no nested * allocation. Most bootstrapping occurs within method * externalSubmit during the first submission to the pool. * * When external threads submit to the common pool, they can * perform subtask processing (see externalHelpComplete and * related methods) upon joins. This caller-helps policy makes it * sensible to set common pool parallelism level to one (or more) * less than the total number of available cores, or even zero for * pure caller-runs. We do not need to record whether external * submissions are to the common pool -- if not, external help * methods return quickly. These submitters would otherwise be * blocked waiting for completion, so the extra effort (with * liberally sprinkled task status checks) in inapplicable cases * amounts to an odd form of limited spin-wait before blocking in * ForkJoinTask.join. * * As a more appropriate default in managed environments, unless * overridden by system properties, we use workers of subclass * InnocuousForkJoinWorkerThread when there is a SecurityManager * present. These workers have no permissions set, do not belong * to any user-defined ThreadGroup, and erase all ThreadLocals * after executing any top-level task (see WorkQueue.runTask). * The associated mechanics (mainly in ForkJoinWorkerThread) may * be JVM-dependent and must access particular Thread class fields * to achieve this effect. * * Style notes * =========== * * Memory ordering relies mainly on Unsafe intrinsics that carry * the further responsibility of explicitly performing null- and * bounds- checks otherwise carried out implicitly by JVMs. This * can be awkward and ugly, but also reflects the need to control * outcomes across the unusual cases that arise in very racy code * with very few invariants. So these explicit checks would exist * in some form anyway. All fields are read into locals before * use, and null-checked if they are references. This is usually * done in a "C"-like style of listing declarations at the heads * of methods or blocks, and using inline assignments on first * encounter. Array bounds-checks are usually performed by * masking with array.length-1, which relies on the invariant that * these arrays are created with positive lengths, which is itself * paranoically checked. Nearly all explicit checks lead to * bypass/return, not exception throws, because they may * legitimately arise due to cancellation/revocation during * shutdown. * * There is a lot of representation-level coupling among classes * ForkJoinPool, ForkJoinWorkerThread, and ForkJoinTask. The * fields of WorkQueue maintain data structures managed by * ForkJoinPool, so are directly accessed. There is little point * trying to reduce this, since any associated future changes in * representations will need to be accompanied by algorithmic * changes anyway. Several methods intrinsically sprawl because * they must accumulate sets of consistent reads of fields held in * local variables. There are also other coding oddities * (including several unnecessary-looking hoisted null checks) * that help some methods perform reasonably even when interpreted * (not compiled). * * The order of declarations in this file is (with a few exceptions): * (1) Static utility functions * (2) Nested (static) classes * (3) Static fields * (4) Fields, along with constants used when unpacking some of them * (5) Internal control methods * (6) Callbacks and other support for ForkJoinTask methods * (7) Exported methods * (8) Static block initializing statics in minimally dependent order */ // Static utilities /** * If there is a security manager, makes sure caller has * permission to modify threads. */ private static void checkPermission() { SecurityManager security = System.getSecurityManager(); if (security != null) security.checkPermission(modifyThreadPermission); } // Nested classes /** * Factory for creating new {@link ForkJoinWorkerThread}s. * A {@code ForkJoinWorkerThreadFactory} must be defined and used * for {@code ForkJoinWorkerThread} subclasses that extend base * functionality or initialize threads with different contexts. */ public static interface ForkJoinWorkerThreadFactory { /** * Returns a new worker thread operating in the given pool. * * @param pool the pool this thread works in * @return the new worker thread * @throws NullPointerException if the pool is null */ public ForkJoinWorkerThread newThread(ForkJoinPool pool); } /** * Default ForkJoinWorkerThreadFactory implementation; creates a * new ForkJoinWorkerThread. */ static final class DefaultForkJoinWorkerThreadFactory implements ForkJoinWorkerThreadFactory { public final ForkJoinWorkerThread newThread(ForkJoinPool pool) { return new ForkJoinWorkerThread(pool); } } /** * Class for artificial tasks that are used to replace the target * of local joins if they are removed from an interior queue slot * in WorkQueue.tryRemoveAndExec. We don't need the proxy to * actually do anything beyond having a unique identity. */ static final class EmptyTask extends ForkJoinTask { private static final long serialVersionUID = -7721805057305804111L; EmptyTask() { status = ForkJoinTask.NORMAL; } // force done public final Void getRawResult() { return null; } public final void setRawResult(Void x) {} public final boolean exec() { return true; } } // Constants shared across ForkJoinPool and WorkQueue // Bounds static final int SMASK = 0xffff; // short bits == max index static final int MAX_CAP = 0x7fff; // max #workers - 1 static final int EVENMASK = 0xfffe; // even short bits static final int SQMASK = 0x007e; // max 64 (even) slots // Masks and units for WorkQueue.scanState and ctl sp subfield static final int SCANNING = 1; // false when running tasks static final int INACTIVE = 1 << 31; // must be negative static final int SS_SEQ = 1 << 16; // version count // Mode bits for ForkJoinPool.config and WorkQueue.config static final int MODE_MASK = 0xffff << 16; // top half of int static final int LIFO_QUEUE = 0; static final int FIFO_QUEUE = 1 << 16; static final int SHARED_QUEUE = 1 << 31; // must be negative /** * Queues supporting work-stealing as well as external task * submission. See above for descriptions and algorithms. * Performance on most platforms is very sensitive to placement of * instances of both WorkQueues and their arrays -- we absolutely * do not want multiple WorkQueue instances or multiple queue * arrays sharing cache lines. The @Contended annotation alerts * JVMs to try to keep instances apart. */ @sun.misc.Contended static final class WorkQueue { /** * Capacity of work-stealing queue array upon initialization. * Must be a power of two; at least 4, but should be larger to * reduce or eliminate cacheline sharing among queues. * Currently, it is much larger, as a partial workaround for * the fact that JVMs often place arrays in locations that * share GC bookkeeping (especially cardmarks) such that * per-write accesses encounter serious memory contention. */ static final int INITIAL_QUEUE_CAPACITY = 1 << 13; /** * Maximum size for queue arrays. Must be a power of two less * than or equal to 1 << (31 - width of array entry) to ensure * lack of wraparound of index calculations, but defined to a * value a bit less than this to help users trap runaway * programs before saturating systems. */ static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M // Instance fields volatile int scanState; // versioned, <0: inactive; odd:scanning int stackPred; // pool stack (ctl) predecessor int nsteals; // number of steals int hint; // randomization and stealer index hint int config; // pool index and mode volatile int qlock; // 1: locked, < 0: terminate; else 0 volatile int base; // index of next slot for poll int top; // index of next slot for push ForkJoinTask[] array; // the elements (initially unallocated) final ForkJoinPool pool; // the containing pool (may be null) final ForkJoinWorkerThread owner; // owning thread or null if shared volatile Thread parker; // == owner during call to park; else null volatile ForkJoinTask currentJoin; // task being joined in awaitJoin volatile ForkJoinTask currentSteal; // mainly used by helpStealer WorkQueue(ForkJoinPool pool, ForkJoinWorkerThread owner) { this.pool = pool; this.owner = owner; // Place indices in the center of array (that is not yet allocated) base = top = INITIAL_QUEUE_CAPACITY >>> 1; } /** * Returns an exportable index (used by ForkJoinWorkerThread). */ final int getPoolIndex() { return (config & 0xffff) >>> 1; // ignore odd/even tag bit } /** * Returns the approximate number of tasks in the queue. */ final int queueSize() { int n = base - top; // non-owner callers must read base first return (n >= 0) ? 0 : -n; // ignore transient negative } /** * Provides a more accurate estimate of whether this queue has * any tasks than does queueSize, by checking whether a * near-empty queue has at least one unclaimed task. */ final boolean isEmpty() { ForkJoinTask[] a; int n, m, s; return ((n = base - (s = top)) >= 0 || (n == -1 && // possibly one task ((a = array) == null || (m = a.length - 1) < 0 || U.getObject (a, (long)((m & (s - 1)) << ASHIFT) + ABASE) == null))); } /** * Pushes a task. Call only by owner in unshared queues. (The * shared-queue version is embedded in method externalPush.) * * @param task the task. Caller must ensure non-null. * @throws RejectedExecutionException if array cannot be resized */ final void push(ForkJoinTask task) { ForkJoinTask[] a; ForkJoinPool p; int b = base, s = top, n; if ((a = array) != null) { // ignore if queue removed int m = a.length - 1; // fenced write for task visibility U.putOrderedObject(a, ((m & s) << ASHIFT) + ABASE, task); U.putOrderedInt(this, QTOP, s + 1); if ((n = s - b) <= 1) { if ((p = pool) != null) p.signalWork(p.workQueues, this); } else if (n >= m) growArray(); } } /** * Initializes or doubles the capacity of array. Call either * by owner or with lock held -- it is OK for base, but not * top, to move while resizings are in progress. */ final ForkJoinTask[] growArray() { ForkJoinTask[] oldA = array; int size = oldA != null ? oldA.length << 1 : INITIAL_QUEUE_CAPACITY; if (size > MAXIMUM_QUEUE_CAPACITY) throw new RejectedExecutionException("Queue capacity exceeded"); int oldMask, t, b; ForkJoinTask[] a = array = new ForkJoinTask[size]; if (oldA != null && (oldMask = oldA.length - 1) >= 0 && (t = top) - (b = base) > 0) { int mask = size - 1; do { // emulate poll from old array, push to new array ForkJoinTask x; int oldj = ((b & oldMask) << ASHIFT) + ABASE; int j = ((b & mask) << ASHIFT) + ABASE; x = (ForkJoinTask)U.getObjectVolatile(oldA, oldj); if (x != null && U.compareAndSwapObject(oldA, oldj, x, null)) U.putObjectVolatile(a, j, x); } while (++b != t); } return a; } /** * Takes next task, if one exists, in LIFO order. Call only * by owner in unshared queues. */ final ForkJoinTask pop() { ForkJoinTask[] a; ForkJoinTask t; int m; if ((a = array) != null && (m = a.length - 1) >= 0) { for (int s; (s = top - 1) - base >= 0;) { long j = ((m & s) << ASHIFT) + ABASE; if ((t = (ForkJoinTask)U.getObject(a, j)) == null) break; if (U.compareAndSwapObject(a, j, t, null)) { U.putOrderedInt(this, QTOP, s); return t; } } } return null; } /** * Takes a task in FIFO order if b is base of queue and a task * can be claimed without contention. Specialized versions * appear in ForkJoinPool methods scan and helpStealer. */ final ForkJoinTask pollAt(int b) { ForkJoinTask t; ForkJoinTask[] a; if ((a = array) != null) { int j = (((a.length - 1) & b) << ASHIFT) + ABASE; if ((t = (ForkJoinTask)U.getObjectVolatile(a, j)) != null && base == b && U.compareAndSwapObject(a, j, t, null)) { base = b + 1; return t; } } return null; } /** * Takes next task, if one exists, in FIFO order. */ final ForkJoinTask poll() { ForkJoinTask[] a; int b; ForkJoinTask t; while ((b = base) - top < 0 && (a = array) != null) { int j = (((a.length - 1) & b) << ASHIFT) + ABASE; t = (ForkJoinTask)U.getObjectVolatile(a, j); if (base == b) { if (t != null) { if (U.compareAndSwapObject(a, j, t, null)) { base = b + 1; return t; } } else if (b + 1 == top) // now empty break; } } return null; } /** * Takes next task, if one exists, in order specified by mode. */ final ForkJoinTask nextLocalTask() { return (config & FIFO_QUEUE) == 0 ? pop() : poll(); } /** * Returns next task, if one exists, in order specified by mode. */ final ForkJoinTask peek() { ForkJoinTask[] a = array; int m; if (a == null || (m = a.length - 1) < 0) return null; int i = (config & FIFO_QUEUE) == 0 ? top - 1 : base; int j = ((i & m) << ASHIFT) + ABASE; return (ForkJoinTask)U.getObjectVolatile(a, j); } /** * Pops the given task only if it is at the current top. * (A shared version is available only via FJP.tryExternalUnpush) */ final boolean tryUnpush(ForkJoinTask t) { ForkJoinTask[] a; int s; if ((a = array) != null && (s = top) != base && U.compareAndSwapObject (a, (((a.length - 1) & --s) << ASHIFT) + ABASE, t, null)) { U.putOrderedInt(this, QTOP, s); return true; } return false; } /** * Removes and cancels all known tasks, ignoring any exceptions. */ final void cancelAll() { ForkJoinTask t; if ((t = currentJoin) != null) { currentJoin = null; ForkJoinTask.cancelIgnoringExceptions(t); } if ((t = currentSteal) != null) { currentSteal = null; ForkJoinTask.cancelIgnoringExceptions(t); } while ((t = poll()) != null) ForkJoinTask.cancelIgnoringExceptions(t); } // Specialized execution methods /** * Polls and runs tasks until empty. */ final void pollAndExecAll() { for (ForkJoinTask t; (t = poll()) != null;) t.doExec(); } /** * Removes and executes all local tasks. If LIFO, invokes * pollAndExecAll. Otherwise implements a specialized pop loop * to exec until empty. */ final void execLocalTasks() { int b = base, m, s; ForkJoinTask[] a = array; if (b - (s = top - 1) <= 0 && a != null && (m = a.length - 1) >= 0) { if ((config & FIFO_QUEUE) == 0) { for (ForkJoinTask t;;) { if ((t = (ForkJoinTask)U.getAndSetObject (a, ((m & s) << ASHIFT) + ABASE, null)) == null) break; U.putOrderedInt(this, QTOP, s); t.doExec(); if (base - (s = top - 1) > 0) break; } } else pollAndExecAll(); } } /** * Executes the given task and any remaining local tasks. */ final void runTask(ForkJoinTask task) { if (task != null) { scanState &= ~SCANNING; // mark as busy (currentSteal = task).doExec(); U.putOrderedObject(this, QCURRENTSTEAL, null); // release for GC execLocalTasks(); ForkJoinWorkerThread thread = owner; if (++nsteals < 0) // collect on overflow transferStealCount(pool); scanState |= SCANNING; if (thread != null) thread.afterTopLevelExec(); } } /** * Adds steal count to pool stealCounter if it exists, and resets. */ final void transferStealCount(ForkJoinPool p) { AtomicLong sc; if (p != null && (sc = p.stealCounter) != null) { int s = nsteals; nsteals = 0; // if negative, correct for overflow sc.getAndAdd((long)(s < 0 ? Integer.MAX_VALUE : s)); } } /** * If present, removes from queue and executes the given task, * or any other cancelled task. Used only by awaitJoin. * * @return true if queue empty and task not known to be done */ final boolean tryRemoveAndExec(ForkJoinTask task) { ForkJoinTask[] a; int m, s, b, n; if ((a = array) != null && (m = a.length - 1) >= 0 && task != null) { while ((n = (s = top) - (b = base)) > 0) { for (ForkJoinTask t;;) { // traverse from s to b long j = ((--s & m) << ASHIFT) + ABASE; if ((t = (ForkJoinTask)U.getObject(a, j)) == null) return s + 1 == top; // shorter than expected else if (t == task) { boolean removed = false; if (s + 1 == top) { // pop if (U.compareAndSwapObject(a, j, task, null)) { U.putOrderedInt(this, QTOP, s); removed = true; } } else if (base == b) // replace with proxy removed = U.compareAndSwapObject( a, j, task, new EmptyTask()); if (removed) task.doExec(); break; } else if (t.status < 0 && s + 1 == top) { if (U.compareAndSwapObject(a, j, t, null)) U.putOrderedInt(this, QTOP, s); break; // was cancelled } if (--n == 0) return false; } if (task.status < 0) return false; } } return true; } /** * Pops task if in the same CC computation as the given task, * in either shared or owned mode. Used only by helpComplete. */ final CountedCompleter popCC(CountedCompleter task, int mode) { int s; ForkJoinTask[] a; Object o; if (base - (s = top) < 0 && (a = array) != null) { long j = (((a.length - 1) & (s - 1)) << ASHIFT) + ABASE; if ((o = U.getObjectVolatile(a, j)) != null && (o instanceof CountedCompleter)) { CountedCompleter t = (CountedCompleter)o; for (CountedCompleter r = t;;) { if (r == task) { if (mode < 0) { // must lock if (U.compareAndSwapInt(this, QLOCK, 0, 1)) { if (top == s && array == a && U.compareAndSwapObject(a, j, t, null)) { U.putOrderedInt(this, QTOP, s - 1); U.putOrderedInt(this, QLOCK, 0); return t; } U.compareAndSwapInt(this, QLOCK, 1, 0); } } else if (U.compareAndSwapObject(a, j, t, null)) { U.putOrderedInt(this, QTOP, s - 1); return t; } break; } else if ((r = r.completer) == null) // try parent break; } } } return null; } /** * Steals and runs a task in the same CC computation as the * given task if one exists and can be taken without * contention. Otherwise returns a checksum/control value for * use by method helpComplete. * * @return 1 if successful, 2 if retryable (lost to another * stealer), -1 if non-empty but no matching task found, else * the base index, forced negative. */ final int pollAndExecCC(CountedCompleter task) { int b, h; ForkJoinTask[] a; Object o; if ((b = base) - top >= 0 || (a = array) == null) h = b | Integer.MIN_VALUE; // to sense movement on re-poll else { long j = (((a.length - 1) & b) << ASHIFT) + ABASE; if ((o = U.getObjectVolatile(a, j)) == null) h = 2; // retryable else if (!(o instanceof CountedCompleter)) h = -1; // unmatchable else { CountedCompleter t = (CountedCompleter)o; for (CountedCompleter r = t;;) { if (r == task) { if (base == b && U.compareAndSwapObject(a, j, t, null)) { base = b + 1; t.doExec(); h = 1; // success } else h = 2; // lost CAS break; } else if ((r = r.completer) == null) { h = -1; // unmatched break; } } } } return h; } /** * Returns true if owned and not known to be blocked. */ final boolean isApparentlyUnblocked() { Thread wt; Thread.State s; return (scanState >= 0 && (wt = owner) != null && (s = wt.getState()) != Thread.State.BLOCKED && s != Thread.State.WAITING && s != Thread.State.TIMED_WAITING); } // Unsafe mechanics. Note that some are (and must be) the same as in FJP private static final sun.misc.Unsafe U; private static final int ABASE; private static final int ASHIFT; private static final long QTOP; private static final long QLOCK; private static final long QCURRENTSTEAL; static { try { U = sun.misc.Unsafe.getUnsafe(); Class wk = WorkQueue.class; Class ak = ForkJoinTask[].class; QTOP = U.objectFieldOffset (wk.getDeclaredField("top")); QLOCK = U.objectFieldOffset (wk.getDeclaredField("qlock")); QCURRENTSTEAL = U.objectFieldOffset (wk.getDeclaredField("currentSteal")); ABASE = U.arrayBaseOffset(ak); int scale = U.arrayIndexScale(ak); if ((scale & (scale - 1)) != 0) throw new Error("data type scale not a power of two"); ASHIFT = 31 - Integer.numberOfLeadingZeros(scale); } catch (Exception e) { throw new Error(e); } } } // static fields (initialized in static initializer below) /** * Creates a new ForkJoinWorkerThread. This factory is used unless * overridden in ForkJoinPool constructors. */ public static final ForkJoinWorkerThreadFactory defaultForkJoinWorkerThreadFactory; /** * Permission required for callers of methods that may start or * kill threads. */ private static final RuntimePermission modifyThreadPermission; /** * Common (static) pool. Non-null for public use unless a static * construction exception, but internal usages null-check on use * to paranoically avoid potential initialization circularities * as well as to simplify generated code. */ static final ForkJoinPool common; /** * Common pool parallelism. To allow simpler use and management * when common pool threads are disabled, we allow the underlying * common.parallelism field to be zero, but in that case still report * parallelism as 1 to reflect resulting caller-runs mechanics. */ static final int commonParallelism; /** * Limit on spare thread construction in tryCompensate. */ private static int commonMaxSpares; /** * Sequence number for creating workerNamePrefix. */ private static int poolNumberSequence; /** * Returns the next sequence number. We don't expect this to * ever contend, so use simple builtin sync. */ private static final synchronized int nextPoolId() { return ++poolNumberSequence; } // static configuration constants /** * Initial timeout value (in nanoseconds) for the thread * triggering quiescence to park waiting for new work. On timeout, * the thread will instead try to shrink the number of * workers. The value should be large enough to avoid overly * aggressive shrinkage during most transient stalls (long GCs * etc). */ private static final long IDLE_TIMEOUT = 2000L * 1000L * 1000L; // 2sec /** * Tolerance for idle timeouts, to cope with timer undershoots */ private static final long TIMEOUT_SLOP = 20L * 1000L * 1000L; // 20ms /** * The initial value for commonMaxSpares during static * initialization. The value is far in excess of normal * requirements, but also far short of MAX_CAP and typical * OS thread limits, so allows JVMs to catch misuse/abuse * before running out of resources needed to do so. */ private static final int DEFAULT_COMMON_MAX_SPARES = 256; /** * Number of times to spin-wait before blocking. The spins (in * awaitRunStateLock and awaitWork) currently use randomized * spins. If/when MWAIT-like intrinsics becomes available, they * may allow quieter spinning. The value of SPINS must be a power * of two, at least 4. The current value causes spinning for a * small fraction of typical context-switch times, well worthwhile * given the typical likelihoods that blocking is not necessary. */ private static final int SPINS = 1 << 11; /** * Increment for seed generators. See class ThreadLocal for * explanation. */ private static final int SEED_INCREMENT = 0x9e3779b9; /* * Bits and masks for field ctl, packed with 4 16 bit subfields: * AC: Number of active running workers minus target parallelism * TC: Number of total workers minus target parallelism * SS: version count and status of top waiting thread * ID: poolIndex of top of Treiber stack of waiters * * When convenient, we can extract the lower 32 stack top bits * (including version bits) as sp=(int)ctl. The offsets of counts * by the target parallelism and the positionings of fields makes * it possible to perform the most common checks via sign tests of * fields: When ac is negative, there are not enough active * workers, when tc is negative, there are not enough total * workers. When sp is non-zero, there are waiting workers. To * deal with possibly negative fields, we use casts in and out of * "short" and/or signed shifts to maintain signedness. * * Because it occupies uppermost bits, we can add one active count * using getAndAddLong of AC_UNIT, rather than CAS, when returning * from a blocked join. Other updates entail multiple subfields * and masking, requiring CAS. */ // Lower and upper word masks private static final long SP_MASK = 0xffffffffL; private static final long UC_MASK = ~SP_MASK; // Active counts private static final int AC_SHIFT = 48; private static final long AC_UNIT = 0x0001L << AC_SHIFT; private static final long AC_MASK = 0xffffL << AC_SHIFT; // Total counts private static final int TC_SHIFT = 32; private static final long TC_UNIT = 0x0001L << TC_SHIFT; private static final long TC_MASK = 0xffffL << TC_SHIFT; private static final long ADD_WORKER = 0x0001L << (TC_SHIFT + 15); // sign // runState bits: SHUTDOWN must be negative, others arbitrary powers of two private static final int RSLOCK = 1; private static final int RSIGNAL = 1 << 1; private static final int STARTED = 1 << 2; private static final int STOP = 1 << 29; private static final int TERMINATED = 1 << 30; private static final int SHUTDOWN = 1 << 31; // Instance fields volatile long ctl; // main pool control volatile int runState; // lockable status final int config; // parallelism, mode int indexSeed; // to generate worker index volatile WorkQueue[] workQueues; // main registry final ForkJoinWorkerThreadFactory factory; final UncaughtExceptionHandler ueh; // per-worker UEH final String workerNamePrefix; // to create worker name string volatile AtomicLong stealCounter; // also used as sync monitor /** * Acquires the runState lock; returns current (locked) runState. */ private int lockRunState() { int rs; return ((((rs = runState) & RSLOCK) != 0 || !U.compareAndSwapInt(this, RUNSTATE, rs, rs |= RSLOCK)) ? awaitRunStateLock() : rs); } /** * Spins and/or blocks until runstate lock is available. See * above for explanation. */ private int awaitRunStateLock() { Object lock; boolean wasInterrupted = false; for (int spins = SPINS, r = 0, rs, ns;;) { if (((rs = runState) & RSLOCK) == 0) { if (U.compareAndSwapInt(this, RUNSTATE, rs, ns = rs | RSLOCK)) { if (wasInterrupted) { try { Thread.currentThread().interrupt(); } catch (SecurityException ignore) { } } return ns; } } else if (r == 0) r = ThreadLocalRandom.nextSecondarySeed(); else if (spins > 0) { r ^= r << 6; r ^= r >>> 21; r ^= r << 7; // xorshift if (r >= 0) --spins; } else if ((rs & STARTED) == 0 || (lock = stealCounter) == null) Thread.yield(); // initialization race else if (U.compareAndSwapInt(this, RUNSTATE, rs, rs | RSIGNAL)) { synchronized (lock) { if ((runState & RSIGNAL) != 0) { try { lock.wait(); } catch (InterruptedException ie) { if (!(Thread.currentThread() instanceof ForkJoinWorkerThread)) wasInterrupted = true; } } else lock.notifyAll(); } } } } /** * Unlocks and sets runState to newRunState. * * @param oldRunState a value returned from lockRunState * @param newRunState the next value (must have lock bit clear). */ private void unlockRunState(int oldRunState, int newRunState) { if (!U.compareAndSwapInt(this, RUNSTATE, oldRunState, newRunState)) { Object lock = stealCounter; runState = newRunState; // clears RSIGNAL bit if (lock != null) synchronized (lock) { lock.notifyAll(); } } } // Creating, registering and deregistering workers /** * Tries to construct and start one worker. Assumes that total * count has already been incremented as a reservation. Invokes * deregisterWorker on any failure. * * @return true if successful */ private boolean createWorker() { ForkJoinWorkerThreadFactory fac = factory; Throwable ex = null; ForkJoinWorkerThread wt = null; try { if (fac != null && (wt = fac.newThread(this)) != null) { wt.start(); return true; } } catch (Throwable rex) { ex = rex; } deregisterWorker(wt, ex); return false; } /** * Tries to add one worker, incrementing ctl counts before doing * so, relying on createWorker to back out on failure. * * @param c incoming ctl value, with total count negative and no * idle workers. On CAS failure, c is refreshed and retried if * this holds (otherwise, a new worker is not needed). */ private void tryAddWorker(long c) { boolean add = false; do { long nc = ((AC_MASK & (c + AC_UNIT)) | (TC_MASK & (c + TC_UNIT))); if (ctl == c) { int rs, stop; // check if terminating if ((stop = (rs = lockRunState()) & STOP) == 0) add = U.compareAndSwapLong(this, CTL, c, nc); unlockRunState(rs, rs & ~RSLOCK); if (stop != 0) break; if (add) { createWorker(); break; } } } while (((c = ctl) & ADD_WORKER) != 0L && (int)c == 0); } /** * Callback from ForkJoinWorkerThread constructor to establish and * record its WorkQueue. * * @param wt the worker thread * @return the worker's queue */ final WorkQueue registerWorker(ForkJoinWorkerThread wt) { UncaughtExceptionHandler handler; wt.setDaemon(true); // configure thread if ((handler = ueh) != null) wt.setUncaughtExceptionHandler(handler); WorkQueue w = new WorkQueue(this, wt); int i = 0; // assign a pool index int mode = config & MODE_MASK; int rs = lockRunState(); try { WorkQueue[] ws; int n; // skip if no array if ((ws = workQueues) != null && (n = ws.length) > 0) { int s = indexSeed += SEED_INCREMENT; // unlikely to collide int m = n - 1; i = ((s << 1) | 1) & m; // odd-numbered indices if (ws[i] != null) { // collision int probes = 0; // step by approx half n int step = (n <= 4) ? 2 : ((n >>> 1) & EVENMASK) + 2; while (ws[i = (i + step) & m] != null) { if (++probes >= n) { workQueues = ws = Arrays.copyOf(ws, n <<= 1); m = n - 1; probes = 0; } } } w.hint = s; // use as random seed w.config = i | mode; w.scanState = i; // publication fence ws[i] = w; } } finally { unlockRunState(rs, rs & ~RSLOCK); } wt.setName(workerNamePrefix.concat(Integer.toString(i >>> 1))); return w; } /** * Final callback from terminating worker, as well as upon failure * to construct or start a worker. Removes record of worker from * array, and adjusts counts. If pool is shutting down, tries to * complete termination. * * @param wt the worker thread, or null if construction failed * @param ex the exception causing failure, or null if none */ final void deregisterWorker(ForkJoinWorkerThread wt, Throwable ex) { WorkQueue w = null; if (wt != null && (w = wt.workQueue) != null) { WorkQueue[] ws; // remove index from array int idx = w.config & SMASK; int rs = lockRunState(); if ((ws = workQueues) != null && ws.length > idx && ws[idx] == w) ws[idx] = null; unlockRunState(rs, rs & ~RSLOCK); } long c; // decrement counts do {} while (!U.compareAndSwapLong (this, CTL, c = ctl, ((AC_MASK & (c - AC_UNIT)) | (TC_MASK & (c - TC_UNIT)) | (SP_MASK & c)))); if (w != null) { w.qlock = -1; // ensure set w.transferStealCount(this); w.cancelAll(); // cancel remaining tasks } for (;;) { // possibly replace WorkQueue[] ws; int m, sp; if (tryTerminate(false, false) || w == null || w.array == null || (runState & STOP) != 0 || (ws = workQueues) == null || (m = ws.length - 1) < 0) // already terminating break; if ((sp = (int)(c = ctl)) != 0) { // wake up replacement if (tryRelease(c, ws[sp & m], AC_UNIT)) break; } else if (ex != null && (c & ADD_WORKER) != 0L) { tryAddWorker(c); // create replacement break; } else // don't need replacement break; } if (ex == null) // help clean on way out ForkJoinTask.helpExpungeStaleExceptions(); else // rethrow ForkJoinTask.rethrow(ex); } // Signalling /** * Tries to create or activate a worker if too few are active. * * @param ws the worker array to use to find signallees * @param q a WorkQueue --if non-null, don't retry if now empty */ final void signalWork(WorkQueue[] ws, WorkQueue q) { long c; int sp, i; WorkQueue v; Thread p; while ((c = ctl) < 0L) { // too few active if ((sp = (int)c) == 0) { // no idle workers if ((c & ADD_WORKER) != 0L) // too few workers tryAddWorker(c); break; } if (ws == null) // unstarted/terminated break; if (ws.length <= (i = sp & SMASK)) // terminated break; if ((v = ws[i]) == null) // terminating break; int vs = (sp + SS_SEQ) & ~INACTIVE; // next scanState int d = sp - v.scanState; // screen CAS long nc = (UC_MASK & (c + AC_UNIT)) | (SP_MASK & v.stackPred); if (d == 0 && U.compareAndSwapLong(this, CTL, c, nc)) { v.scanState = vs; // activate v if ((p = v.parker) != null) U.unpark(p); break; } if (q != null && q.base == q.top) // no more work break; } } /** * Signals and releases worker v if it is top of idle worker * stack. This performs a one-shot version of signalWork only if * there is (apparently) at least one idle worker. * * @param c incoming ctl value * @param v if non-null, a worker * @param inc the increment to active count (zero when compensating) * @return true if successful */ private boolean tryRelease(long c, WorkQueue v, long inc) { int sp = (int)c, vs = (sp + SS_SEQ) & ~INACTIVE; Thread p; if (v != null && v.scanState == sp) { // v is at top of stack long nc = (UC_MASK & (c + inc)) | (SP_MASK & v.stackPred); if (U.compareAndSwapLong(this, CTL, c, nc)) { v.scanState = vs; if ((p = v.parker) != null) U.unpark(p); return true; } } return false; } // Scanning for tasks /** * Top-level runloop for workers, called by ForkJoinWorkerThread.run. */ final void runWorker(WorkQueue w) { w.growArray(); // allocate queue int seed = w.hint; // initially holds randomization hint int r = (seed == 0) ? 1 : seed; // avoid 0 for xorShift for (ForkJoinTask t;;) { if ((t = scan(w, r)) != null) w.runTask(t); else if (!awaitWork(w, r)) break; r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift } } /** * Scans for and tries to steal a top-level task. Scans start at a * random location, randomly moving on apparent contention, * otherwise continuing linearly until reaching two consecutive * empty passes over all queues with the same checksum (summing * each base index of each queue, that moves on each steal), at * which point the worker tries to inactivate and then re-scans, * attempting to re-activate (itself or some other worker) if * finding a task; otherwise returning null to await work. Scans * otherwise touch as little memory as possible, to reduce * disruption on other scanning threads. * * @param w the worker (via its WorkQueue) * @param r a random seed * @return a task, or null if none found */ private ForkJoinTask scan(WorkQueue w, int r) { WorkQueue[] ws; int m; if ((ws = workQueues) != null && (m = ws.length - 1) > 0 && w != null) { int ss = w.scanState; // initially non-negative for (int origin = r & m, k = origin, oldSum = 0, checkSum = 0;;) { WorkQueue q; ForkJoinTask[] a; ForkJoinTask t; int b, n; long c; if ((q = ws[k]) != null) { if ((n = (b = q.base) - q.top) < 0 && (a = q.array) != null) { // non-empty long i = (((a.length - 1) & b) << ASHIFT) + ABASE; if ((t = ((ForkJoinTask) U.getObjectVolatile(a, i))) != null && q.base == b) { if (ss >= 0) { if (U.compareAndSwapObject(a, i, t, null)) { q.base = b + 1; if (n < -1) // signal others signalWork(ws, q); return t; } } else if (oldSum == 0 && // try to activate w.scanState < 0) tryRelease(c = ctl, ws[m & (int)c], AC_UNIT); } if (ss < 0) // refresh ss = w.scanState; r ^= r << 1; r ^= r >>> 3; r ^= r << 10; origin = k = r & m; // move and rescan oldSum = checkSum = 0; continue; } checkSum += b; } if ((k = (k + 1) & m) == origin) { // continue until stable if ((ss >= 0 || (ss == (ss = w.scanState))) && oldSum == (oldSum = checkSum)) { if (ss < 0 || w.qlock < 0) // already inactive break; int ns = ss | INACTIVE; // try to inactivate long nc = ((SP_MASK & ns) | (UC_MASK & ((c = ctl) - AC_UNIT))); w.stackPred = (int)c; // hold prev stack top U.putInt(w, QSCANSTATE, ns); if (U.compareAndSwapLong(this, CTL, c, nc)) ss = ns; else w.scanState = ss; // back out } checkSum = 0; } } } return null; } /** * Possibly blocks worker w waiting for a task to steal, or * returns false if the worker should terminate. If inactivating * w has caused the pool to become quiescent, checks for pool * termination, and, so long as this is not the only worker, waits * for up to a given duration. On timeout, if ctl has not * changed, terminates the worker, which will in turn wake up * another worker to possibly repeat this process. * * @param w the calling worker * @param r a random seed (for spins) * @return false if the worker should terminate */ private boolean awaitWork(WorkQueue w, int r) { if (w == null || w.qlock < 0) // w is terminating return false; for (int pred = w.stackPred, spins = SPINS, ss;;) { if ((ss = w.scanState) >= 0) break; else if (spins > 0) { r ^= r << 6; r ^= r >>> 21; r ^= r << 7; if (r >= 0 && --spins == 0) { // randomize spins WorkQueue v; WorkQueue[] ws; int s, j; AtomicLong sc; if (pred != 0 && (ws = workQueues) != null && (j = pred & SMASK) < ws.length && (v = ws[j]) != null && // see if pred parking (v.parker == null || v.scanState >= 0)) spins = SPINS; // continue spinning } } else if (w.qlock < 0) // recheck after spins return false; else if (!Thread.interrupted()) { long c, prevctl, parkTime, deadline; int ac = (int)((c = ctl) >> AC_SHIFT) + (config & SMASK); if ((ac <= 0 && tryTerminate(false, false)) || (runState & STOP) != 0) // pool terminating return false; if (ac <= 0 && ss == (int)c) { // is last waiter prevctl = (UC_MASK & (c + AC_UNIT)) | (SP_MASK & pred); int t = (short)(c >>> TC_SHIFT); // shrink excess spares if (t > 2 && U.compareAndSwapLong(this, CTL, c, prevctl)) return false; // else use timed wait parkTime = IDLE_TIMEOUT * ((t >= 0) ? 1 : 1 - t); deadline = System.nanoTime() + parkTime - TIMEOUT_SLOP; } else prevctl = parkTime = deadline = 0L; Thread wt = Thread.currentThread(); U.putObject(wt, PARKBLOCKER, this); // emulate LockSupport w.parker = wt; if (w.scanState < 0 && ctl == c) // recheck before park U.park(false, parkTime); U.putOrderedObject(w, QPARKER, null); U.putObject(wt, PARKBLOCKER, null); if (w.scanState >= 0) break; if (parkTime != 0L && ctl == c && deadline - System.nanoTime() <= 0L && U.compareAndSwapLong(this, CTL, c, prevctl)) return false; // shrink pool } } return true; } // Joining tasks /** * Tries to steal and run tasks within the target's computation. * Uses a variant of the top-level algorithm, restricted to tasks * with the given task as ancestor: It prefers taking and running * eligible tasks popped from the worker's own queue (via * popCC). Otherwise it scans others, randomly moving on * contention or execution, deciding to give up based on a * checksum (via return codes frob pollAndExecCC). The maxTasks * argument supports external usages; internal calls use zero, * allowing unbounded steps (external calls trap non-positive * values). * * @param w caller * @param maxTasks if non-zero, the maximum number of other tasks to run * @return task status on exit */ final int helpComplete(WorkQueue w, CountedCompleter task, int maxTasks) { WorkQueue[] ws; int s = 0, m; if ((ws = workQueues) != null && (m = ws.length - 1) >= 0 && task != null && w != null) { int mode = w.config; // for popCC int r = w.hint ^ w.top; // arbitrary seed for origin int origin = r & m; // first queue to scan int h = 1; // 1:ran, >1:contended, <0:hash for (int k = origin, oldSum = 0, checkSum = 0;;) { CountedCompleter p; WorkQueue q; if ((s = task.status) < 0) break; if (h == 1 && (p = w.popCC(task, mode)) != null) { p.doExec(); // run local task if (maxTasks != 0 && --maxTasks == 0) break; origin = k; // reset oldSum = checkSum = 0; } else { // poll other queues if ((q = ws[k]) == null) h = 0; else if ((h = q.pollAndExecCC(task)) < 0) checkSum += h; if (h > 0) { if (h == 1 && maxTasks != 0 && --maxTasks == 0) break; r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift origin = k = r & m; // move and restart oldSum = checkSum = 0; } else if ((k = (k + 1) & m) == origin) { if (oldSum == (oldSum = checkSum)) break; checkSum = 0; } } } } return s; } /** * Tries to locate and execute tasks for a stealer of the given * task, or in turn one of its stealers, Traces currentSteal -> * currentJoin links looking for a thread working on a descendant * of the given task and with a non-empty queue to steal back and * execute tasks from. The first call to this method upon a * waiting join will often entail scanning/search, (which is OK * because the joiner has nothing better to do), but this method * leaves hints in workers to speed up subsequent calls. * * @param w caller * @param task the task to join */ private void helpStealer(WorkQueue w, ForkJoinTask task) { WorkQueue[] ws = workQueues; int oldSum = 0, checkSum, m; if (ws != null && (m = ws.length - 1) >= 0 && w != null && task != null) { do { // restart point checkSum = 0; // for stability check ForkJoinTask subtask; WorkQueue j = w, v; // v is subtask stealer descent: for (subtask = task; subtask.status >= 0; ) { for (int h = j.hint | 1, k = 0, i; ; k += 2) { if (k > m) // can't find stealer break descent; if ((v = ws[i = (h + k) & m]) != null) { if (v.currentSteal == subtask) { j.hint = i; break; } checkSum += v.base; } } for (;;) { // help v or descend ForkJoinTask[] a; int b; checkSum += (b = v.base); ForkJoinTask next = v.currentJoin; if (subtask.status < 0 || j.currentJoin != subtask || v.currentSteal != subtask) // stale break descent; if (b - v.top >= 0 || (a = v.array) == null) { if ((subtask = next) == null) break descent; j = v; break; } int i = (((a.length - 1) & b) << ASHIFT) + ABASE; ForkJoinTask t = ((ForkJoinTask) U.getObjectVolatile(a, i)); if (v.base == b) { if (t == null) // stale break descent; if (U.compareAndSwapObject(a, i, t, null)) { v.base = b + 1; ForkJoinTask ps = w.currentSteal; int top = w.top; do { U.putOrderedObject(w, QCURRENTSTEAL, t); t.doExec(); // clear local tasks too } while (task.status >= 0 && w.top != top && (t = w.pop()) != null); U.putOrderedObject(w, QCURRENTSTEAL, ps); if (w.base != w.top) return; // can't further help } } } } } while (task.status >= 0 && oldSum != (oldSum = checkSum)); } } /** * Tries to decrement active count (sometimes implicitly) and * possibly release or create a compensating worker in preparation * for blocking. Returns false (retryable by caller), on * contention, detected staleness, instability, or termination. * * @param w caller */ private boolean tryCompensate(WorkQueue w) { boolean canBlock; WorkQueue[] ws; long c; int m, pc, sp; if (w == null || w.qlock < 0 || // caller terminating (ws = workQueues) == null || (m = ws.length - 1) <= 0 || (pc = config & SMASK) == 0) // parallelism disabled canBlock = false; else if ((sp = (int)(c = ctl)) != 0) // release idle worker canBlock = tryRelease(c, ws[sp & m], 0L); else { int ac = (int)(c >> AC_SHIFT) + pc; int tc = (short)(c >> TC_SHIFT) + pc; int nbusy = 0; // validate saturation for (int i = 0; i <= m; ++i) { // two passes of odd indices WorkQueue v; if ((v = ws[((i << 1) | 1) & m]) != null) { if ((v.scanState & SCANNING) != 0) break; ++nbusy; } } if (nbusy != (tc << 1) || ctl != c) canBlock = false; // unstable or stale else if (tc >= pc && ac > 1 && w.isEmpty()) { long nc = ((AC_MASK & (c - AC_UNIT)) | (~AC_MASK & c)); // uncompensated canBlock = U.compareAndSwapLong(this, CTL, c, nc); } else if (tc >= MAX_CAP || (this == common && tc >= pc + commonMaxSpares)) throw new RejectedExecutionException( "Thread limit exceeded replacing blocked worker"); else { // similar to tryAddWorker boolean add = false; int rs; // CAS within lock long nc = ((AC_MASK & c) | (TC_MASK & (c + TC_UNIT))); if (((rs = lockRunState()) & STOP) == 0) add = U.compareAndSwapLong(this, CTL, c, nc); unlockRunState(rs, rs & ~RSLOCK); canBlock = add && createWorker(); // throws on exception } } return canBlock; } /** * Helps and/or blocks until the given task is done or timeout. * * @param w caller * @param task the task * @param deadline for timed waits, if nonzero * @return task status on exit */ final int awaitJoin(WorkQueue w, ForkJoinTask task, long deadline) { int s = 0; if (task != null && w != null) { ForkJoinTask prevJoin = w.currentJoin; U.putOrderedObject(w, QCURRENTJOIN, task); CountedCompleter cc = (task instanceof CountedCompleter) ? (CountedCompleter)task : null; for (;;) { if ((s = task.status) < 0) break; if (cc != null) helpComplete(w, cc, 0); else if (w.base == w.top || w.tryRemoveAndExec(task)) helpStealer(w, task); if ((s = task.status) < 0) break; long ms, ns; if (deadline == 0L) ms = 0L; else if ((ns = deadline - System.nanoTime()) <= 0L) break; else if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) <= 0L) ms = 1L; if (tryCompensate(w)) { task.internalWait(ms); U.getAndAddLong(this, CTL, AC_UNIT); } } U.putOrderedObject(w, QCURRENTJOIN, prevJoin); } return s; } // Specialized scanning /** * Returns a (probably) non-empty steal queue, if one is found * during a scan, else null. This method must be retried by * caller if, by the time it tries to use the queue, it is empty. */ private WorkQueue findNonEmptyStealQueue() { WorkQueue[] ws; int m; // one-shot version of scan loop int r = ThreadLocalRandom.nextSecondarySeed(); if ((ws = workQueues) != null && (m = ws.length - 1) >= 0) { for (int origin = r & m, k = origin, oldSum = 0, checkSum = 0;;) { WorkQueue q; int b; if ((q = ws[k]) != null) { if ((b = q.base) - q.top < 0) return q; checkSum += b; } if ((k = (k + 1) & m) == origin) { if (oldSum == (oldSum = checkSum)) break; checkSum = 0; } } } return null; } /** * Runs tasks until {@code isQuiescent()}. We piggyback on * active count ctl maintenance, but rather than blocking * when tasks cannot be found, we rescan until all others cannot * find tasks either. */ final void helpQuiescePool(WorkQueue w) { ForkJoinTask ps = w.currentSteal; // save context for (boolean active = true;;) { long c; WorkQueue q; ForkJoinTask t; int b; w.execLocalTasks(); // run locals before each scan if ((q = findNonEmptyStealQueue()) != null) { if (!active) { // re-establish active count active = true; U.getAndAddLong(this, CTL, AC_UNIT); } if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null) { U.putOrderedObject(w, QCURRENTSTEAL, t); t.doExec(); if (++w.nsteals < 0) w.transferStealCount(this); } } else if (active) { // decrement active count without queuing long nc = (AC_MASK & ((c = ctl) - AC_UNIT)) | (~AC_MASK & c); if ((int)(nc >> AC_SHIFT) + (config & SMASK) <= 0) break; // bypass decrement-then-increment if (U.compareAndSwapLong(this, CTL, c, nc)) active = false; } else if ((int)((c = ctl) >> AC_SHIFT) + (config & SMASK) <= 0 && U.compareAndSwapLong(this, CTL, c, c + AC_UNIT)) break; } U.putOrderedObject(w, QCURRENTSTEAL, ps); } /** * Gets and removes a local or stolen task for the given worker. * * @return a task, if available */ final ForkJoinTask nextTaskFor(WorkQueue w) { for (ForkJoinTask t;;) { WorkQueue q; int b; if ((t = w.nextLocalTask()) != null) return t; if ((q = findNonEmptyStealQueue()) == null) return null; if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null) return t; } } /** * Returns a cheap heuristic guide for task partitioning when * programmers, frameworks, tools, or languages have little or no * idea about task granularity. In essence, by offering this * method, we ask users only about tradeoffs in overhead vs * expected throughput and its variance, rather than how finely to * partition tasks. * * In a steady state strict (tree-structured) computation, each * thread makes available for stealing enough tasks for other * threads to remain active. Inductively, if all threads play by * the same rules, each thread should make available only a * constant number of tasks. * * The minimum useful constant is just 1. But using a value of 1 * would require immediate replenishment upon each steal to * maintain enough tasks, which is infeasible. Further, * partitionings/granularities of offered tasks should minimize * steal rates, which in general means that threads nearer the top * of computation tree should generate more than those nearer the * bottom. In perfect steady state, each thread is at * approximately the same level of computation tree. However, * producing extra tasks amortizes the uncertainty of progress and * diffusion assumptions. * * So, users will want to use values larger (but not much larger) * than 1 to both smooth over transient shortages and hedge * against uneven progress; as traded off against the cost of * extra task overhead. We leave the user to pick a threshold * value to compare with the results of this call to guide * decisions, but recommend values such as 3. * * When all threads are active, it is on average OK to estimate * surplus strictly locally. In steady-state, if one thread is * maintaining say 2 surplus tasks, then so are others. So we can * just use estimated queue length. However, this strategy alone * leads to serious mis-estimates in some non-steady-state * conditions (ramp-up, ramp-down, other stalls). We can detect * many of these by further considering the number of "idle" * threads, that are known to have zero queued tasks, so * compensate by a factor of (#idle/#active) threads. */ static int getSurplusQueuedTaskCount() { Thread t; ForkJoinWorkerThread wt; ForkJoinPool pool; WorkQueue q; if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)) { int p = (pool = (wt = (ForkJoinWorkerThread)t).pool). config & SMASK; int n = (q = wt.workQueue).top - q.base; int a = (int)(pool.ctl >> AC_SHIFT) + p; return n - (a > (p >>>= 1) ? 0 : a > (p >>>= 1) ? 1 : a > (p >>>= 1) ? 2 : a > (p >>>= 1) ? 4 : 8); } return 0; } // Termination /** * Possibly initiates and/or completes termination. * * @param now if true, unconditionally terminate, else only * if no work and no active workers * @param enable if true, enable shutdown when next possible * @return true if now terminating or terminated */ private boolean tryTerminate(boolean now, boolean enable) { int rs; if (this == common) // cannot shut down return false; if ((rs = runState) >= 0) { if (!enable) return false; rs = lockRunState(); // enter SHUTDOWN phase unlockRunState(rs, (rs & ~RSLOCK) | SHUTDOWN); } if ((rs & STOP) == 0) { if (!now) { // check quiescence for (long oldSum = 0L;;) { // repeat until stable WorkQueue[] ws; WorkQueue w; int m, b; long c; long checkSum = ctl; if ((int)(checkSum >> AC_SHIFT) + (config & SMASK) > 0) return false; // still active workers if ((ws = workQueues) == null || (m = ws.length - 1) <= 0) break; // check queues for (int i = 0; i <= m; ++i) { if ((w = ws[i]) != null) { if ((b = w.base) != w.top || w.scanState >= 0 || w.currentSteal != null) { tryRelease(c = ctl, ws[m & (int)c], AC_UNIT); return false; // arrange for recheck } checkSum += b; if ((i & 1) == 0) w.qlock = -1; // try to disable external } } if (oldSum == (oldSum = checkSum)) break; } } if ((runState & STOP) == 0) { rs = lockRunState(); // enter STOP phase unlockRunState(rs, (rs & ~RSLOCK) | STOP); } } int pass = 0; // 3 passes to help terminate for (long oldSum = 0L;;) { // or until done or stable WorkQueue[] ws; WorkQueue w; ForkJoinWorkerThread wt; int m; long checkSum = ctl; if ((short)(checkSum >>> TC_SHIFT) + (config & SMASK) <= 0 || (ws = workQueues) == null || (m = ws.length - 1) <= 0) { if ((runState & TERMINATED) == 0) { rs = lockRunState(); // done unlockRunState(rs, (rs & ~RSLOCK) | TERMINATED); synchronized (this) { notifyAll(); } // for awaitTermination } break; } for (int i = 0; i <= m; ++i) { if ((w = ws[i]) != null) { checkSum += w.base; w.qlock = -1; // try to disable if (pass > 0) { w.cancelAll(); // clear queue if (pass > 1 && (wt = w.owner) != null) { if (!wt.isInterrupted()) { try { // unblock join wt.interrupt(); } catch (Throwable ignore) { } } if (w.scanState < 0) U.unpark(wt); // wake up } } } } if (checkSum != oldSum) { // unstable oldSum = checkSum; pass = 0; } else if (pass > 3 && pass > m) // can't further help break; else if (++pass > 1) { // try to dequeue long c; int j = 0, sp; // bound attempts while (j++ <= m && (sp = (int)(c = ctl)) != 0) tryRelease(c, ws[sp & m], AC_UNIT); } } return true; } // External operations /** * Full version of externalPush, handling uncommon cases, as well * as performing secondary initialization upon the first * submission of the first task to the pool. It also detects * first submission by an external thread and creates a new shared * queue if the one at index if empty or contended. * * @param task the task. Caller must ensure non-null. */ private void externalSubmit(ForkJoinTask task) { int r; // initialize caller's probe if ((r = ThreadLocalRandom.getProbe()) == 0) { ThreadLocalRandom.localInit(); r = ThreadLocalRandom.getProbe(); } for (;;) { WorkQueue[] ws; WorkQueue q; int rs, m, k; boolean move = false; if ((rs = runState) < 0) { tryTerminate(false, false); // help terminate throw new RejectedExecutionException(); } else if ((rs & STARTED) == 0 || // initialize ((ws = workQueues) == null || (m = ws.length - 1) < 0)) { int ns = 0; rs = lockRunState(); try { if ((rs & STARTED) == 0) { U.compareAndSwapObject(this, STEALCOUNTER, null, new AtomicLong()); // create workQueues array with size a power of two int p = config & SMASK; // ensure at least 2 slots int n = (p > 1) ? p - 1 : 1; n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8; n |= n >>> 16; n = (n + 1) << 1; workQueues = new WorkQueue[n]; ns = STARTED; } } finally { unlockRunState(rs, (rs & ~RSLOCK) | ns); } } else if ((q = ws[k = r & m & SQMASK]) != null) { if (q.qlock == 0 && U.compareAndSwapInt(q, QLOCK, 0, 1)) { ForkJoinTask[] a = q.array; int s = q.top; boolean submitted = false; // initial submission or resizing try { // locked version of push if ((a != null && a.length > s + 1 - q.base) || (a = q.growArray()) != null) { int j = (((a.length - 1) & s) << ASHIFT) + ABASE; U.putOrderedObject(a, j, task); U.putOrderedInt(q, QTOP, s + 1); submitted = true; } } finally { U.compareAndSwapInt(q, QLOCK, 1, 0); } if (submitted) { signalWork(ws, q); return; } } move = true; // move on failure } else if (((rs = runState) & RSLOCK) == 0) { // create new queue q = new WorkQueue(this, null); q.hint = r; q.config = k | SHARED_QUEUE; q.scanState = INACTIVE; rs = lockRunState(); // publish index if (rs > 0 && (ws = workQueues) != null && k < ws.length && ws[k] == null) ws[k] = q; // else terminated unlockRunState(rs, rs & ~RSLOCK); } else move = true; // move if busy if (move) r = ThreadLocalRandom.advanceProbe(r); } } /** * Tries to add the given task to a submission queue at * submitter's current queue. Only the (vastly) most common path * is directly handled in this method, while screening for need * for externalSubmit. * * @param task the task. Caller must ensure non-null. */ final void externalPush(ForkJoinTask task) { WorkQueue[] ws; WorkQueue q; int m; int r = ThreadLocalRandom.getProbe(); int rs = runState; if ((ws = workQueues) != null && (m = (ws.length - 1)) >= 0 && (q = ws[m & r & SQMASK]) != null && r != 0 && rs > 0 && U.compareAndSwapInt(q, QLOCK, 0, 1)) { ForkJoinTask[] a; int am, n, s; if ((a = q.array) != null && (am = a.length - 1) > (n = (s = q.top) - q.base)) { int j = ((am & s) << ASHIFT) + ABASE; U.putOrderedObject(a, j, task); U.putOrderedInt(q, QTOP, s + 1); U.putOrderedInt(q, QLOCK, 0); if (n <= 1) signalWork(ws, q); return; } U.compareAndSwapInt(q, QLOCK, 1, 0); } externalSubmit(task); } /** * Returns common pool queue for an external thread. */ static WorkQueue commonSubmitterQueue() { ForkJoinPool p = common; int r = ThreadLocalRandom.getProbe(); WorkQueue[] ws; int m; return (p != null && (ws = p.workQueues) != null && (m = ws.length - 1) >= 0) ? ws[m & r & SQMASK] : null; } /** * Performs tryUnpush for an external submitter: Finds queue, * locks if apparently non-empty, validates upon locking, and * adjusts top. Each check can fail but rarely does. */ final boolean tryExternalUnpush(ForkJoinTask task) { WorkQueue[] ws; WorkQueue w; ForkJoinTask[] a; int m, s; int r = ThreadLocalRandom.getProbe(); if ((ws = workQueues) != null && (m = ws.length - 1) >= 0 && (w = ws[m & r & SQMASK]) != null && (a = w.array) != null && (s = w.top) != w.base) { long j = (((a.length - 1) & (s - 1)) << ASHIFT) + ABASE; if (U.compareAndSwapInt(w, QLOCK, 0, 1)) { if (w.top == s && w.array == a && U.getObject(a, j) == task && U.compareAndSwapObject(a, j, task, null)) { U.putOrderedInt(w, QTOP, s - 1); U.putOrderedInt(w, QLOCK, 0); return true; } U.compareAndSwapInt(w, QLOCK, 1, 0); } } return false; } /** * Performs helpComplete for an external submitter. */ final int externalHelpComplete(CountedCompleter task, int maxTasks) { WorkQueue[] ws; int n; int r = ThreadLocalRandom.getProbe(); return ((ws = workQueues) == null || (n = ws.length) == 0) ? 0 : helpComplete(ws[(n - 1) & r & SQMASK], task, maxTasks); } // Exported methods // Constructors /** * Creates a {@code ForkJoinPool} with parallelism equal to {@link * java.lang.Runtime#availableProcessors}, using the {@linkplain * #defaultForkJoinWorkerThreadFactory default thread factory}, * no UncaughtExceptionHandler, and non-async LIFO processing mode. * * @throws SecurityException if a security manager exists and * the caller is not permitted to modify threads * because it does not hold {@link * java.lang.RuntimePermission}{@code ("modifyThread")} */ public ForkJoinPool() { this(Math.min(MAX_CAP, Runtime.getRuntime().availableProcessors()), defaultForkJoinWorkerThreadFactory, null, false); } /** * Creates a {@code ForkJoinPool} with the indicated parallelism * level, the {@linkplain * #defaultForkJoinWorkerThreadFactory default thread factory}, * no UncaughtExceptionHandler, and non-async LIFO processing mode. * * @param parallelism the parallelism level * @throws IllegalArgumentException if parallelism less than or * equal to zero, or greater than implementation limit * @throws SecurityException if a security manager exists and * the caller is not permitted to modify threads * because it does not hold {@link * java.lang.RuntimePermission}{@code ("modifyThread")} */ public ForkJoinPool(int parallelism) { this(parallelism, defaultForkJoinWorkerThreadFactory, null, false); } /** * Creates a {@code ForkJoinPool} with the given parameters. * * @param parallelism the parallelism level. For default value, * use {@link java.lang.Runtime#availableProcessors}. * @param factory the factory for creating new threads. For default value, * use {@link #defaultForkJoinWorkerThreadFactory}. * @param handler the handler for internal worker threads that * terminate due to unrecoverable errors encountered while executing * tasks. For default value, use {@code null}. * @param asyncMode if true, * establishes local first-in-first-out scheduling mode for forked * tasks that are never joined. This mode may be more appropriate * than default locally stack-based mode in applications in which * worker threads only process event-style asynchronous tasks. * For default value, use {@code false}. * @throws IllegalArgumentException if parallelism less than or * equal to zero, or greater than implementation limit * @throws NullPointerException if the factory is null * @throws SecurityException if a security manager exists and * the caller is not permitted to modify threads * because it does not hold {@link * java.lang.RuntimePermission}{@code ("modifyThread")} */ public ForkJoinPool(int parallelism, ForkJoinWorkerThreadFactory factory, UncaughtExceptionHandler handler, boolean asyncMode) { this(checkParallelism(parallelism), checkFactory(factory), handler, asyncMode ? FIFO_QUEUE : LIFO_QUEUE, "ForkJoinPool-" + nextPoolId() + "-worker-"); checkPermission(); } private static int checkParallelism(int parallelism) { if (parallelism <= 0 || parallelism > MAX_CAP) throw new IllegalArgumentException(); return parallelism; } private static ForkJoinWorkerThreadFactory checkFactory (ForkJoinWorkerThreadFactory factory) { if (factory == null) throw new NullPointerException(); return factory; } /** * Creates a {@code ForkJoinPool} with the given parameters, without * any security checks or parameter validation. Invoked directly by * makeCommonPool. */ private ForkJoinPool(int parallelism, ForkJoinWorkerThreadFactory factory, UncaughtExceptionHandler handler, int mode, String workerNamePrefix) { this.workerNamePrefix = workerNamePrefix; this.factory = factory; this.ueh = handler; this.config = (parallelism & SMASK) | mode; long np = (long)(-parallelism); // offset ctl counts this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK); } /** * Returns the common pool instance. This pool is statically * constructed; its run state is unaffected by attempts to {@link * #shutdown} or {@link #shutdownNow}. However this pool and any * ongoing processing are automatically terminated upon program * {@link System#exit}. Any program that relies on asynchronous * task processing to complete before program termination should * invoke {@code commonPool().}{@link #awaitQuiescence awaitQuiescence}, * before exit. * * @return the common pool instance * @since 1.8 */ public static ForkJoinPool commonPool() { // assert common != null : "static init error"; return common; } // Execution methods /** * Performs the given task, returning its result upon completion. * If the computation encounters an unchecked Exception or Error, * it is rethrown as the outcome of this invocation. Rethrown * exceptions behave in the same way as regular exceptions, but, * when possible, contain stack traces (as displayed for example * using {@code ex.printStackTrace()}) of both the current thread * as well as the thread actually encountering the exception; * minimally only the latter. * * @param task the task * @param the type of the task's result * @return the task's result * @throws NullPointerException if the task is null * @throws RejectedExecutionException if the task cannot be * scheduled for execution */ public T invoke(ForkJoinTask task) { if (task == null) throw new NullPointerException(); externalPush(task); return task.join(); } /** * Arranges for (asynchronous) execution of the given task. * * @param task the task * @throws NullPointerException if the task is null * @throws RejectedExecutionException if the task cannot be * scheduled for execution */ public void execute(ForkJoinTask task) { if (task == null) throw new NullPointerException(); externalPush(task); } // AbstractExecutorService methods /** * @throws NullPointerException if the task is null * @throws RejectedExecutionException if the task cannot be * scheduled for execution */ public void execute(Runnable task) { if (task == null) throw new NullPointerException(); ForkJoinTask job; if (task instanceof ForkJoinTask) // avoid re-wrap job = (ForkJoinTask) task; else job = new ForkJoinTask.RunnableExecuteAction(task); externalPush(job); } /** * Submits a ForkJoinTask for execution. * * @param task the task to submit * @param the type of the task's result * @return the task * @throws NullPointerException if the task is null * @throws RejectedExecutionException if the task cannot be * scheduled for execution */ public ForkJoinTask submit(ForkJoinTask task) { if (task == null) throw new NullPointerException(); externalPush(task); return task; } /** * @throws NullPointerException if the task is null * @throws RejectedExecutionException if the task cannot be * scheduled for execution */ public ForkJoinTask submit(Callable task) { ForkJoinTask job = new ForkJoinTask.AdaptedCallable(task); externalPush(job); return job; } /** * @throws NullPointerException if the task is null * @throws RejectedExecutionException if the task cannot be * scheduled for execution */ public ForkJoinTask submit(Runnable task, T result) { ForkJoinTask job = new ForkJoinTask.AdaptedRunnable(task, result); externalPush(job); return job; } /** * @throws NullPointerException if the task is null * @throws RejectedExecutionException if the task cannot be * scheduled for execution */ public ForkJoinTask submit(Runnable task) { if (task == null) throw new NullPointerException(); ForkJoinTask job; if (task instanceof ForkJoinTask) // avoid re-wrap job = (ForkJoinTask) task; else job = new ForkJoinTask.AdaptedRunnableAction(task); externalPush(job); return job; } /** * @throws NullPointerException {@inheritDoc} * @throws RejectedExecutionException {@inheritDoc} */ public List> invokeAll(Collection> tasks) { // In previous versions of this class, this method constructed // a task to run ForkJoinTask.invokeAll, but now external // invocation of multiple tasks is at least as efficient. ArrayList> futures = new ArrayList<>(tasks.size()); boolean done = false; try { for (Callable t : tasks) { ForkJoinTask f = new ForkJoinTask.AdaptedCallable(t); futures.add(f); externalPush(f); } for (int i = 0, size = futures.size(); i < size; i++) ((ForkJoinTask)futures.get(i)).quietlyJoin(); done = true; return futures; } finally { if (!done) for (int i = 0, size = futures.size(); i < size; i++) futures.get(i).cancel(false); } } /** * Returns the factory used for constructing new workers. * * @return the factory used for constructing new workers */ public ForkJoinWorkerThreadFactory getFactory() { return factory; } /** * Returns the handler for internal worker threads that terminate * due to unrecoverable errors encountered while executing tasks. * * @return the handler, or {@code null} if none */ public UncaughtExceptionHandler getUncaughtExceptionHandler() { return ueh; } /** * Returns the targeted parallelism level of this pool. * * @return the targeted parallelism level of this pool */ public int getParallelism() { int par; return ((par = config & SMASK) > 0) ? par : 1; } /** * Returns the targeted parallelism level of the common pool. * * @return the targeted parallelism level of the common pool * @since 1.8 */ public static int getCommonPoolParallelism() { return commonParallelism; } /** * Returns the number of worker threads that have started but not * yet terminated. The result returned by this method may differ * from {@link #getParallelism} when threads are created to * maintain parallelism when others are cooperatively blocked. * * @return the number of worker threads */ public int getPoolSize() { return (config & SMASK) + (short)(ctl >>> TC_SHIFT); } /** * Returns {@code true} if this pool uses local first-in-first-out * scheduling mode for forked tasks that are never joined. * * @return {@code true} if this pool uses async mode */ public boolean getAsyncMode() { return (config & FIFO_QUEUE) != 0; } /** * Returns an estimate of the number of worker threads that are * not blocked waiting to join tasks or for other managed * synchronization. This method may overestimate the * number of running threads. * * @return the number of worker threads */ public int getRunningThreadCount() { int rc = 0; WorkQueue[] ws; WorkQueue w; if ((ws = workQueues) != null) { for (int i = 1; i < ws.length; i += 2) { if ((w = ws[i]) != null && w.isApparentlyUnblocked()) ++rc; } } return rc; } /** * Returns an estimate of the number of threads that are currently * stealing or executing tasks. This method may overestimate the * number of active threads. * * @return the number of active threads */ public int getActiveThreadCount() { int r = (config & SMASK) + (int)(ctl >> AC_SHIFT); return (r <= 0) ? 0 : r; // suppress momentarily negative values } /** * Returns {@code true} if all worker threads are currently idle. * An idle worker is one that cannot obtain a task to execute * because none are available to steal from other threads, and * there are no pending submissions to the pool. This method is * conservative; it might not return {@code true} immediately upon * idleness of all threads, but will eventually become true if * threads remain inactive. * * @return {@code true} if all threads are currently idle */ public boolean isQuiescent() { return (config & SMASK) + (int)(ctl >> AC_SHIFT) <= 0; } /** * Returns an estimate of the total number of tasks stolen from * one thread's work queue by another. The reported value * underestimates the actual total number of steals when the pool * is not quiescent. This value may be useful for monitoring and * tuning fork/join programs: in general, steal counts should be * high enough to keep threads busy, but low enough to avoid * overhead and contention across threads. * * @return the number of steals */ public long getStealCount() { AtomicLong sc = stealCounter; long count = (sc == null) ? 0L : sc.get(); WorkQueue[] ws; WorkQueue w; if ((ws = workQueues) != null) { for (int i = 1; i < ws.length; i += 2) { if ((w = ws[i]) != null) count += w.nsteals; } } return count; } /** * Returns an estimate of the total number of tasks currently held * in queues by worker threads (but not including tasks submitted * to the pool that have not begun executing). This value is only * an approximation, obtained by iterating across all threads in * the pool. This method may be useful for tuning task * granularities. * * @return the number of queued tasks */ public long getQueuedTaskCount() { long count = 0; WorkQueue[] ws; WorkQueue w; if ((ws = workQueues) != null) { for (int i = 1; i < ws.length; i += 2) { if ((w = ws[i]) != null) count += w.queueSize(); } } return count; } /** * Returns an estimate of the number of tasks submitted to this * pool that have not yet begun executing. This method may take * time proportional to the number of submissions. * * @return the number of queued submissions */ public int getQueuedSubmissionCount() { int count = 0; WorkQueue[] ws; WorkQueue w; if ((ws = workQueues) != null) { for (int i = 0; i < ws.length; i += 2) { if ((w = ws[i]) != null) count += w.queueSize(); } } return count; } /** * Returns {@code true} if there are any tasks submitted to this * pool that have not yet begun executing. * * @return {@code true} if there are any queued submissions */ public boolean hasQueuedSubmissions() { WorkQueue[] ws; WorkQueue w; if ((ws = workQueues) != null) { for (int i = 0; i < ws.length; i += 2) { if ((w = ws[i]) != null && !w.isEmpty()) return true; } } return false; } /** * Removes and returns the next unexecuted submission if one is * available. This method may be useful in extensions to this * class that re-assign work in systems with multiple pools. * * @return the next submission, or {@code null} if none */ protected ForkJoinTask pollSubmission() { WorkQueue[] ws; WorkQueue w; ForkJoinTask t; if ((ws = workQueues) != null) { for (int i = 0; i < ws.length; i += 2) { if ((w = ws[i]) != null && (t = w.poll()) != null) return t; } } return null; } /** * Removes all available unexecuted submitted and forked tasks * from scheduling queues and adds them to the given collection, * without altering their execution status. These may include * artificially generated or wrapped tasks. This method is * designed to be invoked only when the pool is known to be * quiescent. Invocations at other times may not remove all * tasks. A failure encountered while attempting to add elements * to collection {@code c} may result in elements being in * neither, either or both collections when the associated * exception is thrown. The behavior of this operation is * undefined if the specified collection is modified while the * operation is in progress. * * @param c the collection to transfer elements into * @return the number of elements transferred */ protected int drainTasksTo(Collection> c) { int count = 0; WorkQueue[] ws; WorkQueue w; ForkJoinTask t; if ((ws = workQueues) != null) { for (int i = 0; i < ws.length; ++i) { if ((w = ws[i]) != null) { while ((t = w.poll()) != null) { c.add(t); ++count; } } } } return count; } /** * Returns a string identifying this pool, as well as its state, * including indications of run state, parallelism level, and * worker and task counts. * * @return a string identifying this pool, as well as its state */ public String toString() { // Use a single pass through workQueues to collect counts long qt = 0L, qs = 0L; int rc = 0; AtomicLong sc = stealCounter; long st = (sc == null) ? 0L : sc.get(); long c = ctl; WorkQueue[] ws; WorkQueue w; if ((ws = workQueues) != null) { for (int i = 0; i < ws.length; ++i) { if ((w = ws[i]) != null) { int size = w.queueSize(); if ((i & 1) == 0) qs += size; else { qt += size; st += w.nsteals; if (w.isApparentlyUnblocked()) ++rc; } } } } int pc = (config & SMASK); int tc = pc + (short)(c >>> TC_SHIFT); int ac = pc + (int)(c >> AC_SHIFT); if (ac < 0) // ignore transient negative ac = 0; int rs = runState; String level = ((rs & TERMINATED) != 0 ? "Terminated" : (rs & STOP) != 0 ? "Terminating" : (rs & SHUTDOWN) != 0 ? "Shutting down" : "Running"); return super.toString() + "[" + level + ", parallelism = " + pc + ", size = " + tc + ", active = " + ac + ", running = " + rc + ", steals = " + st + ", tasks = " + qt + ", submissions = " + qs + "]"; } /** * Possibly initiates an orderly shutdown in which previously * submitted tasks are executed, but no new tasks will be * accepted. Invocation has no effect on execution state if this * is the {@link #commonPool()}, and no additional effect if * already shut down. Tasks that are in the process of being * submitted concurrently during the course of this method may or * may not be rejected. * * @throws SecurityException if a security manager exists and * the caller is not permitted to modify threads * because it does not hold {@link * java.lang.RuntimePermission}{@code ("modifyThread")} */ public void shutdown() { checkPermission(); tryTerminate(false, true); } /** * Possibly attempts to cancel and/or stop all tasks, and reject * all subsequently submitted tasks. Invocation has no effect on * execution state if this is the {@link #commonPool()}, and no * additional effect if already shut down. Otherwise, tasks that * are in the process of being submitted or executed concurrently * during the course of this method may or may not be * rejected. This method cancels both existing and unexecuted * tasks, in order to permit termination in the presence of task * dependencies. So the method always returns an empty list * (unlike the case for some other Executors). * * @return an empty list * @throws SecurityException if a security manager exists and * the caller is not permitted to modify threads * because it does not hold {@link * java.lang.RuntimePermission}{@code ("modifyThread")} */ public List shutdownNow() { checkPermission(); tryTerminate(true, true); return Collections.emptyList(); } /** * Returns {@code true} if all tasks have completed following shut down. * * @return {@code true} if all tasks have completed following shut down */ public boolean isTerminated() { return (runState & TERMINATED) != 0; } /** * Returns {@code true} if the process of termination has * commenced but not yet completed. This method may be useful for * debugging. A return of {@code true} reported a sufficient * period after shutdown may indicate that submitted tasks have * ignored or suppressed interruption, or are waiting for I/O, * causing this executor not to properly terminate. (See the * advisory notes for class {@link ForkJoinTask} stating that * tasks should not normally entail blocking operations. But if * they do, they must abort them on interrupt.) * * @return {@code true} if terminating but not yet terminated */ public boolean isTerminating() { int rs = runState; return (rs & STOP) != 0 && (rs & TERMINATED) == 0; } /** * Returns {@code true} if this pool has been shut down. * * @return {@code true} if this pool has been shut down */ public boolean isShutdown() { return (runState & SHUTDOWN) != 0; } /** * Blocks until all tasks have completed execution after a * shutdown request, or the timeout occurs, or the current thread * is interrupted, whichever happens first. Because the {@link * #commonPool()} never terminates until program shutdown, when * applied to the common pool, this method is equivalent to {@link * #awaitQuiescence(long, TimeUnit)} but always returns {@code false}. * * @param timeout the maximum time to wait * @param unit the time unit of the timeout argument * @return {@code true} if this executor terminated and * {@code false} if the timeout elapsed before termination * @throws InterruptedException if interrupted while waiting */ public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); if (this == common) { awaitQuiescence(timeout, unit); return false; } long nanos = unit.toNanos(timeout); if (isTerminated()) return true; if (nanos <= 0L) return false; long deadline = System.nanoTime() + nanos; synchronized (this) { for (;;) { if (isTerminated()) return true; if (nanos <= 0L) return false; long millis = TimeUnit.NANOSECONDS.toMillis(nanos); wait(millis > 0L ? millis : 1L); nanos = deadline - System.nanoTime(); } } } /** * If called by a ForkJoinTask operating in this pool, equivalent * in effect to {@link ForkJoinTask#helpQuiesce}. Otherwise, * waits and/or attempts to assist performing tasks until this * pool {@link #isQuiescent} or the indicated timeout elapses. * * @param timeout the maximum time to wait * @param unit the time unit of the timeout argument * @return {@code true} if quiescent; {@code false} if the * timeout elapsed. */ public boolean awaitQuiescence(long timeout, TimeUnit unit) { long nanos = unit.toNanos(timeout); ForkJoinWorkerThread wt; Thread thread = Thread.currentThread(); if ((thread instanceof ForkJoinWorkerThread) && (wt = (ForkJoinWorkerThread)thread).pool == this) { helpQuiescePool(wt.workQueue); return true; } long startTime = System.nanoTime(); WorkQueue[] ws; int r = 0, m; boolean found = true; while (!isQuiescent() && (ws = workQueues) != null && (m = ws.length - 1) >= 0) { if (!found) { if ((System.nanoTime() - startTime) > nanos) return false; Thread.yield(); // cannot block } found = false; for (int j = (m + 1) << 2; j >= 0; --j) { ForkJoinTask t; WorkQueue q; int b, k; if ((k = r++ & m) <= m && k >= 0 && (q = ws[k]) != null && (b = q.base) - q.top < 0) { found = true; if ((t = q.pollAt(b)) != null) t.doExec(); break; } } } return true; } /** * Waits and/or attempts to assist performing tasks indefinitely * until the {@link #commonPool()} {@link #isQuiescent}. */ static void quiesceCommonPool() { common.awaitQuiescence(Long.MAX_VALUE, TimeUnit.NANOSECONDS); } /** * Interface for extending managed parallelism for tasks running * in {@link ForkJoinPool}s. * *

A {@code ManagedBlocker} provides two methods. Method * {@link #isReleasable} must return {@code true} if blocking is * not necessary. Method {@link #block} blocks the current thread * if necessary (perhaps internally invoking {@code isReleasable} * before actually blocking). These actions are performed by any * thread invoking {@link ForkJoinPool#managedBlock(ManagedBlocker)}. * The unusual methods in this API accommodate synchronizers that * may, but don't usually, block for long periods. Similarly, they * allow more efficient internal handling of cases in which * additional workers may be, but usually are not, needed to * ensure sufficient parallelism. Toward this end, * implementations of method {@code isReleasable} must be amenable * to repeated invocation. * *

For example, here is a ManagedBlocker based on a * ReentrantLock: *

 {@code
     * class ManagedLocker implements ManagedBlocker {
     *   final ReentrantLock lock;
     *   boolean hasLock = false;
     *   ManagedLocker(ReentrantLock lock) { this.lock = lock; }
     *   public boolean block() {
     *     if (!hasLock)
     *       lock.lock();
     *     return true;
     *   }
     *   public boolean isReleasable() {
     *     return hasLock || (hasLock = lock.tryLock());
     *   }
     * }}
* *

Here is a class that possibly blocks waiting for an * item on a given queue: *

 {@code
     * class QueueTaker implements ManagedBlocker {
     *   final BlockingQueue queue;
     *   volatile E item = null;
     *   QueueTaker(BlockingQueue q) { this.queue = q; }
     *   public boolean block() throws InterruptedException {
     *     if (item == null)
     *       item = queue.take();
     *     return true;
     *   }
     *   public boolean isReleasable() {
     *     return item != null || (item = queue.poll()) != null;
     *   }
     *   public E getItem() { // call after pool.managedBlock completes
     *     return item;
     *   }
     * }}
*/ public static interface ManagedBlocker { /** * Possibly blocks the current thread, for example waiting for * a lock or condition. * * @return {@code true} if no additional blocking is necessary * (i.e., if isReleasable would return true) * @throws InterruptedException if interrupted while waiting * (the method is not required to do so, but is allowed to) */ boolean block() throws InterruptedException; /** * Returns {@code true} if blocking is unnecessary. * @return {@code true} if blocking is unnecessary */ boolean isReleasable(); } /** * Runs the given possibly blocking task. When {@linkplain * ForkJoinTask#inForkJoinPool() running in a ForkJoinPool}, this * method possibly arranges for a spare thread to be activated if * necessary to ensure sufficient parallelism while the current * thread is blocked in {@link ManagedBlocker#block blocker.block()}. * *

This method repeatedly calls {@code blocker.isReleasable()} and * {@code blocker.block()} until either method returns {@code true}. * Every call to {@code blocker.block()} is preceded by a call to * {@code blocker.isReleasable()} that returned {@code false}. * *

If not running in a ForkJoinPool, this method is * behaviorally equivalent to *

 {@code
     * while (!blocker.isReleasable())
     *   if (blocker.block())
     *     break;}
* * If running in a ForkJoinPool, the pool may first be expanded to * ensure sufficient parallelism available during the call to * {@code blocker.block()}. * * @param blocker the blocker task * @throws InterruptedException if {@code blocker.block()} did so */ public static void managedBlock(ManagedBlocker blocker) throws InterruptedException { ForkJoinPool p; ForkJoinWorkerThread wt; Thread t = Thread.currentThread(); if ((t instanceof ForkJoinWorkerThread) && (p = (wt = (ForkJoinWorkerThread)t).pool) != null) { WorkQueue w = wt.workQueue; while (!blocker.isReleasable()) { if (p.tryCompensate(w)) { try { do {} while (!blocker.isReleasable() && !blocker.block()); } finally { U.getAndAddLong(p, CTL, AC_UNIT); } break; } } } else { do {} while (!blocker.isReleasable() && !blocker.block()); } } // AbstractExecutorService overrides. These rely on undocumented // fact that ForkJoinTask.adapt returns ForkJoinTasks that also // implement RunnableFuture. protected RunnableFuture newTaskFor(Runnable runnable, T value) { return new ForkJoinTask.AdaptedRunnable(runnable, value); } protected RunnableFuture newTaskFor(Callable callable) { return new ForkJoinTask.AdaptedCallable(callable); } // Unsafe mechanics private static final sun.misc.Unsafe U; private static final int ABASE; private static final int ASHIFT; private static final long CTL; private static final long RUNSTATE; private static final long STEALCOUNTER; private static final long PARKBLOCKER; private static final long QTOP; private static final long QLOCK; private static final long QSCANSTATE; private static final long QPARKER; private static final long QCURRENTSTEAL; private static final long QCURRENTJOIN; static { // initialize field offsets for CAS etc try { U = sun.misc.Unsafe.getUnsafe(); Class k = ForkJoinPool.class; CTL = U.objectFieldOffset (k.getDeclaredField("ctl")); RUNSTATE = U.objectFieldOffset (k.getDeclaredField("runState")); STEALCOUNTER = U.objectFieldOffset (k.getDeclaredField("stealCounter")); Class tk = Thread.class; PARKBLOCKER = U.objectFieldOffset (tk.getDeclaredField("parkBlocker")); Class wk = WorkQueue.class; QTOP = U.objectFieldOffset (wk.getDeclaredField("top")); QLOCK = U.objectFieldOffset (wk.getDeclaredField("qlock")); QSCANSTATE = U.objectFieldOffset (wk.getDeclaredField("scanState")); QPARKER = U.objectFieldOffset (wk.getDeclaredField("parker")); QCURRENTSTEAL = U.objectFieldOffset (wk.getDeclaredField("currentSteal")); QCURRENTJOIN = U.objectFieldOffset (wk.getDeclaredField("currentJoin")); Class ak = ForkJoinTask[].class; ABASE = U.arrayBaseOffset(ak); int scale = U.arrayIndexScale(ak); if ((scale & (scale - 1)) != 0) throw new Error("data type scale not a power of two"); ASHIFT = 31 - Integer.numberOfLeadingZeros(scale); } catch (Exception e) { throw new Error(e); } commonMaxSpares = DEFAULT_COMMON_MAX_SPARES; defaultForkJoinWorkerThreadFactory = new DefaultForkJoinWorkerThreadFactory(); modifyThreadPermission = new RuntimePermission("modifyThread"); common = java.security.AccessController.doPrivileged (new java.security.PrivilegedAction() { public ForkJoinPool run() { return makeCommonPool(); }}); int par = common.config & SMASK; // report 1 even if threads disabled commonParallelism = par > 0 ? par : 1; } /** * Creates and returns the common pool, respecting user settings * specified via system properties. */ private static ForkJoinPool makeCommonPool() { int parallelism = -1; ForkJoinWorkerThreadFactory factory = null; UncaughtExceptionHandler handler = null; try { // ignore exceptions in accessing/parsing properties String pp = System.getProperty ("java.util.concurrent.ForkJoinPool.common.parallelism"); String fp = System.getProperty ("java.util.concurrent.ForkJoinPool.common.threadFactory"); String hp = System.getProperty ("java.util.concurrent.ForkJoinPool.common.exceptionHandler"); if (pp != null) parallelism = Integer.parseInt(pp); if (fp != null) factory = ((ForkJoinWorkerThreadFactory)ClassLoader. getSystemClassLoader().loadClass(fp).newInstance()); if (hp != null) handler = ((UncaughtExceptionHandler)ClassLoader. getSystemClassLoader().loadClass(hp).newInstance()); } catch (Exception ignore) { } if (factory == null) { if (System.getSecurityManager() == null) factory = defaultForkJoinWorkerThreadFactory; else // use security-managed default factory = new InnocuousForkJoinWorkerThreadFactory(); } if (parallelism < 0 && // default 1 less than #cores (parallelism = Runtime.getRuntime().availableProcessors() - 1) <= 0) parallelism = 1; if (parallelism > MAX_CAP) parallelism = MAX_CAP; return new ForkJoinPool(parallelism, factory, handler, LIFO_QUEUE, "ForkJoinPool.commonPool-worker-"); } /** * Factory for innocuous worker threads */ static final class InnocuousForkJoinWorkerThreadFactory implements ForkJoinWorkerThreadFactory { /** * An ACC to restrict permissions for the factory itself. * The constructed workers have no permissions set. */ private static final AccessControlContext innocuousAcc; static { Permissions innocuousPerms = new Permissions(); innocuousPerms.add(modifyThreadPermission); innocuousPerms.add(new RuntimePermission( "enableContextClassLoaderOverride")); innocuousPerms.add(new RuntimePermission( "modifyThreadGroup")); innocuousAcc = new AccessControlContext(new ProtectionDomain[] { new ProtectionDomain(null, innocuousPerms) }); } public final ForkJoinWorkerThread newThread(ForkJoinPool pool) { return (ForkJoinWorkerThread.InnocuousForkJoinWorkerThread) java.security.AccessController.doPrivileged( new java.security.PrivilegedAction() { public ForkJoinWorkerThread run() { return new ForkJoinWorkerThread. InnocuousForkJoinWorkerThread(pool); }}, innocuousAcc); } } }