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/*
* 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.
*
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/*
* 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.security.AccessControlContext;
import java.security.Permissions;
import java.security.ProtectionDomain;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.LockSupport;
/**
* 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.
*
* <p>A {@code ForkJoinPool} differs from other kinds of {@link
* ExecutorService} mainly by virtue of employing
* <em>work-stealing</em>: 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 <em>asyncMode</em> to true in constructors, {@code
* ForkJoinPool}s may also be appropriate for use with event-style
* tasks that are never joined.
*
* <p>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).
*
* <p>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.
*
* <p>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.
*
* <p>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.
*
* <table BORDER CELLPADDING=3 CELLSPACING=1>
* <caption>Summary of task execution methods</caption>
* <tr>
* <td></td>
* <td ALIGN=CENTER> <b>Call from non-fork/join clients</b></td>
* <td ALIGN=CENTER> <b>Call from within fork/join computations</b></td>
* </tr>
* <tr>
* <td> <b>Arrange async execution</b></td>
* <td> {@link #execute(ForkJoinTask)}</td>
* <td> {@link ForkJoinTask#fork}</td>
* </tr>
* <tr>
* <td> <b>Await and obtain result</b></td>
* <td> {@link #invoke(ForkJoinTask)}</td>
* <td> {@link ForkJoinTask#invoke}</td>
* </tr>
* <tr>
* <td> <b>Arrange exec and obtain Future</b></td>
* <td> {@link #submit(ForkJoinTask)}</td>
* <td> {@link ForkJoinTask#fork} (ForkJoinTasks <em>are</em> Futures)</td>
* </tr>
* </table>
*
* <p>The common pool is by default constructed with default
* parameters, but these may be controlled by setting three
* {@linkplain System#getProperty system properties}:
* <ul>
* <li>{@code java.util.concurrent.ForkJoinPool.common.parallelism}
* - the parallelism level, a non-negative integer
* <li>{@code java.util.concurrent.ForkJoinPool.common.threadFactory}
* - the class name of a {@link ForkJoinWorkerThreadFactory}
* <li>{@code java.util.concurrent.ForkJoinPool.common.exceptionHandler}
* - the class name of a {@link UncaughtExceptionHandler}
* <li>{@code java.util.concurrent.ForkJoinPool.common.maximumSpares}
* - the maximum number of allowed extra threads to maintain target
* parallelism (default 256).
* </ul>
* 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.
*
* <p><b>Implementation notes</b>: 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}.
*
* <p>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
*/
// Android-removed: @Contended, this hint is not used by the Android runtime.
//@jdk.internal.vm.annotation.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)
* in a circular buffer:
* q.array[q.top++ % length] = task;
*
* (The actual code needs to null-check and size-check the array,
* uses masking, not mod, for indexing a power-of-two-sized array,
* properly fences accesses, and possibly signals 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 ((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 ((the task at base slot is not null) and
* (CAS slot to null))
* increment base and return task;
*
* There are several variants of each of these; for example most
* versions of poll pre-screen the CAS by rechecking that the base
* has not changed since reading the slot, and most methods only
* attempt the CAS if base appears not to be equal to top.
*
* 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 (but different than) the one used here.
* Extracting tasks in array slots via (fully fenced) CAS provides
* primary synchronization. The base and top indices imprecisely
* guide where to extract from. We do not always require strict
* orderings of array and index updates, so sometimes let them be
* subject to compiler and processor reorderings. However, the
* volatile "base" index also serves as a basis for memory
* ordering: Slot accesses are preceded by a read of base,
* ensuring happens-before ordering with respect to stealers (so
* the slots themselves can be read via plain array reads.) The
* only other memory orderings relied on are maintained in the
* course of signalling and activation (see below). A check that
* base == top indicates (momentary) emptiness, but otherwise may
* err on the side of possibly making the queue appear nonempty
* when a push, pop, or poll have not fully committed, or making
* it appear empty when an update of top has not yet been visibly
* written. (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) visibly completes.
* However, in the aggregate, we ensure at least probabilistic
* non-blockingness. If an attempted steal fails, a scanning
* thief 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.
*
* 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 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. Because it is used only as
* a spinlock, unlocking requires only a "releasing" store (using
* putOrderedInt). The qlock is also used during termination
* detection, in which case it is forced to a negative
* non-lockable value.
*
* 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 lifetime status, atomically and
* monotonically setting STARTED, SHUTDOWN, STOP, and finally
* TERMINATED bits.
*
* Field "auxState" is a ReentrantLock subclass that also
* opportunistically holds some other bookkeeping fields accessed
* only when locked. It is mainly used to lock (infrequent)
* updates to workQueues. The auxState instance is itself lazily
* constructed (see tryInitialize), requiring a double-check-style
* bootstrapping use of field runState, and locking a private
* static.
*
* Field "workQueues" holds references to WorkQueues. It is
* updated (only during worker creation and termination) under the
* lock, but is otherwise concurrently readable, and accessed
* directly. We also ensure that reads of the array reference
* itself never become too stale (for example, re-reading before
* each scan). 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 block/unblock 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.
*
* 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 (see
* externalPush).
*
* WorkQueue field scanState is used by both workers and the pool
* to manage and track whether a worker is UNSIGNALLED (possibly
* blocked waiting for a signal). 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.
*
* The ctl field also serves as the basis for memory
* synchronization surrounding activation. This uses a more
* efficient version of a Dekker-like rule that task producers and
* consumers sync with each other by both writing/CASing ctl (even
* if to its current value). This would be extremely costly. So
* we relax it in several ways: (1) Producers only signal when
* their queue is empty. Other workers propagate this signal (in
* method scan) when they find tasks. (2) Workers only enqueue
* after scanning (see below) and not finding any tasks. (3)
* Rather than CASing ctl to its current value in the common case
* where no action is required, we reduce write contention by
* equivalently prefacing signalWork when called by an external
* task producer using a memory access with full-volatile
* semantics or a "fullFence". (4) For internal task producers we
* rely on the fact that even if no other workers awaken, the
* producer itself will eventually see the task and execute it.
*
* Almost always, too many signals are issued. A task producer
* cannot in general tell if some existing worker is in the midst
* of finishing one task (or already scanning) and ready to take
* another without being signalled. So the producer might instead
* activate a different worker that does not find any work, and
* then inactivates. This scarcely matters in steady-state
* computations involving all workers, but can create contention
* and bookkeeping bottlenecks during ramp-up, ramp-down, and small
* computations involving only a few workers.
*
* Scanning. Method scan() performs top-level scanning for tasks.
* Each scan traverses (and tries to poll from) each queue in
* pseudorandom permutation order by randomly selecting an origin
* index and a step value. (The pseudorandom generator need not
* have high-quality statistical properties in the long term, but
* just within computations; We use 64bit and 32bit Marsaglia
* XorShifts, which are cheap and suffice here.) Scanning also
* employs contention reduction: When scanning workers fail a CAS
* polling for work, they soon restart with a different
* pseudorandom scan order (thus likely retrying at different
* intervals). This improves throughput when many threads are
* trying to take tasks from few queues. Scans do not otherwise
* explicitly take into account core affinities, loads, cache
* localities, etc, However, they do exploit temporal locality
* (which usually approximates these) by preferring to re-poll (up
* to POLL_LIMIT times) from the same queue after a successful
* poll before trying others. Restricted forms of scanning occur
* in methods helpComplete and findNonEmptyStealQueue, and take
* similar but simpler forms.
*
* Deactivation and waiting. Queuing encounters several intrinsic
* races; most notably that an inactivating scanning worker can
* miss seeing a task produced during a scan. So when a worker
* cannot find a task to steal, it inactivates and enqueues, and
* then rescans to ensure that it didn't miss one, reactivating
* upon seeing one with probability approximately proportional to
* probability of a miss. (In most cases, the worker will be
* signalled before self-signalling, avoiding cascades of multiple
* signals for the same task).
*
* Workers block (in method awaitWork) using park/unpark;
* advertising the need for signallers to unpark by setting their
* "parker" fields.
*
* 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 given by IDLE_TIMEOUT_MS, increasing the
* period as the number of threads decreases, eventually removing
* all workers.
*
* 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. 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.
*
* Spare threads are removed as soon as they notice that the
* target parallelism level has been exceeded, in method
* tryDropSpare. (Method scan arranges returns for rechecks upon
* each probe via the "bound" parameter.)
*
* The compensation mechanism may be bounded. Bounds for the
* commonPool (see COMMON_MAX_SPARES) 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, or {@code null} if the request
* to create a thread is rejected
* @throws NullPointerException if the pool is null
*/
public ForkJoinWorkerThread newThread(ForkJoinPool pool);
}
/**
* Default ForkJoinWorkerThreadFactory implementation; creates a
* new ForkJoinWorkerThread.
*/
private 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.
*/
private static final class EmptyTask extends ForkJoinTask<Void> {
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; }
}
/**
* Additional fields and lock created upon initialization.
*/
private static final class AuxState extends ReentrantLock {
private static final long serialVersionUID = -6001602636862214147L;
volatile long stealCount; // cumulative steal count
long indexSeed; // index bits for registerWorker
AuxState() {}
}
// 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 UNSIGNALLED = 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 SPARE_WORKER = 1 << 17; // set if tc > 0 on creation
static final int UNREGISTERED = 1 << 18; // to skip some of deregister
static final int FIFO_QUEUE = 1 << 31; // must be negative
static final int LIFO_QUEUE = 0; // for clarity
static final int IS_OWNED = 1; // low bit 0 if shared
/**
* The maximum number of task executions from the same queue
* before checking other queues, bounding unfairness and impact of
* infinite user task recursion. Must be a power of two minus 1.
*/
static final int POLL_LIMIT = (1 << 10) - 1;
/**
* 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.
*/
// Android-removed: @Contended, this hint is not used by the Android runtime.
//@jdk.internal.vm.annotation.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, negative if inactive
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
// Android-removed: @Contended, this hint is not used by the Android runtime.
// @jdk.internal.vm.annotation.Contended("group2") // segregate
volatile ForkJoinTask<?> currentSteal; // nonnull when running some task
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; // 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, al, s;
return ((n = base - (s = top)) >= 0 || // possibly one task
(n == -1 && ((a = array) == null ||
(al = a.length) == 0 ||
a[(al - 1) & (s - 1)] == null)));
}
/**
* Pushes a task. Call only by owner in unshared queues.
*
* @param task the task. Caller must ensure non-null.
* @throws RejectedExecutionException if array cannot be resized
*/
final void push(ForkJoinTask<?> task) {
U.storeFence(); // ensure safe publication
int s = top, al, d; ForkJoinTask<?>[] a;
if ((a = array) != null && (al = a.length) > 0) {
a[(al - 1) & s] = task; // relaxed writes OK
top = s + 1;
ForkJoinPool p = pool;
if ((d = base - s) == 0 && p != null) {
U.fullFence();
p.signalWork();
}
else if (al + d == 1)
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 < INITIAL_QUEUE_CAPACITY || 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
int index = b & oldMask;
long offset = ((long)index << ASHIFT) + ABASE;
ForkJoinTask<?> x = (ForkJoinTask<?>)
U.getObjectVolatile(oldA, offset);
if (x != null &&
U.compareAndSwapObject(oldA, offset, x, null))
a[b & mask] = x;
} while (++b != t);
U.storeFence();
}
return a;
}
/**
* Takes next task, if one exists, in LIFO order. Call only
* by owner in unshared queues.
*/
final ForkJoinTask<?> pop() {
int b = base, s = top, al, i; ForkJoinTask<?>[] a;
if ((a = array) != null && b != s && (al = a.length) > 0) {
int index = (al - 1) & --s;
long offset = ((long)index << ASHIFT) + ABASE;
ForkJoinTask<?> t = (ForkJoinTask<?>)
U.getObject(a, offset);
if (t != null &&
U.compareAndSwapObject(a, offset, t, null)) {
top = 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<?>[] a; int al;
if ((a = array) != null && (al = a.length) > 0) {
int index = (al - 1) & b;
long offset = ((long)index << ASHIFT) + ABASE;
ForkJoinTask<?> t = (ForkJoinTask<?>)
U.getObjectVolatile(a, offset);
if (t != null && b++ == base &&
U.compareAndSwapObject(a, offset, t, null)) {
base = b;
return t;
}
}
return null;
}
/**
* Takes next task, if one exists, in FIFO order.
*/
final ForkJoinTask<?> poll() {
for (;;) {
int b = base, s = top, d, al; ForkJoinTask<?>[] a;
if ((a = array) != null && (d = b - s) < 0 &&
(al = a.length) > 0) {
int index = (al - 1) & b;
long offset = ((long)index << ASHIFT) + ABASE;
ForkJoinTask<?> t = (ForkJoinTask<?>)
U.getObjectVolatile(a, offset);
if (b++ == base) {
if (t != null) {
if (U.compareAndSwapObject(a, offset, t, null)) {
base = b;
return t;
}
}
else if (d == -1)
break; // now empty
}
}
else
break;
}
return null;
}
/**
* Takes next task, if one exists, in order specified by mode.
*/
final ForkJoinTask<?> nextLocalTask() {
return (config < 0) ? poll() : pop();
}
/**
* Returns next task, if one exists, in order specified by mode.
*/
final ForkJoinTask<?> peek() {
int al; ForkJoinTask<?>[] a;
return ((a = array) != null && (al = a.length) > 0) ?
a[(al - 1) & (config < 0 ? base : top - 1)] : null;
}
/**
* Pops the given task only if it is at the current top.
*/
final boolean tryUnpush(ForkJoinTask<?> task) {
int b = base, s = top, al; ForkJoinTask<?>[] a;
if ((a = array) != null && b != s && (al = a.length) > 0) {
int index = (al - 1) & --s;
long offset = ((long)index << ASHIFT) + ABASE;
if (U.compareAndSwapObject(a, offset, task, null)) {
top = s;
return true;
}
}
return false;
}
/**
* Shared version of push. Fails if already locked.
*
* @return status: > 0 locked, 0 possibly was empty, < 0 was nonempty
*/
final int sharedPush(ForkJoinTask<?> task) {
int stat;
if (U.compareAndSwapInt(this, QLOCK, 0, 1)) {
int b = base, s = top, al, d; ForkJoinTask<?>[] a;
if ((a = array) != null && (al = a.length) > 0 &&
al - 1 + (d = b - s) > 0) {
a[(al - 1) & s] = task;
top = s + 1; // relaxed writes OK here
qlock = 0;
stat = (d < 0 && b == base) ? d : 0;
}
else {
growAndSharedPush(task);
stat = 0;
}
}
else
stat = 1;
return stat;
}
/**
* Helper for sharedPush; called only when locked and resize
* needed.
*/
private void growAndSharedPush(ForkJoinTask<?> task) {
try {
growArray();
int s = top, al; ForkJoinTask<?>[] a;
if ((a = array) != null && (al = a.length) > 0) {
a[(al - 1) & s] = task;
top = s + 1;
}
} finally {
qlock = 0;
}
}
/**
* Shared version of tryUnpush.
*/
final boolean trySharedUnpush(ForkJoinTask<?> task) {
boolean popped = false;
int s = top - 1, al; ForkJoinTask<?>[] a;
if ((a = array) != null && (al = a.length) > 0) {
int index = (al - 1) & s;
long offset = ((long)index << ASHIFT) + ABASE;
ForkJoinTask<?> t = (ForkJoinTask<?>) U.getObject(a, offset);
if (t == task &&
U.compareAndSwapInt(this, QLOCK, 0, 1)) {
if (top == s + 1 && array == a &&
U.compareAndSwapObject(a, offset, task, null)) {
popped = true;
top = s;
}
U.putOrderedInt(this, QLOCK, 0);
}
}
return popped;
}
/**
* 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
/**
* Pops and executes up to POLL_LIMIT tasks or until empty.
*/
final void localPopAndExec() {
for (int nexec = 0;;) {
int b = base, s = top, al; ForkJoinTask<?>[] a;
if ((a = array) != null && b != s && (al = a.length) > 0) {
int index = (al - 1) & --s;
long offset = ((long)index << ASHIFT) + ABASE;
ForkJoinTask<?> t = (ForkJoinTask<?>)
U.getAndSetObject(a, offset, null);
if (t != null) {
top = s;
(currentSteal = t).doExec();
if (++nexec > POLL_LIMIT)
break;
}
else
break;
}
else
break;
}
}
/**
* Polls and executes up to POLL_LIMIT tasks or until empty.
*/
final void localPollAndExec() {
for (int nexec = 0;;) {
int b = base, s = top, al; ForkJoinTask<?>[] a;
if ((a = array) != null && b != s && (al = a.length) > 0) {
int index = (al - 1) & b++;
long offset = ((long)index << ASHIFT) + ABASE;
ForkJoinTask<?> t = (ForkJoinTask<?>)
U.getAndSetObject(a, offset, null);
if (t != null) {
base = b;
t.doExec();
if (++nexec > POLL_LIMIT)
break;
}
}
else
break;
}
}
/**
* Executes the given task and (some) remaining local tasks.
*/
final void runTask(ForkJoinTask<?> task) {
if (task != null) {
task.doExec();
if (config < 0)
localPollAndExec();
else
localPopAndExec();
int ns = ++nsteals;
ForkJoinWorkerThread thread = owner;
currentSteal = null;
if (ns < 0) // collect on overflow
transferStealCount(pool);
if (thread != null)
thread.afterTopLevelExec();
}
}
/**
* Adds steal count to pool steal count if it exists, and resets.
*/
final void transferStealCount(ForkJoinPool p) {
AuxState aux;
if (p != null && (aux = p.auxState) != null) {
long s = nsteals;
nsteals = 0; // if negative, correct for overflow
if (s < 0) s = Integer.MAX_VALUE;
aux.lock();
try {
aux.stealCount += s;
} finally {
aux.unlock();
}
}
}
/**
* 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) {
if (task != null && task.status >= 0) {
int b, s, d, al; ForkJoinTask<?>[] a;
while ((d = (b = base) - (s = top)) < 0 &&
(a = array) != null && (al = a.length) > 0) {
for (;;) { // traverse from s to b
int index = --s & (al - 1);
long offset = (index << ASHIFT) + ABASE;
ForkJoinTask<?> t = (ForkJoinTask<?>)
U.getObjectVolatile(a, offset);
if (t == null)
break; // restart
else if (t == task) {
boolean removed = false;
if (s + 1 == top) { // pop
if (U.compareAndSwapObject(a, offset, t, null)) {
top = s;
removed = true;
}
}
else if (base == b) // replace with proxy
removed = U.compareAndSwapObject(a, offset, t,
new EmptyTask());
if (removed) {
ForkJoinTask<?> ps = currentSteal;
(currentSteal = task).doExec();
currentSteal = ps;
}
break;
}
else if (t.status < 0 && s + 1 == top) {
if (U.compareAndSwapObject(a, offset, t, null)) {
top = s;
}
break; // was cancelled
}
else if (++d == 0) {
if (base != b) // rescan
break;
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 b = base, s = top, al; ForkJoinTask<?>[] a;
if ((a = array) != null && b != s && (al = a.length) > 0) {
int index = (al - 1) & (s - 1);
long offset = ((long)index << ASHIFT) + ABASE;
ForkJoinTask<?> o = (ForkJoinTask<?>)
U.getObjectVolatile(a, offset);
if (o instanceof CountedCompleter) {
CountedCompleter<?> t = (CountedCompleter<?>)o;
for (CountedCompleter<?> r = t;;) {
if (r == task) {
if ((mode & IS_OWNED) == 0) {
boolean popped = false;
if (U.compareAndSwapInt(this, QLOCK, 0, 1)) {
if (top == s && array == a &&
U.compareAndSwapObject(a, offset,
t, null)) {
popped = true;
top = s - 1;
}
U.putOrderedInt(this, QLOCK, 0);
if (popped)
return t;
}
}
else if (U.compareAndSwapObject(a, offset,
t, null)) {
top = 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) {
ForkJoinTask<?>[] a;
int b = base, s = top, al, h;
if ((a = array) != null && b != s && (al = a.length) > 0) {
int index = (al - 1) & b;
long offset = ((long)index << ASHIFT) + ABASE;
ForkJoinTask<?> o = (ForkJoinTask<?>)
U.getObjectVolatile(a, offset);
if (o == 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 (b++ == base &&
U.compareAndSwapObject(a, offset, t, null)) {
base = b;
t.doExec();
h = 1; // success
}
else
h = 2; // lost CAS
break;
}
else if ((r = r.completer) == null) {
h = -1; // unmatched
break;
}
}
}
}
else
h = b | Integer.MIN_VALUE; // to sense movement on re-poll
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 = sun.misc.Unsafe.getUnsafe();
private static final long QLOCK;
private static final int ABASE;
private static final int ASHIFT;
static {
try {
QLOCK = U.objectFieldOffset
(WorkQueue.class.getDeclaredField("qlock"));
ABASE = U.arrayBaseOffset(ForkJoinTask[].class);
int scale = U.arrayIndexScale(ForkJoinTask[].class);
if ((scale & (scale - 1)) != 0)
throw new Error("array index scale not a power of two");
ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
} catch (ReflectiveOperationException 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. Also used as a static lock in tryInitialize.
*/
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 COMMON_PARALLELISM;
/**
* Limit on spare thread construction in tryCompensate.
*/
private static final int COMMON_MAX_SPARES;
/**
* 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 milliseconds) 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_MS = 2000L; // 2sec
/**
* Tolerance for idle timeouts, to cope with timer undershoots.
*/
private static final long TIMEOUT_SLOP_MS = 20L; // 20ms
/**
* The default value for COMMON_MAX_SPARES. Overridable using the
* "java.util.concurrent.ForkJoinPool.common.maximumSpares" system
* property. The default 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;
/**
* 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 STARTED = 1;
private static final int STOP = 1 << 1;
private static final int TERMINATED = 1 << 2;
private static final int SHUTDOWN = 1 << 31;
// Instance fields
volatile long ctl; // main pool control
volatile int runState;
final int config; // parallelism, mode
AuxState auxState; // lock, steal counts
volatile WorkQueue[] workQueues; // main registry
final String workerNamePrefix; // to create worker name string
final ForkJoinWorkerThreadFactory factory;
final UncaughtExceptionHandler ueh; // per-worker UEH
/**
* Instantiates fields upon first submission, or upon shutdown if
* no submissions. If checkTermination true, also responds to
* termination by external calls submitting tasks.
*/
private void tryInitialize(boolean checkTermination) {
if (runState == 0) { // bootstrap by locking static field
int p = config & SMASK;
int n = (p > 1) ? p - 1 : 1; // ensure at least 2 slots
n |= n >>> 1; // create workQueues array with size a power of two
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
n = ((n + 1) << 1) & SMASK;
AuxState aux = new AuxState();
WorkQueue[] ws = new WorkQueue[n];
synchronized (modifyThreadPermission) { // double-check
if (runState == 0) {
workQueues = ws;
auxState = aux;
runState = STARTED;
}
}
}
if (checkTermination && runState < 0) {
tryTerminate(false, false); // help terminate
throw new RejectedExecutionException();
}
}
// 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.
*
* @param isSpare true if this is a spare thread
* @return true if successful
*/
private boolean createWorker(boolean isSpare) {
ForkJoinWorkerThreadFactory fac = factory;
Throwable ex = null;
ForkJoinWorkerThread wt = null;
WorkQueue q;
try {
if (fac != null && (wt = fac.newThread(this)) != null) {
if (isSpare && (q = wt.workQueue) != null)
q.config |= SPARE_WORKER;
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) {
do {
long nc = ((AC_MASK & (c + AC_UNIT)) |
(TC_MASK & (c + TC_UNIT)));
if (ctl == c && U.compareAndSwapLong(this, CTL, c, nc)) {
createWorker(false);
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;
AuxState aux;
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;
if ((aux = auxState) != null) {
aux.lock();
try {
int s = (int)(aux.indexSeed += SEED_INCREMENT), n, m;
WorkQueue[] ws = workQueues;
if (ws != null && (n = ws.length) > 0) {
i = (m = n - 1) & ((s << 1) | 1); // 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 | (s & 0x7fff0000); // random seq bits
ws[i] = w;
}
} finally {
aux.unlock();
}
}
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) {
AuxState aux; WorkQueue[] ws; // remove index from array
int idx = w.config & SMASK;
int ns = w.nsteals;
if ((aux = auxState) != null) {
aux.lock();
try {
if ((ws = workQueues) != null && ws.length > idx &&
ws[idx] == w)
ws[idx] = null;
aux.stealCount += ns;
} finally {
aux.unlock();
}
}
}
if (w == null || (w.config & UNREGISTERED) == 0) { // else pre-adjusted
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.currentSteal = null;
w.qlock = -1; // ensure set
w.cancelAll(); // cancel remaining tasks
}
while (tryTerminate(false, false) >= 0) { // possibly replace
WorkQueue[] ws; int wl, sp; long c;
if (w == null || w.array == null ||
(ws = workQueues) == null || (wl = ws.length) <= 0)
break;
else if ((sp = (int)(c = ctl)) != 0) { // wake up replacement
if (tryRelease(c, ws[(wl - 1) & sp], 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.
*/
final void signalWork() {
for (;;) {
long c; int sp, i; WorkQueue v; WorkQueue[] ws;
if ((c = ctl) >= 0L) // enough workers
break;
else if ((sp = (int)c) == 0) { // no idle workers
if ((c & ADD_WORKER) != 0L) // too few workers
tryAddWorker(c);
break;
}
else if ((ws = workQueues) == null)
break; // unstarted/terminated
else if (ws.length <= (i = sp & SMASK))
break; // terminated
else if ((v = ws[i]) == null)
break; // terminating
else {
int ns = sp & ~UNSIGNALLED;
int vs = v.scanState;
long nc = (v.stackPred & SP_MASK) | (UC_MASK & (c + AC_UNIT));
if (sp == vs && U.compareAndSwapLong(this, CTL, c, nc)) {
v.scanState = ns;
LockSupport.unpark(v.parker);
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, ns = sp & ~UNSIGNALLED;
if (v != null) {
int vs = v.scanState;
long nc = (v.stackPred & SP_MASK) | (UC_MASK & (c + inc));
if (sp == vs && U.compareAndSwapLong(this, CTL, c, nc)) {
v.scanState = ns;
LockSupport.unpark(v.parker);
return true;
}
}
return false;
}
/**
* With approx probability of a missed signal, tries (once) to
* reactivate worker w (or some other worker), failing if stale or
* known to be already active.
*
* @param w the worker
* @param ws the workQueue array to use
* @param r random seed
*/
private void tryReactivate(WorkQueue w, WorkQueue[] ws, int r) {
long c; int sp, wl; WorkQueue v;
if ((sp = (int)(c = ctl)) != 0 && w != null &&
ws != null && (wl = ws.length) > 0 &&
((sp ^ r) & SS_SEQ) == 0 &&
(v = ws[(wl - 1) & sp]) != null) {
long nc = (v.stackPred & SP_MASK) | (UC_MASK & (c + AC_UNIT));
int ns = sp & ~UNSIGNALLED;
if (w.scanState < 0 &&
v.scanState == sp &&
U.compareAndSwapLong(this, CTL, c, nc)) {
v.scanState = ns;
LockSupport.unpark(v.parker);
}
}
}
/**
* If worker w exists and is active, enqueues and sets status to inactive.
*
* @param w the worker
* @param ss current (non-negative) scanState
*/
private void inactivate(WorkQueue w, int ss) {
int ns = (ss + SS_SEQ) | UNSIGNALLED;
long lc = ns & SP_MASK, nc, c;
if (w != null) {
w.scanState = ns;
do {
nc = lc | (UC_MASK & ((c = ctl) - AC_UNIT));
w.stackPred = (int)c;
} while (!U.compareAndSwapLong(this, CTL, c, nc));
}
}
/**
* Possibly blocks worker w waiting for signal, or returns
* negative status if the worker should terminate. May return
* without status change if multiple stale unparks and/or
* interrupts occur.
*
* @param w the calling worker
* @return negative if w should terminate
*/
private int awaitWork(WorkQueue w) {
int stat = 0;
if (w != null && w.scanState < 0) {
long c = ctl;
if ((int)(c >> AC_SHIFT) + (config & SMASK) <= 0)
stat = timedAwaitWork(w, c); // possibly quiescent
else if ((runState & STOP) != 0)
stat = w.qlock = -1; // pool terminating
else if (w.scanState < 0) {
w.parker = Thread.currentThread();
if (w.scanState < 0) // recheck after write
LockSupport.park(this);
w.parker = null;
if ((runState & STOP) != 0)
stat = w.qlock = -1; // recheck
else if (w.scanState < 0)
Thread.interrupted(); // clear status
}
}
return stat;
}
/**
* Possibly triggers shutdown and tries (once) to block worker
* when pool is (or may be) quiescent. Waits up to a duration
* determined by number of workers. 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
* @return negative if w should terminate
*/
private int timedAwaitWork(WorkQueue w, long c) {
int stat = 0;
int scale = 1 - (short)(c >>> TC_SHIFT);
long deadline = (((scale <= 0) ? 1 : scale) * IDLE_TIMEOUT_MS +
System.currentTimeMillis());
if ((runState >= 0 || (stat = tryTerminate(false, false)) > 0) &&
w != null && w.scanState < 0) {
int ss; AuxState aux;
w.parker = Thread.currentThread();
if (w.scanState < 0)
LockSupport.parkUntil(this, deadline);
w.parker = null;
if ((runState & STOP) != 0)
stat = w.qlock = -1; // pool terminating
else if ((ss = w.scanState) < 0 && !Thread.interrupted() &&
(int)c == ss && (aux = auxState) != null && ctl == c &&
deadline - System.currentTimeMillis() <= TIMEOUT_SLOP_MS) {
aux.lock();
try { // pre-deregister
WorkQueue[] ws;
int cfg = w.config, idx = cfg & SMASK;
long nc = ((UC_MASK & (c - TC_UNIT)) |
(SP_MASK & w.stackPred));
if ((runState & STOP) == 0 &&
(ws = workQueues) != null &&
idx < ws.length && idx >= 0 && ws[idx] == w &&
U.compareAndSwapLong(this, CTL, c, nc)) {
ws[idx] = null;
w.config = cfg | UNREGISTERED;
stat = w.qlock = -1;
}
} finally {
aux.unlock();
}
}
}
return stat;
}
/**
* If the given worker is a spare with no queued tasks, and there
* are enough existing workers, drops it from ctl counts and sets
* its state to terminated.
*
* @param w the calling worker -- must be a spare
* @return true if dropped (in which case it must not process more tasks)
*/
private boolean tryDropSpare(WorkQueue w) {
if (w != null && w.isEmpty()) { // no local tasks
long c; int sp, wl; WorkQueue[] ws; WorkQueue v;
while ((short)((c = ctl) >> TC_SHIFT) > 0 &&
((sp = (int)c) != 0 || (int)(c >> AC_SHIFT) > 0) &&
(ws = workQueues) != null && (wl = ws.length) > 0) {
boolean dropped, canDrop;
if (sp == 0) { // no queued workers
long nc = ((AC_MASK & (c - AC_UNIT)) |
(TC_MASK & (c - TC_UNIT)) | (SP_MASK & c));
dropped = U.compareAndSwapLong(this, CTL, c, nc);
}
else if (
(v = ws[(wl - 1) & sp]) == null || v.scanState != sp)
dropped = false; // stale; retry
else {
long nc = v.stackPred & SP_MASK;
if (w == v || w.scanState >= 0) {
canDrop = true; // w unqueued or topmost
nc |= ((AC_MASK & c) | // ensure replacement
(TC_MASK & (c - TC_UNIT)));
}
else { // w may be queued
canDrop = false; // help uncover
nc |= ((AC_MASK & (c + AC_UNIT)) |
(TC_MASK & c));
}
if (U.compareAndSwapLong(this, CTL, c, nc)) {
v.scanState = sp & ~UNSIGNALLED;
LockSupport.unpark(v.parker);
dropped = canDrop;
}
else
dropped = false;
}
if (dropped) { // pre-deregister
int cfg = w.config, idx = cfg & SMASK;
if (idx >= 0 && idx < ws.length && ws[idx] == w)
ws[idx] = null;
w.config = cfg | UNREGISTERED;
w.qlock = -1;
return true;
}
}
}
return false;
}
/**
* Top-level runloop for workers, called by ForkJoinWorkerThread.run.
*/
final void runWorker(WorkQueue w) {
w.growArray(); // allocate queue
int bound = (w.config & SPARE_WORKER) != 0 ? 0 : POLL_LIMIT;
long seed = w.hint * 0xdaba0b6eb09322e3L; // initial random seed
if ((runState & STOP) == 0) {
for (long r = (seed == 0L) ? 1L : seed;;) { // ensure nonzero
if (bound == 0 && tryDropSpare(w))
break;
// high bits of prev seed for step; current low bits for idx
int step = (int)(r >>> 48) | 1;
r ^= r >>> 12; r ^= r << 25; r ^= r >>> 27; // xorshift
if (scan(w, bound, step, (int)r) < 0 && awaitWork(w) < 0)
break;
}
}
}
// Scanning for tasks
/**
* Repeatedly scans for and tries to steal and execute (via
* workQueue.runTask) a queued task. Each scan traverses queues in
* pseudorandom permutation. Upon finding a non-empty queue, makes
* at most the given bound attempts to re-poll (fewer if
* contended) on the same queue before returning (impossible
* scanState value) 0 to restart scan. Else returns after at least
* 1 and at most 32 full scans.
*
* @param w the worker (via its WorkQueue)
* @param bound repoll bound as bitmask (0 if spare)
* @param step (circular) index increment per iteration (must be odd)
* @param r a random seed for origin index
* @return negative if should await signal
*/
private int scan(WorkQueue w, int bound, int step, int r) {
int stat = 0, wl; WorkQueue[] ws;
if ((ws = workQueues) != null && w != null && (wl = ws.length) > 0) {
for (int m = wl - 1,
origin = m & r, idx = origin,
npolls = 0,
ss = w.scanState;;) { // negative if inactive
WorkQueue q; ForkJoinTask<?>[] a; int b, al;
if ((q = ws[idx]) != null && (b = q.base) - q.top < 0 &&
(a = q.array) != null && (al = a.length) > 0) {
int index = (al - 1) & b;
long offset = ((long)index << ASHIFT) + ABASE;
ForkJoinTask<?> t = (ForkJoinTask<?>)
U.getObjectVolatile(a, offset);
if (t == null)
break; // empty or busy
else if (b++ != q.base)
break; // busy
else if (ss < 0) {
tryReactivate(w, ws, r);
break; // retry upon rescan
}
else if (!U.compareAndSwapObject(a, offset, t, null))
break; // contended
else {
q.base = b;
w.currentSteal = t;
if (b != q.top) // propagate signal
signalWork();
w.runTask(t);
if (++npolls > bound)
break;
}
}
else if (npolls != 0) // rescan
break;
else if ((idx = (idx + step) & m) == origin) {
if (ss < 0) { // await signal
stat = ss;
break;
}
else if (r >= 0) {
inactivate(w, ss);
break;
}
else
r <<= 1; // at most 31 rescans
}
}
}
return stat;
}
// 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 from 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, wl;
if ((ws = workQueues) != null && (wl = ws.length) > 1 &&
task != null && w != null) {
for (int m = wl - 1,
mode = w.config,
r = ~mode, // scanning seed
origin = r & m, k = origin, // first queue to scan
step = 3, // first scan step
h = 1, // 1:ran, >1:contended, <0:hash
oldSum = 0, checkSum = 0;;) {
CountedCompleter<?> p; WorkQueue q; int i;
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 worker queues
if ((i = k | 1) < 0 || i > m || (q = ws[i]) == null)
h = 0;
else if ((h = q.pollAndExecCC(task)) < 0)
checkSum += h;
if (h > 0) {
if (h == 1 && maxTasks != 0 && --maxTasks == 0)
break;
step = (r >>> 16) | 3;
r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift
k = origin = r & m; // move and restart
oldSum = checkSum = 0;
}
else if ((k = (k + step) & 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) {
if (task != null && w != null) {
ForkJoinTask<?> ps = w.currentSteal;
WorkQueue[] ws; int wl, oldSum = 0;
outer: while (w.tryRemoveAndExec(task) && task.status >= 0 &&
(ws = workQueues) != null && (wl = ws.length) > 0) {
ForkJoinTask<?> subtask;
int m = wl - 1, checkSum = 0; // for stability check
WorkQueue j = w, v; // v is subtask stealer
descent: for (subtask = task; subtask.status >= 0; ) {
for (int h = j.hint | 1, k = 0, i;;) {
if ((v = ws[i = (h + (k << 1)) & m]) != null) {
if (v.currentSteal == subtask) {
j.hint = i;
break;
}
checkSum += v.base;
}
if (++k > m) // can't find stealer
break outer;
}
for (;;) { // help v or descend
ForkJoinTask<?>[] a; int b, al;
if (subtask.status < 0) // too late to help
break descent;
checkSum += (b = v.base);
ForkJoinTask<?> next = v.currentJoin;
ForkJoinTask<?> t = null;
if ((a = v.array) != null && (al = a.length) > 0) {
int index = (al - 1) & b;
long offset = ((long)index << ASHIFT) + ABASE;
t = (ForkJoinTask<?>)
U.getObjectVolatile(a, offset);
if (t != null && b++ == v.base) {
if (j.currentJoin != subtask ||
v.currentSteal != subtask ||
subtask.status < 0)
break descent; // stale
if (U.compareAndSwapObject(a, offset, t, null)) {
v.base = b;
w.currentSteal = t;
for (int top = w.top;;) {
t.doExec(); // help
w.currentSteal = ps;
if (task.status < 0)
break outer;
if (w.top == top)
break; // run local tasks
if ((t = w.pop()) == null)
break descent;
w.currentSteal = t;
}
}
}
}
if (t == null && b == v.base && b - v.top >= 0) {
if ((subtask = next) == null) { // try to descend
if (next == v.currentJoin &&
oldSum == (oldSum = checkSum))
break outer;
break descent;
}
j = v;
break;
}
}
}
}
}
}
/**
* 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; int wl;
long c = ctl;
WorkQueue[] ws = workQueues;
int pc = config & SMASK;
int ac = pc + (int)(c >> AC_SHIFT);
int tc = pc + (short)(c >> TC_SHIFT);
if (w == null || w.qlock < 0 || pc == 0 || // terminating or disabled
ws == null || (wl = ws.length) <= 0)
canBlock = false;
else {
int m = wl - 1, sp;
boolean busy = true; // validate ac
for (int i = 0; i <= m; ++i) {
int k; WorkQueue v;
if ((k = (i << 1) | 1) <= m && k >= 0 && (v = ws[k]) != null &&
v.scanState >= 0 && v.currentSteal == null) {
busy = false;
break;
}
}
if (!busy || ctl != c)
canBlock = false; // unstable or stale
else if ((sp = (int)c) != 0) // release idle worker
canBlock = tryRelease(c, ws[m & sp], 0L);
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 + COMMON_MAX_SPARES))
throw new RejectedExecutionException(
"Thread limit exceeded replacing blocked worker");
else { // similar to tryAddWorker
boolean isSpare = (tc >= pc);
long nc = (AC_MASK & c) | (TC_MASK & (c + TC_UNIT));
canBlock = (U.compareAndSwapLong(this, CTL, c, nc) &&
createWorker(isSpare)); // 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 (w != null) {
ForkJoinTask<?> prevJoin = w.currentJoin;
if (task != null && (s = task.status) >= 0) {
w.currentJoin = task;
CountedCompleter<?> cc = (task instanceof CountedCompleter) ?
(CountedCompleter<?>)task : null;
for (;;) {
if (cc != null)
helpComplete(w, cc, 0);
else
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);
}
if ((s = task.status) < 0)
break;
}
w.currentJoin = 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 wl; // one-shot version of scan loop
int r = ThreadLocalRandom.nextSecondarySeed();
if ((ws = workQueues) != null && (wl = ws.length) > 0) {
int m = wl - 1, origin = r & m;
for (int 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
int wc = w.config;
for (boolean active = true;;) {
long c; WorkQueue q; ForkJoinTask<?> t;
if (wc >= 0 && (t = w.pop()) != null) { // run locals if LIFO
(w.currentSteal = t).doExec();
w.currentSteal = ps;
}
else if ((q = findNonEmptyStealQueue()) != null) {
if (!active) { // re-establish active count
active = true;
U.getAndAddLong(this, CTL, AC_UNIT);
}
if ((t = q.pollAt(q.base)) != null) {
(w.currentSteal = t).doExec();
w.currentSteal = ps;
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 (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;
}
}
/**
* 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;
if ((t = w.nextLocalTask()) != null)
return t;
if ((q = findNonEmptyStealQueue()) == null)
return null;
if ((t = q.pollAt(q.base)) != 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, terminate when next possible
* @return -1: terminating/terminated, 0: retry if internal caller, else 1
*/
private int tryTerminate(boolean now, boolean enable) {
int rs; // 3 phases: try to set SHUTDOWN, then STOP, then TERMINATED
while ((rs = runState) >= 0) {
if (!enable || this == common) // cannot shutdown
return 1;
else if (rs == 0)
tryInitialize(false); // ensure initialized
else
U.compareAndSwapInt(this, RUNSTATE, rs, rs | SHUTDOWN);
}
if ((rs & STOP) == 0) { // try to initiate termination
if (!now) { // check quiescence
for (long oldSum = 0L;;) { // repeat until stable
WorkQueue[] ws; WorkQueue w; int b;
long checkSum = ctl;
if ((int)(checkSum >> AC_SHIFT) + (config & SMASK) > 0)
return 0; // still active workers
if ((ws = workQueues) != null) {
for (int i = 0; i < ws.length; ++i) {
if ((w = ws[i]) != null) {
checkSum += (b = w.base);
if (w.currentSteal != null || b != w.top)
return 0; // retry if internal caller
}
}
}
if (oldSum == (oldSum = checkSum))
break;
}
}
do {} while (!U.compareAndSwapInt(this, RUNSTATE,
rs = runState, rs | STOP));
}
for (long oldSum = 0L;;) { // repeat until stable
WorkQueue[] ws; WorkQueue w; ForkJoinWorkerThread wt;
long checkSum = ctl;
if ((ws = workQueues) != null) { // help terminate others
for (int i = 0; i < ws.length; ++i) {
if ((w = ws[i]) != null) {
w.cancelAll(); // clear queues
checkSum += w.base;
if (w.qlock >= 0) {
w.qlock = -1; // racy set OK
if ((wt = w.owner) != null) {
try { // unblock join or park
wt.interrupt();
} catch (Throwable ignore) {
}
}
}
}
}
}
if (oldSum == (oldSum = checkSum))
break;
}
if ((short)(ctl >>> TC_SHIFT) + (config & SMASK) <= 0) {
runState = (STARTED | SHUTDOWN | STOP | TERMINATED); // final write
synchronized (this) {
notifyAll(); // for awaitTermination
}
}
return -1;
}
// External operations
/**
* Constructs and tries to install a new external queue,
* failing if the workQueues array already has a queue at
* the given index.
*
* @param index the index of the new queue
*/
private void tryCreateExternalQueue(int index) {
AuxState aux;
if ((aux = auxState) != null && index >= 0) {
WorkQueue q = new WorkQueue(this, null);
q.config = index;
q.scanState = ~UNSIGNALLED;
q.qlock = 1; // lock queue
boolean installed = false;
aux.lock();
try { // lock pool to install
WorkQueue[] ws;
if ((ws = workQueues) != null && index < ws.length &&
ws[index] == null) {
ws[index] = q; // else throw away
installed = true;
}
} finally {
aux.unlock();
}
if (installed) {
try {
q.growArray();
} finally {
q.qlock = 0;
}
}
}
}
/**
* Adds the given task to a submission queue at submitter's
* current queue. Also performs secondary initialization upon the
* first submission of the first task to the pool, and 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.
*/
final void externalPush(ForkJoinTask<?> task) {
int r; // initialize caller's probe
if ((r = ThreadLocalRandom.getProbe()) == 0) {
ThreadLocalRandom.localInit();
r = ThreadLocalRandom.getProbe();
}
for (;;) {
WorkQueue q; int wl, k, stat;
int rs = runState;
WorkQueue[] ws = workQueues;
if (rs <= 0 || ws == null || (wl = ws.length) <= 0)
tryInitialize(true);
else if ((q = ws[k = (wl - 1) & r & SQMASK]) == null)
tryCreateExternalQueue(k);
else if ((stat = q.sharedPush(task)) < 0)
break;
else if (stat == 0) {
signalWork();
break;
}
else // move if busy
r = ThreadLocalRandom.advanceProbe(r);
}
}
/**
* Pushes a possibly-external submission.
*/
private <T> ForkJoinTask<T> externalSubmit(ForkJoinTask<T> task) {
Thread t; ForkJoinWorkerThread w; WorkQueue q;
if (task == null)
throw new NullPointerException();
if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) &&
(w = (ForkJoinWorkerThread)t).pool == this &&
(q = w.workQueue) != null)
q.push(task);
else
externalPush(task);
return task;
}
/**
* Returns common pool queue for an external thread.
*/
static WorkQueue commonSubmitterQueue() {
ForkJoinPool p = common;
int r = ThreadLocalRandom.getProbe();
WorkQueue[] ws; int wl;
return (p != null && (ws = p.workQueues) != null &&
(wl = ws.length) > 0) ?
ws[(wl - 1) & r & SQMASK] : null;
}
/**
* Performs tryUnpush for an external submitter.
*/
final boolean tryExternalUnpush(ForkJoinTask<?> task) {
int r = ThreadLocalRandom.getProbe();
WorkQueue[] ws; WorkQueue w; int wl;
return ((ws = workQueues) != null &&
(wl = ws.length) > 0 &&
(w = ws[(wl - 1) & r & SQMASK]) != null &&
w.trySharedUnpush(task));
}
/**
* Performs helpComplete for an external submitter.
*/
final int externalHelpComplete(CountedCompleter<?> task, int maxTasks) {
WorkQueue[] ws; int wl;
int r = ThreadLocalRandom.getProbe();
return ((ws = workQueues) != null && (wl = ws.length) > 0) ?
helpComplete(ws[(wl - 1) & r & SQMASK], task, maxTasks) : 0;
}
// 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 <T> 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> T invoke(ForkJoinTask<T> task) {
if (task == null)
throw new NullPointerException();
externalSubmit(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) {
externalSubmit(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);
externalSubmit(job);
}
/**
* Submits a ForkJoinTask for execution.
*
* @param task the task to submit
* @param <T> 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 <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
return externalSubmit(task);
}
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public <T> ForkJoinTask<T> submit(Callable<T> task) {
return externalSubmit(new ForkJoinTask.AdaptedCallable<T>(task));
}
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public <T> ForkJoinTask<T> submit(Runnable task, T result) {
return externalSubmit(new ForkJoinTask.AdaptedRunnable<T>(task, result));
}
/**
* @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);
return externalSubmit(job);
}
/**
* @throws NullPointerException {@inheritDoc}
* @throws RejectedExecutionException {@inheritDoc}
*/
public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> 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<Future<T>> futures = new ArrayList<>(tasks.size());
try {
for (Callable<T> t : tasks) {
ForkJoinTask<T> f = new ForkJoinTask.AdaptedCallable<T>(t);
futures.add(f);
externalSubmit(f);
}
for (int i = 0, size = futures.size(); i < size; i++)
((ForkJoinTask<?>)futures.get(i)).quietlyJoin();
return futures;
} catch (Throwable t) {
for (int i = 0, size = futures.size(); i < size; i++)
futures.get(i).cancel(false);
throw t;
}
}
/**
* 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 COMMON_PARALLELISM;
}
/**
* 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() {
AuxState sc = auxState;
long count = (sc == null) ? 0L : sc.stealCount;
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; int wl; WorkQueue w; ForkJoinTask<?> t;
int r = ThreadLocalRandom.nextSecondarySeed();
if ((ws = workQueues) != null && (wl = ws.length) > 0) {
for (int m = wl - 1, i = 0; i < wl; ++i) {
if ((w = ws[(i << 1) & m]) != 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<? super ForkJoinTask<?>> 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;
AuxState sc = auxState;
long st = (sc == null) ? 0L : sc.stealCount;
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<Runnable> 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, wl;
boolean found = true;
while (!isQuiescent() && (ws = workQueues) != null &&
(wl = ws.length) > 0) {
if (!found) {
if ((System.nanoTime() - startTime) > nanos)
return false;
Thread.yield(); // cannot block
}
found = false;
for (int m = wl - 1, 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.
*
* <p>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.
*
* <p>For example, here is a ManagedBlocker based on a
* ReentrantLock:
* <pre> {@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());
* }
* }}</pre>
*
* <p>Here is a class that possibly blocks waiting for an
* item on a given queue:
* <pre> {@code
* class QueueTaker<E> implements ManagedBlocker {
* final BlockingQueue<E> queue;
* volatile E item = null;
* QueueTaker(BlockingQueue<E> 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;
* }
* }}</pre>
*/
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()}.
*
* <p>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}.
*
* <p>If not running in a ForkJoinPool, this method is
* behaviorally equivalent to
* <pre> {@code
* while (!blocker.isReleasable())
* if (blocker.block())
* break;}</pre>
*
* 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 <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
return new ForkJoinTask.AdaptedRunnable<T>(runnable, value);
}
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
return new ForkJoinTask.AdaptedCallable<T>(callable);
}
// Unsafe mechanics
private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
private static final long CTL;
private static final long RUNSTATE;
private static final int ABASE;
private static final int ASHIFT;
static {
try {
CTL = U.objectFieldOffset
(ForkJoinPool.class.getDeclaredField("ctl"));
RUNSTATE = U.objectFieldOffset
(ForkJoinPool.class.getDeclaredField("runState"));
ABASE = U.arrayBaseOffset(ForkJoinTask[].class);
int scale = U.arrayIndexScale(ForkJoinTask[].class);
if ((scale & (scale - 1)) != 0)
throw new Error("array index scale not a power of two");
ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
} catch (ReflectiveOperationException e) {
throw new Error(e);
}
// Reduce the risk of rare disastrous classloading in first call to
// LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
Class<?> ensureLoaded = LockSupport.class;
int commonMaxSpares = DEFAULT_COMMON_MAX_SPARES;
try {
String p = System.getProperty
("java.util.concurrent.ForkJoinPool.common.maximumSpares");
if (p != null)
commonMaxSpares = Integer.parseInt(p);
} catch (Exception ignore) {}
COMMON_MAX_SPARES = commonMaxSpares;
defaultForkJoinWorkerThreadFactory =
new DefaultForkJoinWorkerThreadFactory();
modifyThreadPermission = new RuntimePermission("modifyThread");
common = java.security.AccessController.doPrivileged
(new java.security.PrivilegedAction<ForkJoinPool>() {
public ForkJoinPool run() { return makeCommonPool(); }});
// report 1 even if threads disabled
COMMON_PARALLELISM = Math.max(common.config & SMASK, 1);
}
/**
* Creates and returns the common pool, respecting user settings
* specified via system properties.
*/
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.
*/
private 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 java.security.AccessController.doPrivileged(
new java.security.PrivilegedAction<ForkJoinWorkerThread>() {
public ForkJoinWorkerThread run() {
return new ForkJoinWorkerThread.
InnocuousForkJoinWorkerThread(pool);
}}, innocuousAcc);
}
}
}