/*
* 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.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.AbstractExecutorService;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.RunnableFuture;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.TimeUnit;
/**
* An {@link ExecutorService} for running {@link ForkJoinTask}s.
* A {@code ForkJoinPool} provides the entry point for submissions
* from non-{@code ForkJoinTask} clients, as well as management and
* monitoring operations.
*
* A {@code ForkJoinPool} differs from other kinds of {@link
* ExecutorService} mainly by virtue of employing
* work-stealing: all threads in the pool attempt to find and
* execute tasks submitted to the pool and/or created by other active
* tasks (eventually blocking waiting for work if none exist). This
* enables efficient processing when most tasks spawn other subtasks
* (as do most {@code ForkJoinTask}s), as well as when many small
* tasks are submitted to the pool from external clients. Especially
* when setting asyncMode to true in constructors, {@code
* ForkJoinPool}s may also be appropriate for use with event-style
* tasks that are never joined.
*
*
A static {@link #commonPool()} is available and appropriate for
* most applications. The common pool is used by any ForkJoinTask that
* is not explicitly submitted to a specified pool. Using the common
* pool normally reduces resource usage (its threads are slowly
* reclaimed during periods of non-use, and reinstated upon subsequent
* use).
*
*
For applications that require separate or custom pools, a {@code
* ForkJoinPool} may be constructed with a given target parallelism
* level; by default, equal to the number of available processors. The
* pool attempts to maintain enough active (or available) threads by
* dynamically adding, suspending, or resuming internal worker
* threads, even if some tasks are stalled waiting to join others.
* However, no such adjustments are guaranteed in the face of blocked
* I/O or other unmanaged synchronization. The nested {@link
* ManagedBlocker} interface enables extension of the kinds of
* synchronization accommodated.
*
*
In addition to execution and lifecycle control methods, this
* class provides status check methods (for example
* {@link #getStealCount}) that are intended to aid in developing,
* tuning, and monitoring fork/join applications. Also, method
* {@link #toString} returns indications of pool state in a
* convenient form for informal monitoring.
*
*
As is the case with other ExecutorServices, there are three
* main task execution methods summarized in the following table.
* These are designed to be used primarily by clients not already
* engaged in fork/join computations in the current pool. The main
* forms of these methods accept instances of {@code ForkJoinTask},
* but overloaded forms also allow mixed execution of plain {@code
* Runnable}- or {@code Callable}- based activities as well. However,
* tasks that are already executing in a pool should normally instead
* use the within-computation forms listed in the table unless using
* async event-style tasks that are not usually joined, in which case
* there is little difference among choice of methods.
*
*
* Summary of task execution methods
*
* |
* Call from non-fork/join clients |
* Call from within fork/join computations |
*
*
* | Arrange async execution |
* {@link #execute(ForkJoinTask)} |
* {@link ForkJoinTask#fork} |
*
*
* | Await and obtain result |
* {@link #invoke(ForkJoinTask)} |
* {@link ForkJoinTask#invoke} |
*
*
* | Arrange exec and obtain Future |
* {@link #submit(ForkJoinTask)} |
* {@link ForkJoinTask#fork} (ForkJoinTasks are Futures) |
*
*
*
* The common pool is by default constructed with default
* parameters, but these may be controlled by setting three
* {@linkplain System#getProperty system properties}:
*
* - {@code java.util.concurrent.ForkJoinPool.common.parallelism}
* - the parallelism level, a non-negative integer
*
- {@code java.util.concurrent.ForkJoinPool.common.threadFactory}
* - the class name of a {@link ForkJoinWorkerThreadFactory}
*
- {@code java.util.concurrent.ForkJoinPool.common.exceptionHandler}
* - the class name of a {@link UncaughtExceptionHandler}
*
* The system class loader is used to load these classes.
* Upon any error in establishing these settings, default parameters
* are used. It is possible to disable or limit the use of threads in
* the common pool by setting the parallelism property to zero, and/or
* using a factory that may return {@code null}. However doing so may
* cause unjoined tasks to never be executed.
*
* Implementation notes: This implementation restricts the
* maximum number of running threads to 32767. Attempts to create
* pools with greater than the maximum number result in
* {@code IllegalArgumentException}.
*
*
This implementation rejects submitted tasks (that is, by throwing
* {@link RejectedExecutionException}) only when the pool is shut down
* or internal resources have been exhausted.
*
* @since 1.7
* @author Doug Lea
*/
@sun.misc.Contended
public class ForkJoinPool extends AbstractExecutorService {
/*
* Implementation Overview
*
* This class and its nested classes provide the main
* functionality and control for a set of worker threads:
* Submissions from non-FJ threads enter into submission queues.
* Workers take these tasks and typically split them into subtasks
* that may be stolen by other workers. Preference rules give
* first priority to processing tasks from their own queues (LIFO
* or FIFO, depending on mode), then to randomized FIFO steals of
* tasks in other queues.
*
* 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://labs.oracle.com/pls/apex/f?p=labs:10:0) and
* "Idempotent work stealing" by Michael, Saraswat, and Vechev,
* PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186).
* See also "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 (atomic, volatile etc) issues. 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. So, both a successful pop and poll
* mainly entail a CAS of a slot from non-null to null. Because
* we rely on CASes of references, we do not need tag bits on base
* or top. They are simple ints as used in any circular
* array-based queue (see for example ArrayDeque). Updates to the
* indices must still be ordered in a way that guarantees that top
* == base means the queue is empty, but otherwise may err on the
* side of possibly making the queue appear nonempty when a push,
* pop, or poll have not fully committed. Note that this means
* that the poll operation, considered individually, is not
* wait-free. One thief cannot successfully continue until another
* in-progress one (or, if previously empty, a push) completes.
* However, in the aggregate, we ensure at least probabilistic
* non-blockingness. If an attempted steal fails, a thief always
* chooses a different random victim target to try next. So, in
* order for one thief to progress, it suffices for any
* in-progress poll or new push on any empty queue to
* complete. (This is why we normally use method pollAt and its
* variants that try once at the apparent base index, else
* consider alternative actions, rather than method poll.)
*
* This approach also enables support of a user mode in which local
* task processing is in FIFO, not LIFO order, simply by using
* poll rather than pop. This can be useful in message-passing
* frameworks in which tasks are never joined. However neither
* mode considers affinities, loads, cache localities, etc, so
* rarely provide the best possible performance on a given
* machine, but portably provide good throughput by averaging over
* these factors. (Further, even if we did try to use such
* information, we do not usually have a basis for exploiting it.
* For example, some sets of tasks profit from cache affinities,
* but others are harmed by cache pollution effects.)
*
* WorkQueues are also used in a similar way for tasks submitted
* to the pool. We cannot mix these tasks in the same queues used
* for work-stealing (this would contaminate lifo/fifo
* processing). 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). However, because most
* shared/external queue operations are more expensive than
* internal, and because, at steady state, external submitters
* will compete for CPU with workers, ForkJoinTask.join and
* related methods disable them from repeatedly helping to process
* tasks if all workers are active. Insertion of tasks in shared
* mode requires a lock (mainly to protect in the case of
* resizing) but we use only a simple spinlock (using bits in
* 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.
*
* Management
* ==========
*
* The main throughput advantages of work-stealing stem from
* decentralized control -- workers mostly take tasks from
* themselves or each other. We cannot negate this in the
* implementation of other management responsibilities. The main
* tactic for avoiding bottlenecks is packing nearly all
* essentially atomic control state into two volatile variables
* that are by far most often read (not written) as status and
* consistency checks.
*
* Field "ctl" contains 64 bits holding all the 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
* fields.
*
* Field "plock" is a form of sequence lock with a saturating
* shutdown bit (similarly for per-queue "qlocks"), mainly
* protecting updates to the workQueues array, as well as to
* enable shutdown. When used as a lock, it is normally only very
* briefly held, so is nearly always available after at most a
* brief spin, but we use a monitor-based backup strategy to
* block when needed.
*
* Recording WorkQueues. WorkQueues are recorded in the
* "workQueues" array that is created upon first use and expanded
* if necessary. Updates to the array while recording new workers
* and unrecording terminated ones are protected from each other
* by a lock but the array is otherwise concurrently readable, and
* accessed directly. 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 wait queue
* field of ctl stores indices, not references. Access to the
* workQueues in associated methods (for example signalWork) must
* both index-check and null-check the IDs. All such accesses
* ignore bad IDs by returning out early from what they are doing,
* since this can only be associated with termination, in which
* case it is OK to give up. All uses of the workQueues array
* also check that it is non-null (even if previously
* non-null). This allows nulling during termination, which is
* currently not necessary, but remains an option for
* resource-revocation-based shutdown schemes. It also helps
* reduce JIT issuance of uncommon-trap code, which tends to
* unnecessarily complicate control flow in some methods.
*
* Event Queuing. 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 (this is
* compounded at program start-up by JIT compilation and
* allocation). So we try to streamline this as much as possible.
* We park/unpark workers after placing in an event wait queue
* when they cannot find work. This "queue" is actually a simple
* Treiber stack, headed by the "id" field of ctl, plus a 15bit
* counter value (that reflects the number of times a worker has
* been inactivated) to avoid ABA effects (we need only as many
* version numbers as worker threads). Successors are held in
* field WorkQueue.nextWait. Queuing deals with several intrinsic
* races, mainly that a task-producing thread can miss seeing (and
* signalling) another thread that gave up looking for work but
* has not yet entered the wait queue. We solve this by requiring
* a full sweep of all workers (via repeated calls to method
* scan()) both before and after a newly waiting worker is added
* to the wait queue. Because enqueued workers may actually be
* rescanning rather than waiting, we set and clear the "parker"
* field of WorkQueues to reduce unnecessary calls to unpark.
* (This requires a secondary recheck to avoid missed signals.)
* Note the unusual conventions about Thread.interrupts
* surrounding parking and other blocking: Because interrupts are
* used solely to alert threads to check termination, which is
* checked anyway upon blocking, we clear status (using
* Thread.interrupted) before any call to park, so that park does
* not immediately return due to status being set via some other
* unrelated call to interrupt in user code.
*
* Signalling. We create or wake up workers only when there
* appears to be at least one task they might be able to find and
* execute. When a submission is added or another worker adds a
* task to a queue that has fewer than two tasks, they signal
* waiting workers (or trigger creation of new ones if fewer than
* the given parallelism level -- signalWork). These primary
* signals are buttressed by others whenever other threads remove
* a task from a queue and notice that there are other tasks there
* as well. So in general, pools will be over-signalled. On most
* platforms, signalling (unpark) overhead time is noticeably
* long, and the time between signalling a thread and it actually
* making progress can be very noticeably long, so it is worth
* offloading these delays from critical paths as much as
* possible. Additionally, workers spin-down gradually, by staying
* alive so long as they see the ctl state changing. Similar
* stability-sensing techniques are also used before blocking in
* awaitJoin and helpComplete.
*
* 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 if the pool has remained quiescent for a
* given period -- a short period if there are more threads than
* parallelism, longer as the number of threads decreases. This
* will slowly propagate, eventually terminating all workers after
* periods of non-use.
*
* Shutdown and Termination. A call to shutdownNow atomically sets
* a plock bit and then (non-atomically) sets each worker's
* qlock status, cancels all unprocessed tasks, and wakes up
* all waiting workers. Detecting whether termination should
* commence after a non-abrupt shutdown() call requires more work
* and bookkeeping. We need consensus about quiescence (i.e., that
* there is no more work). The active count provides a primary
* indication but non-abrupt shutdown still requires a rechecking
* scan for any workers that are inactive but not queued.
*
* 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 sometimes 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 tryHelpStealer entails a form of "linear"
* helping: Each worker records (in field currentSteal) the most
* recent task it stole from some other worker. Plus, it records
* (in field currentJoin) the task it is currently actively
* joining. Method tryHelpStealer 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.
* In essence, the joiner executes a task that would be on its own
* local deque had the to-be-joined task not been stolen. This may
* be seen as a conservative variant of the approach 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 (see MAX_HELP) and fall back to suspending the
* worker and if necessary replacing it with another.
*
* Helping actions for CountedCompleters are much simpler: Method
* helpComplete can take and execute any task with the same root
* as the task being waited on. However, this still entails some
* traversal of completer chains, so is less efficient than using
* CountedCompleters without explicit joins.
*
* It is impossible to keep exactly the target parallelism number
* of threads running at any given time. Determining the
* existence of conservatively safe helping targets, the
* availability of already-created spares, and the apparent need
* to create new spares are all racy, so we rely on multiple
* retries of each. Compensation in the apparent absence of
* helping opportunities is challenging to control on JVMs, where
* GC and other activities can stall progress of tasks that in
* turn stall out many other dependent tasks, without us being
* able to determine whether they will ever require compensation.
* Even though work-stealing otherwise encounters little
* degradation in the presence of more threads than cores,
* aggressively adding new threads in such cases entails risk of
* unwanted positive feedback control loops in which more threads
* cause more dependent stalls (as well as delayed progress of
* unblocked threads to the point that we know they are available)
* leading to more situations requiring more threads, and so
* on. This aspect of control can be seen as an (analytically
* intractable) game with an opponent that may choose the worst
* (for us) active thread to stall at any time. We take several
* precautions to bound losses (and thus bound gains), mainly in
* methods tryCompensate and awaitJoin.
*
* 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
* fullExternalPush during the first submission to the pool.
*
* When external threads submit to the common pool, they can
* perform subtask processing (see externalHelpJoin and related
* methods). 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, externalHelpJoin
* returns quickly (at the most helping to signal some common pool
* workers). 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.
*
* Style notes
* ===========
*
* 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 volatiles held
* in local variables. Methods signalWork() and scan() are the
* main bottlenecks, so are especially heavily
* micro-optimized/mangled. There are lots of inline assignments
* (of form "while ((local = field) != 0)") which are usually the
* simplest way to ensure the required read orderings (which are
* sometimes critical). This leads to a "C"-like style of listing
* declarations of these locals at the heads of methods or blocks.
* There are several occurrences of the unusual "do {} while
* (!cas...)" which is the simplest way to force an update of a
* CAS'ed variable. 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:
* (1) Static utility functions
* (2) Nested (static) classes
* (3) Static fields
* (4) Fields, along with constants used when unpacking some of them
* (5) Internal control methods
* (6) Callbacks and other support for ForkJoinTask methods
* (7) Exported methods
* (8) Static block initializing statics in minimally dependent order
*/
// Static utilities
/**
* If there is a security manager, makes sure caller has
* permission to modify threads.
*/
private static void checkPermission() {
SecurityManager security = System.getSecurityManager();
if (security != null)
security.checkPermission(modifyThreadPermission);
}
// Nested classes
/**
* Factory for creating new {@link ForkJoinWorkerThread}s.
* A {@code ForkJoinWorkerThreadFactory} must be defined and used
* for {@code ForkJoinWorkerThread} subclasses that extend base
* functionality or initialize threads with different contexts.
*/
public static interface ForkJoinWorkerThreadFactory {
/**
* Returns a new worker thread operating in the given pool.
*
* @param pool the pool this thread works in
* @return the new worker thread
* @throws NullPointerException if the pool is null
*/
public ForkJoinWorkerThread newThread(ForkJoinPool pool);
}
/**
* Default ForkJoinWorkerThreadFactory implementation; creates a
* new ForkJoinWorkerThread.
*/
static final class DefaultForkJoinWorkerThreadFactory
implements ForkJoinWorkerThreadFactory {
public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
return new ForkJoinWorkerThread(pool);
}
}
/**
* Class for artificial tasks that are used to replace the target
* of local joins if they are removed from an interior queue slot
* in WorkQueue.tryRemoveAndExec. We don't need the proxy to
* actually do anything beyond having a unique identity.
*/
static final class EmptyTask extends ForkJoinTask {
private static final long serialVersionUID = -7721805057305804111L;
EmptyTask() { status = ForkJoinTask.NORMAL; } // force done
public final Void getRawResult() { return null; }
public final void setRawResult(Void x) {}
public final boolean exec() { return true; }
}
/**
* Queues supporting work-stealing as well as external task
* submission. See above for main rationale and algorithms.
* Implementation relies heavily on "Unsafe" intrinsics
* and selective use of "volatile":
*
* Field "base" is the index (mod array.length) of the least valid
* queue slot, which is always the next position to steal (poll)
* from if nonempty. Reads and writes require volatile orderings
* but not CAS, because updates are only performed after slot
* CASes.
*
* Field "top" is the index (mod array.length) of the next queue
* slot to push to or pop from. It is written only by owner thread
* for push, or under lock for external/shared push, and accessed
* by other threads only after reading (volatile) base. Both top
* and base are allowed to wrap around on overflow, but (top -
* base) (or more commonly -(base - top) to force volatile read of
* base before top) still estimates size. The lock ("qlock") is
* forced to -1 on termination, causing all further lock attempts
* to fail. (Note: we don't need CAS for termination state because
* upon pool shutdown, all shared-queues will stop being used
* anyway.) Nearly all lock bodies are set up so that exceptions
* within lock bodies are "impossible" (modulo JVM errors that
* would cause failure anyway.)
*
* The array slots are read and written using the emulation of
* volatiles/atomics provided by Unsafe. Insertions must in
* general use putOrderedObject as a form of releasing store to
* ensure that all writes to the task object are ordered before
* its publication in the queue. All removals entail a CAS to
* null. The array is always a power of two. To ensure safety of
* Unsafe array operations, all accesses perform explicit null
* checks and implicit bounds checks via power-of-two masking.
*
* In addition to basic queuing support, this class contains
* fields described elsewhere to control execution. It turns out
* to work better memory-layout-wise to include them in this class
* rather than a separate class.
*
* 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. (It would be best for queue objects
* and their arrays to share, but there is nothing available to
* help arrange that). The @Contended annotation alerts JVMs to
* try to keep instances apart.
*/
@sun.misc.Contended
static final class WorkQueue {
/**
* Capacity of work-stealing queue array upon initialization.
* Must be a power of two; at least 4, but should be larger to
* reduce or eliminate cacheline sharing among queues.
* Currently, it is much larger, as a partial workaround for
* the fact that JVMs often place arrays in locations that
* share GC bookkeeping (especially cardmarks) such that
* per-write accesses encounter serious memory contention.
*/
static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
/**
* Maximum size for queue arrays. Must be a power of two less
* than or equal to 1 << (31 - width of array entry) to ensure
* lack of wraparound of index calculations, but defined to a
* value a bit less than this to help users trap runaway
* programs before saturating systems.
*/
static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M
volatile int eventCount; // encoded inactivation count; < 0 if inactive
int nextWait; // encoded record of next event waiter
int nsteals; // number of steals
int hint; // steal index hint
short poolIndex; // index of this queue in pool
final short mode; // 0: lifo, > 0: fifo, < 0: shared
volatile int qlock; // 1: locked, -1: 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
ForkJoinTask currentSteal; // current non-local task being executed
WorkQueue(ForkJoinPool pool, ForkJoinWorkerThread owner, int mode,
int seed) {
this.pool = pool;
this.owner = owner;
this.mode = (short)mode;
this.hint = seed; // store initial seed for runWorker
// Place indices in the center of array (that is not yet allocated)
base = top = INITIAL_QUEUE_CAPACITY >>> 1;
}
/**
* Returns the approximate number of tasks in the queue.
*/
final int queueSize() {
int n = base - top; // non-owner callers must read base first
return (n >= 0) ? 0 : -n; // ignore transient negative
}
/**
* Provides a more accurate estimate of whether this queue has
* any tasks than does queueSize, by checking whether a
* near-empty queue has at least one unclaimed task.
*/
final boolean isEmpty() {
ForkJoinTask[] a; int m, s;
int n = base - (s = top);
return (n >= 0 ||
(n == -1 &&
((a = array) == null ||
(m = a.length - 1) < 0 ||
U.getObject
(a, (long)((m & (s - 1)) << ASHIFT) + ABASE) == null)));
}
/**
* Pushes a task. Call only by owner in unshared queues. (The
* shared-queue version is embedded in method externalPush.)
*
* @param task the task. Caller must ensure non-null.
* @throws RejectedExecutionException if array cannot be resized
*/
final void push(ForkJoinTask task) {
ForkJoinTask[] a; ForkJoinPool p;
int s = top, n;
if ((a = array) != null) { // ignore if queue removed
int m = a.length - 1;
U.putOrderedObject(a, ((m & s) << ASHIFT) + ABASE, task);
if ((n = (top = s + 1) - base) <= 2)
(p = pool).signalWork(p.workQueues, this);
else if (n >= m)
growArray();
}
}
/**
* Initializes or doubles the capacity of array. Call either
* by owner or with lock held -- it is OK for base, but not
* top, to move while resizings are in progress.
*/
final ForkJoinTask[] growArray() {
ForkJoinTask[] oldA = array;
int size = oldA != null ? oldA.length << 1 : INITIAL_QUEUE_CAPACITY;
if (size > MAXIMUM_QUEUE_CAPACITY)
throw new RejectedExecutionException("Queue capacity exceeded");
int oldMask, t, b;
ForkJoinTask[] a = array = new ForkJoinTask[size];
if (oldA != null && (oldMask = oldA.length - 1) >= 0 &&
(t = top) - (b = base) > 0) {
int mask = size - 1;
do {
ForkJoinTask x;
int oldj = ((b & oldMask) << ASHIFT) + ABASE;
int j = ((b & mask) << ASHIFT) + ABASE;
x = (ForkJoinTask)U.getObjectVolatile(oldA, oldj);
if (x != null &&
U.compareAndSwapObject(oldA, oldj, x, null))
U.putObjectVolatile(a, j, x);
} while (++b != t);
}
return a;
}
/**
* Takes next task, if one exists, in LIFO order. Call only
* by owner in unshared queues.
*/
final ForkJoinTask pop() {
ForkJoinTask[] a; ForkJoinTask t; int m;
if ((a = array) != null && (m = a.length - 1) >= 0) {
for (int s; (s = top - 1) - base >= 0;) {
long j = ((m & s) << ASHIFT) + ABASE;
if ((t = (ForkJoinTask)U.getObject(a, j)) == null)
break;
if (U.compareAndSwapObject(a, j, t, null)) {
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 tryHelpStealer.
*/
final ForkJoinTask pollAt(int b) {
ForkJoinTask t; ForkJoinTask[] a;
if ((a = array) != null) {
int j = (((a.length - 1) & b) << ASHIFT) + ABASE;
if ((t = (ForkJoinTask)U.getObjectVolatile(a, j)) != null &&
base == b && U.compareAndSwapObject(a, j, t, null)) {
U.putOrderedInt(this, QBASE, b + 1);
return t;
}
}
return null;
}
/**
* Takes next task, if one exists, in FIFO order.
*/
final ForkJoinTask poll() {
ForkJoinTask[] a; int b; ForkJoinTask t;
while ((b = base) - top < 0 && (a = array) != null) {
int j = (((a.length - 1) & b) << ASHIFT) + ABASE;
t = (ForkJoinTask)U.getObjectVolatile(a, j);
if (t != null) {
if (U.compareAndSwapObject(a, j, t, null)) {
U.putOrderedInt(this, QBASE, b + 1);
return t;
}
}
else if (base == b) {
if (b + 1 == top)
break;
Thread.yield(); // wait for lagging update (very rare)
}
}
return null;
}
/**
* Takes next task, if one exists, in order specified by mode.
*/
final ForkJoinTask nextLocalTask() {
return mode == 0 ? pop() : poll();
}
/**
* Returns next task, if one exists, in order specified by mode.
*/
final ForkJoinTask peek() {
ForkJoinTask[] a = array; int m;
if (a == null || (m = a.length - 1) < 0)
return null;
int i = mode == 0 ? top - 1 : base;
int j = ((i & m) << ASHIFT) + ABASE;
return (ForkJoinTask)U.getObjectVolatile(a, j);
}
/**
* Pops the given task only if it is at the current top.
* (A shared version is available only via FJP.tryExternalUnpush)
*/
final boolean tryUnpush(ForkJoinTask t) {
ForkJoinTask[] a; int s;
if ((a = array) != null && (s = top) != base &&
U.compareAndSwapObject
(a, (((a.length - 1) & --s) << ASHIFT) + ABASE, t, null)) {
top = s;
return true;
}
return false;
}
/**
* Removes and cancels all known tasks, ignoring any exceptions.
*/
final void cancelAll() {
ForkJoinTask.cancelIgnoringExceptions(currentJoin);
ForkJoinTask.cancelIgnoringExceptions(currentSteal);
for (ForkJoinTask t; (t = poll()) != null; )
ForkJoinTask.cancelIgnoringExceptions(t);
}
// Specialized execution methods
/**
* Polls and runs tasks until empty.
*/
final void pollAndExecAll() {
for (ForkJoinTask t; (t = poll()) != null;)
t.doExec();
}
/**
* Executes a top-level task and any local tasks remaining
* after execution.
*/
final void runTask(ForkJoinTask task) {
if ((currentSteal = task) != null) {
task.doExec();
ForkJoinTask[] a = array;
int md = mode;
++nsteals;
currentSteal = null;
if (md != 0)
pollAndExecAll();
else if (a != null) {
int s, m = a.length - 1;
ForkJoinTask t;
while ((s = top - 1) - base >= 0 &&
(t = (ForkJoinTask)U.getAndSetObject
(a, ((m & s) << ASHIFT) + ABASE, null)) != null) {
top = s;
t.doExec();
}
}
}
}
/**
* If present, removes from queue and executes the given task,
* or any other cancelled task. Returns (true) on any CAS
* or consistency check failure so caller can retry.
*
* @return false if no progress can be made, else true
*/
final boolean tryRemoveAndExec(ForkJoinTask task) {
boolean stat;
ForkJoinTask[] a; int m, s, b, n;
if (task != null && (a = array) != null && (m = a.length - 1) >= 0 &&
(n = (s = top) - (b = base)) > 0) {
boolean removed = false, empty = true;
stat = true;
for (ForkJoinTask t;;) { // traverse from s to b
long j = ((--s & m) << ASHIFT) + ABASE;
t = (ForkJoinTask)U.getObject(a, j);
if (t == null) // inconsistent length
break;
else if (t == task) {
if (s + 1 == top) { // pop
if (!U.compareAndSwapObject(a, j, task, null))
break;
top = s;
removed = true;
}
else if (base == b) // replace with proxy
removed = U.compareAndSwapObject(a, j, task,
new EmptyTask());
break;
}
else if (t.status >= 0)
empty = false;
else if (s + 1 == top) { // pop and throw away
if (U.compareAndSwapObject(a, j, t, null))
top = s;
break;
}
if (--n == 0) {
if (!empty && base == b)
stat = false;
break;
}
}
if (removed)
task.doExec();
}
else
stat = false;
return stat;
}
/**
* Tries to poll for and execute the given task or any other
* task in its CountedCompleter computation.
*/
final boolean pollAndExecCC(CountedCompleter root) {
ForkJoinTask[] a; int b; Object o; CountedCompleter t, r;
if ((b = base) - top < 0 && (a = array) != null) {
long j = (((a.length - 1) & b) << ASHIFT) + ABASE;
if ((o = U.getObjectVolatile(a, j)) == null)
return true; // retry
if (o instanceof CountedCompleter) {
for (t = (CountedCompleter)o, r = t;;) {
if (r == root) {
if (base == b &&
U.compareAndSwapObject(a, j, t, null)) {
U.putOrderedInt(this, QBASE, b + 1);
t.doExec();
}
return true;
}
else if ((r = r.completer) == null)
break; // not part of root computation
}
}
}
return false;
}
/**
* Tries to pop and execute the given task or any other task
* in its CountedCompleter computation.
*/
final boolean externalPopAndExecCC(CountedCompleter root) {
ForkJoinTask[] a; int s; Object o; CountedCompleter t, r;
if (base - (s = top) < 0 && (a = array) != null) {
long j = (((a.length - 1) & (s - 1)) << ASHIFT) + ABASE;
if ((o = U.getObject(a, j)) instanceof CountedCompleter) {
for (t = (CountedCompleter)o, r = t;;) {
if (r == root) {
if (U.compareAndSwapInt(this, QLOCK, 0, 1)) {
if (top == s && array == a &&
U.compareAndSwapObject(a, j, t, null)) {
top = s - 1;
qlock = 0;
t.doExec();
}
else
qlock = 0;
}
return true;
}
else if ((r = r.completer) == null)
break;
}
}
}
return false;
}
/**
* Internal version
*/
final boolean internalPopAndExecCC(CountedCompleter root) {
ForkJoinTask[] a; int s; Object o; CountedCompleter t, r;
if (base - (s = top) < 0 && (a = array) != null) {
long j = (((a.length - 1) & (s - 1)) << ASHIFT) + ABASE;
if ((o = U.getObject(a, j)) instanceof CountedCompleter) {
for (t = (CountedCompleter)o, r = t;;) {
if (r == root) {
if (U.compareAndSwapObject(a, j, t, null)) {
top = s - 1;
t.doExec();
}
return true;
}
else if ((r = r.completer) == null)
break;
}
}
}
return false;
}
/**
* Returns true if owned and not known to be blocked.
*/
final boolean isApparentlyUnblocked() {
Thread wt; Thread.State s;
return (eventCount >= 0 &&
(wt = owner) != null &&
(s = wt.getState()) != Thread.State.BLOCKED &&
s != Thread.State.WAITING &&
s != Thread.State.TIMED_WAITING);
}
// Unsafe mechanics
private static final sun.misc.Unsafe U;
private static final long QBASE;
private static final long QLOCK;
private static final int ABASE;
private static final int ASHIFT;
static {
try {
U = sun.misc.Unsafe.getUnsafe();
Class k = WorkQueue.class;
Class ak = ForkJoinTask[].class;
QBASE = U.objectFieldOffset
(k.getDeclaredField("base"));
QLOCK = U.objectFieldOffset
(k.getDeclaredField("qlock"));
ABASE = U.arrayBaseOffset(ak);
int scale = U.arrayIndexScale(ak);
if ((scale & (scale - 1)) != 0)
throw new Error("data type scale not a power of two");
ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
} catch (Exception e) {
throw new Error(e);
}
}
}
// static fields (initialized in static initializer below)
/**
* Creates a new ForkJoinWorkerThread. This factory is used unless
* overridden in ForkJoinPool constructors.
*/
public static final ForkJoinWorkerThreadFactory
defaultForkJoinWorkerThreadFactory;
/**
* Permission required for callers of methods that may start or
* kill threads.
*/
private static final RuntimePermission modifyThreadPermission;
/**
* Common (static) pool. Non-null for public use unless a static
* construction exception, but internal usages null-check on use
* to paranoically avoid potential initialization circularities
* as well as to simplify generated code.
*/
static final ForkJoinPool common;
/**
* Common pool parallelism. To allow simpler use and management
* when common pool threads are disabled, we allow the underlying
* common.parallelism field to be zero, but in that case still report
* parallelism as 1 to reflect resulting caller-runs mechanics.
*/
static final int commonParallelism;
/**
* 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 constants
/**
* Initial timeout value (in nanoseconds) for the thread
* triggering quiescence to park waiting for new work. On timeout,
* the thread will instead try to shrink the number of
* workers. The value should be large enough to avoid overly
* aggressive shrinkage during most transient stalls (long GCs
* etc).
*/
private static final long IDLE_TIMEOUT = 2000L * 1000L * 1000L; // 2sec
/**
* Timeout value when there are more threads than parallelism level
*/
private static final long FAST_IDLE_TIMEOUT = 200L * 1000L * 1000L;
/**
* Tolerance for idle timeouts, to cope with timer undershoots
*/
private static final long TIMEOUT_SLOP = 2000000L;
/**
* The maximum stolen->joining link depth allowed in method
* tryHelpStealer. Must be a power of two. Depths for legitimate
* chains are unbounded, but we use a fixed constant to avoid
* (otherwise unchecked) cycles and to bound staleness of
* traversal parameters at the expense of sometimes blocking when
* we could be helping.
*/
private static final int MAX_HELP = 64;
/**
* Increment for seed generators. See class ThreadLocal for
* explanation.
*/
private static final int SEED_INCREMENT = 0x61c88647;
/*
* Bits and masks for control variables
*
* Field ctl is a long packed with:
* AC: Number of active running workers minus target parallelism (16 bits)
* TC: Number of total workers minus target parallelism (16 bits)
* ST: true if pool is terminating (1 bit)
* EC: the wait count of top waiting thread (15 bits)
* ID: poolIndex of top of Treiber stack of waiters (16 bits)
*
* When convenient, we can extract the upper 32 bits of counts and
* the lower 32 bits of queue state, u = (int)(ctl >>> 32) and e =
* (int)ctl. The ec field is never accessed alone, but always
* together with id and st. 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, and when e is
* negative, the pool is terminating. To deal with these possibly
* negative fields, we use casts in and out of "short" and/or
* signed shifts to maintain signedness.
*
* When a thread is queued (inactivated), its eventCount field is
* set negative, which is the only way to tell if a worker is
* prevented from executing tasks, even though it must continue to
* scan for them to avoid queuing races. Note however that
* eventCount updates lag releases so usage requires care.
*
* Field plock is an int packed with:
* SHUTDOWN: true if shutdown is enabled (1 bit)
* SEQ: a sequence lock, with PL_LOCK bit set if locked (30 bits)
* SIGNAL: set when threads may be waiting on the lock (1 bit)
*
* The sequence number enables simple consistency checks:
* Staleness of read-only operations on the workQueues array can
* be checked by comparing plock before vs after the reads.
*/
// bit positions/shifts for fields
private static final int AC_SHIFT = 48;
private static final int TC_SHIFT = 32;
private static final int ST_SHIFT = 31;
private static final int EC_SHIFT = 16;
// bounds
private static final int SMASK = 0xffff; // short bits
private static final int MAX_CAP = 0x7fff; // max #workers - 1
private static final int EVENMASK = 0xfffe; // even short bits
private static final int SQMASK = 0x007e; // max 64 (even) slots
private static final int SHORT_SIGN = 1 << 15;
private static final int INT_SIGN = 1 << 31;
// masks
private static final long STOP_BIT = 0x0001L << ST_SHIFT;
private static final long AC_MASK = ((long)SMASK) << AC_SHIFT;
private static final long TC_MASK = ((long)SMASK) << TC_SHIFT;
// units for incrementing and decrementing
private static final long TC_UNIT = 1L << TC_SHIFT;
private static final long AC_UNIT = 1L << AC_SHIFT;
// masks and units for dealing with u = (int)(ctl >>> 32)
private static final int UAC_SHIFT = AC_SHIFT - 32;
private static final int UTC_SHIFT = TC_SHIFT - 32;
private static final int UAC_MASK = SMASK << UAC_SHIFT;
private static final int UTC_MASK = SMASK << UTC_SHIFT;
private static final int UAC_UNIT = 1 << UAC_SHIFT;
private static final int UTC_UNIT = 1 << UTC_SHIFT;
// masks and units for dealing with e = (int)ctl
private static final int E_MASK = 0x7fffffff; // no STOP_BIT
private static final int E_SEQ = 1 << EC_SHIFT;
// plock bits
private static final int SHUTDOWN = 1 << 31;
private static final int PL_LOCK = 2;
private static final int PL_SIGNAL = 1;
private static final int PL_SPINS = 1 << 8;
// access mode for WorkQueue
static final int LIFO_QUEUE = 0;
static final int FIFO_QUEUE = 1;
static final int SHARED_QUEUE = -1;
// Instance fields
volatile long stealCount; // collects worker counts
volatile long ctl; // main pool control
volatile int plock; // shutdown status and seqLock
volatile int indexSeed; // worker/submitter index seed
final short parallelism; // parallelism level
final short mode; // LIFO/FIFO
WorkQueue[] workQueues; // main registry
final ForkJoinWorkerThreadFactory factory;
final UncaughtExceptionHandler ueh; // per-worker UEH
final String workerNamePrefix; // to create worker name string
/**
* Acquires the plock lock to protect worker array and related
* updates. This method is called only if an initial CAS on plock
* fails. This acts as a spinlock for normal cases, but falls back
* to builtin monitor to block when (rarely) needed. This would be
* a terrible idea for a highly contended lock, but works fine as
* a more conservative alternative to a pure spinlock.
*/
private int acquirePlock() {
int spins = PL_SPINS, ps, nps;
for (;;) {
if (((ps = plock) & PL_LOCK) == 0 &&
U.compareAndSwapInt(this, PLOCK, ps, nps = ps + PL_LOCK))
return nps;
else if (spins >= 0) {
if (ThreadLocalRandom.nextSecondarySeed() >= 0)
--spins;
}
else if (U.compareAndSwapInt(this, PLOCK, ps, ps | PL_SIGNAL)) {
synchronized (this) {
if ((plock & PL_SIGNAL) != 0) {
try {
wait();
} catch (InterruptedException ie) {
try {
Thread.currentThread().interrupt();
} catch (SecurityException ignore) {
}
}
}
else
notifyAll();
}
}
}
}
/**
* Unlocks and signals any thread waiting for plock. Called only
* when CAS of seq value for unlock fails.
*/
private void releasePlock(int ps) {
plock = ps;
synchronized (this) { notifyAll(); }
}
/**
* Tries to create and start one worker if fewer than target
* parallelism level exist. Adjusts counts etc on failure.
*/
private void tryAddWorker() {
long c; int u, e;
while ((u = (int)((c = ctl) >>> 32)) < 0 &&
(u & SHORT_SIGN) != 0 && (e = (int)c) >= 0) {
long nc = ((long)(((u + UTC_UNIT) & UTC_MASK) |
((u + UAC_UNIT) & UAC_MASK)) << 32) | (long)e;
if (U.compareAndSwapLong(this, CTL, c, nc)) {
ForkJoinWorkerThreadFactory fac;
Throwable ex = null;
ForkJoinWorkerThread wt = null;
try {
if ((fac = factory) != null &&
(wt = fac.newThread(this)) != null) {
wt.start();
break;
}
} catch (Throwable rex) {
ex = rex;
}
deregisterWorker(wt, ex);
break;
}
}
}
// Registering and deregistering workers
/**
* Callback from ForkJoinWorkerThread to establish and record its
* WorkQueue. To avoid scanning bias due to packing entries in
* front of the workQueues array, we treat the array as a simple
* power-of-two hash table using per-thread seed as hash,
* expanding as needed.
*
* @param wt the worker thread
* @return the worker's queue
*/
final WorkQueue registerWorker(ForkJoinWorkerThread wt) {
UncaughtExceptionHandler handler; WorkQueue[] ws; int s, ps;
wt.setDaemon(true);
if ((handler = ueh) != null)
wt.setUncaughtExceptionHandler(handler);
do {} while (!U.compareAndSwapInt(this, INDEXSEED, s = indexSeed,
s += SEED_INCREMENT) ||
s == 0); // skip 0
WorkQueue w = new WorkQueue(this, wt, mode, s);
if (((ps = plock) & PL_LOCK) != 0 ||
!U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
ps = acquirePlock();
int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
try {
if ((ws = workQueues) != null) { // skip if shutting down
int n = ws.length, m = n - 1;
int r = (s << 1) | 1; // use odd-numbered indices
if (ws[r &= m] != null) { // collision
int probes = 0; // step by approx half size
int step = (n <= 4) ? 2 : ((n >>> 1) & EVENMASK) + 2;
while (ws[r = (r + step) & m] != null) {
if (++probes >= n) {
workQueues = ws = Arrays.copyOf(ws, n <<= 1);
m = n - 1;
probes = 0;
}
}
}
w.poolIndex = (short)r;
w.eventCount = r; // volatile write orders
ws[r] = w;
}
} finally {
if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
releasePlock(nps);
}
wt.setName(workerNamePrefix.concat(Integer.toString(w.poolIndex >>> 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) {
int ps;
w.qlock = -1; // ensure set
U.getAndAddLong(this, STEALCOUNT, w.nsteals); // collect steals
if (((ps = plock) & PL_LOCK) != 0 ||
!U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
ps = acquirePlock();
int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
try {
int idx = w.poolIndex;
WorkQueue[] ws = workQueues;
if (ws != null && idx >= 0 && idx < ws.length && ws[idx] == w)
ws[idx] = null;
} finally {
if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
releasePlock(nps);
}
}
long c; // adjust ctl counts
do {} while (!U.compareAndSwapLong
(this, CTL, c = ctl, (((c - AC_UNIT) & AC_MASK) |
((c - TC_UNIT) & TC_MASK) |
(c & ~(AC_MASK|TC_MASK)))));
if (!tryTerminate(false, false) && w != null && w.array != null) {
w.cancelAll(); // cancel remaining tasks
WorkQueue[] ws; WorkQueue v; Thread p; int u, i, e;
while ((u = (int)((c = ctl) >>> 32)) < 0 && (e = (int)c) >= 0) {
if (e > 0) { // activate or create replacement
if ((ws = workQueues) == null ||
(i = e & SMASK) >= ws.length ||
(v = ws[i]) == null)
break;
long nc = (((long)(v.nextWait & E_MASK)) |
((long)(u + UAC_UNIT) << 32));
if (v.eventCount != (e | INT_SIGN))
break;
if (U.compareAndSwapLong(this, CTL, c, nc)) {
v.eventCount = (e + E_SEQ) & E_MASK;
if ((p = v.parker) != null)
U.unpark(p);
break;
}
}
else {
if ((short)u < 0)
tryAddWorker();
break;
}
}
}
if (ex == null) // help clean refs on way out
ForkJoinTask.helpExpungeStaleExceptions();
else // rethrow
ForkJoinTask.rethrow(ex);
}
// Submissions
/**
* Unless shutting down, adds the given task to a submission queue
* at submitter's current queue index (modulo submission
* range). Only the most common path is directly handled in this
* method. All others are relayed to fullExternalPush.
*
* @param task the task. Caller must ensure non-null.
*/
final void externalPush(ForkJoinTask task) {
WorkQueue q; int m, s, n, am; ForkJoinTask[] a;
int r = ThreadLocalRandom.getProbe();
int ps = plock;
WorkQueue[] ws = workQueues;
if (ps > 0 && ws != null && (m = (ws.length - 1)) >= 0 &&
(q = ws[m & r & SQMASK]) != null && r != 0 &&
U.compareAndSwapInt(q, QLOCK, 0, 1)) { // lock
if ((a = q.array) != null &&
(am = a.length - 1) > (n = (s = q.top) - q.base)) {
int j = ((am & s) << ASHIFT) + ABASE;
U.putOrderedObject(a, j, task);
q.top = s + 1; // push on to deque
q.qlock = 0;
if (n <= 1)
signalWork(ws, q);
return;
}
q.qlock = 0;
}
fullExternalPush(task);
}
/**
* Full version of externalPush. This method is called, among
* other times, upon the first submission of the first task to the
* pool, so must perform secondary initialization. It also
* detects first submission by an external thread by looking up
* its ThreadLocal, and creates a new shared queue if the one at
* index if empty or contended. The plock lock body must be
* exception-free (so no try/finally) so we optimistically
* allocate new queues outside the lock and throw them away if
* (very rarely) not needed.
*
* Secondary initialization occurs when plock is zero, to create
* workQueue array and set plock to a valid value. This lock body
* must also be exception-free. Because the plock seq value can
* eventually wrap around zero, this method harmlessly fails to
* reinitialize if workQueues exists, while still advancing plock.
*/
private void fullExternalPush(ForkJoinTask task) {
int r;
if ((r = ThreadLocalRandom.getProbe()) == 0) {
ThreadLocalRandom.localInit();
r = ThreadLocalRandom.getProbe();
}
for (;;) {
WorkQueue[] ws; WorkQueue q; int ps, m, k;
boolean move = false;
if ((ps = plock) < 0)
throw new RejectedExecutionException();
else if (ps == 0 || (ws = workQueues) == null ||
(m = ws.length - 1) < 0) { // initialize workQueues
int p = parallelism; // find power of two table size
int n = (p > 1) ? p - 1 : 1; // ensure at least 2 slots
n |= n >>> 1; n |= n >>> 2; n |= n >>> 4;
n |= n >>> 8; n |= n >>> 16; n = (n + 1) << 1;
WorkQueue[] nws = ((ws = workQueues) == null || ws.length == 0 ?
new WorkQueue[n] : null);
if (((ps = plock) & PL_LOCK) != 0 ||
!U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
ps = acquirePlock();
if (((ws = workQueues) == null || ws.length == 0) && nws != null)
workQueues = nws;
int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
releasePlock(nps);
}
else if ((q = ws[k = r & m & SQMASK]) != null) {
if (q.qlock == 0 && U.compareAndSwapInt(q, QLOCK, 0, 1)) {
ForkJoinTask[] a = q.array;
int s = q.top;
boolean submitted = false;
try { // locked version of push
if ((a != null && a.length > s + 1 - q.base) ||
(a = q.growArray()) != null) { // must presize
int j = (((a.length - 1) & s) << ASHIFT) + ABASE;
U.putOrderedObject(a, j, task);
q.top = s + 1;
submitted = true;
}
} finally {
q.qlock = 0; // unlock
}
if (submitted) {
signalWork(ws, q);
return;
}
}
move = true; // move on failure
}
else if (((ps = plock) & PL_LOCK) == 0) { // create new queue
q = new WorkQueue(this, null, SHARED_QUEUE, r);
q.poolIndex = (short)k;
if (((ps = plock) & PL_LOCK) != 0 ||
!U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
ps = acquirePlock();
if ((ws = workQueues) != null && k < ws.length && ws[k] == null)
ws[k] = q;
int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
releasePlock(nps);
}
else
move = true; // move if busy
if (move)
r = ThreadLocalRandom.advanceProbe(r);
}
}
// Maintaining ctl counts
/**
* Increments active count; mainly called upon return from blocking.
*/
final void incrementActiveCount() {
long c;
do {} while (!U.compareAndSwapLong
(this, CTL, c = ctl, ((c & ~AC_MASK) |
((c & AC_MASK) + AC_UNIT))));
}
/**
* Tries to create or activate a worker if too few are active.
*
* @param ws the worker array to use to find signallees
* @param q if non-null, the queue holding tasks to be processed
*/
final void signalWork(WorkQueue[] ws, WorkQueue q) {
for (;;) {
long c; int e, u, i; WorkQueue w; Thread p;
if ((u = (int)((c = ctl) >>> 32)) >= 0)
break;
if ((e = (int)c) <= 0) {
if ((short)u < 0)
tryAddWorker();
break;
}
if (ws == null || ws.length <= (i = e & SMASK) ||
(w = ws[i]) == null)
break;
long nc = (((long)(w.nextWait & E_MASK)) |
((long)(u + UAC_UNIT)) << 32);
int ne = (e + E_SEQ) & E_MASK;
if (w.eventCount == (e | INT_SIGN) &&
U.compareAndSwapLong(this, CTL, c, nc)) {
w.eventCount = ne;
if ((p = w.parker) != null)
U.unpark(p);
break;
}
if (q != null && q.base >= q.top)
break;
}
}
// Scanning for tasks
/**
* Top-level runloop for workers, called by ForkJoinWorkerThread.run.
*/
final void runWorker(WorkQueue w) {
w.growArray(); // allocate queue
for (int r = w.hint; scan(w, r) == 0; ) {
r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift
}
}
/**
* Scans for and, if found, runs one task, else possibly
* inactivates the worker. This method operates on single reads of
* volatile state and is designed to be re-invoked continuously,
* in part because it returns upon detecting inconsistencies,
* contention, or state changes that indicate possible success on
* re-invocation.
*
* The scan searches for tasks across queues starting at a random
* index, checking each at least twice. The scan terminates upon
* either finding a non-empty queue, or completing the sweep. If
* the worker is not inactivated, it takes and runs a task from
* this queue. Otherwise, if not activated, it tries to activate
* itself or some other worker by signalling. On failure to find a
* task, returns (for retry) if pool state may have changed during
* an empty scan, or tries to inactivate if active, else possibly
* blocks or terminates via method awaitWork.
*
* @param w the worker (via its WorkQueue)
* @param r a random seed
* @return worker qlock status if would have waited, else 0
*/
private final int scan(WorkQueue w, int r) {
WorkQueue[] ws; int m;
long c = ctl; // for consistency check
if ((ws = workQueues) != null && (m = ws.length - 1) >= 0 && w != null) {
for (int j = m + m + 1, ec = w.eventCount;;) {
WorkQueue q; int b, e; ForkJoinTask[] a; ForkJoinTask t;
if ((q = ws[(r - j) & m]) != null &&
(b = q.base) - q.top < 0 && (a = q.array) != null) {
long i = (((a.length - 1) & b) << ASHIFT) + ABASE;
if ((t = ((ForkJoinTask)
U.getObjectVolatile(a, i))) != null) {
if (ec < 0)
helpRelease(c, ws, w, q, b);
else if (q.base == b &&
U.compareAndSwapObject(a, i, t, null)) {
U.putOrderedInt(q, QBASE, b + 1);
if ((b + 1) - q.top < 0)
signalWork(ws, q);
w.runTask(t);
}
}
break;
}
else if (--j < 0) {
if ((ec | (e = (int)c)) < 0) // inactive or terminating
return awaitWork(w, c, ec);
else if (ctl == c) { // try to inactivate and enqueue
long nc = (long)ec | ((c - AC_UNIT) & (AC_MASK|TC_MASK));
w.nextWait = e;
w.eventCount = ec | INT_SIGN;
if (!U.compareAndSwapLong(this, CTL, c, nc))
w.eventCount = ec; // back out
}
break;
}
}
}
return 0;
}
/**
* A continuation of scan(), possibly blocking or terminating
* worker w. Returns without blocking if pool state has apparently
* changed since last invocation. Also, if inactivating w has
* caused the pool to become quiescent, checks for pool
* termination, and, so long as this is not the only worker, waits
* for event for up to a given duration. On timeout, if ctl has
* not changed, terminates the worker, which will in turn wake up
* another worker to possibly repeat this process.
*
* @param w the calling worker
* @param c the ctl value on entry to scan
* @param ec the worker's eventCount on entry to scan
*/
private final int awaitWork(WorkQueue w, long c, int ec) {
int stat, ns; long parkTime, deadline;
if ((stat = w.qlock) >= 0 && w.eventCount == ec && ctl == c &&
!Thread.interrupted()) {
int e = (int)c;
int u = (int)(c >>> 32);
int d = (u >> UAC_SHIFT) + parallelism; // active count
if (e < 0 || (d <= 0 && tryTerminate(false, false)))
stat = w.qlock = -1; // pool is terminating
else if ((ns = w.nsteals) != 0) { // collect steals and retry
w.nsteals = 0;
U.getAndAddLong(this, STEALCOUNT, (long)ns);
}
else {
long pc = ((d > 0 || ec != (e | INT_SIGN)) ? 0L :
((long)(w.nextWait & E_MASK)) | // ctl to restore
((long)(u + UAC_UNIT)) << 32);
if (pc != 0L) { // timed wait if last waiter
int dc = -(short)(c >>> TC_SHIFT);
parkTime = (dc < 0 ? FAST_IDLE_TIMEOUT:
(dc + 1) * IDLE_TIMEOUT);
deadline = System.nanoTime() + parkTime - TIMEOUT_SLOP;
}
else
parkTime = deadline = 0L;
if (w.eventCount == ec && ctl == c) {
Thread wt = Thread.currentThread();
U.putObject(wt, PARKBLOCKER, this);
w.parker = wt; // emulate LockSupport.park
if (w.eventCount == ec && ctl == c)
U.park(false, parkTime); // must recheck before park
w.parker = null;
U.putObject(wt, PARKBLOCKER, null);
if (parkTime != 0L && ctl == c &&
deadline - System.nanoTime() <= 0L &&
U.compareAndSwapLong(this, CTL, c, pc))
stat = w.qlock = -1; // shrink pool
}
}
}
return stat;
}
/**
* Possibly releases (signals) a worker. Called only from scan()
* when a worker with apparently inactive status finds a non-empty
* queue. This requires revalidating all of the associated state
* from caller.
*/
private final void helpRelease(long c, WorkQueue[] ws, WorkQueue w,
WorkQueue q, int b) {
WorkQueue v; int e, i; Thread p;
if (w != null && w.eventCount < 0 && (e = (int)c) > 0 &&
ws != null && ws.length > (i = e & SMASK) &&
(v = ws[i]) != null && ctl == c) {
long nc = (((long)(v.nextWait & E_MASK)) |
((long)((int)(c >>> 32) + UAC_UNIT)) << 32);
int ne = (e + E_SEQ) & E_MASK;
if (q != null && q.base == b && w.eventCount < 0 &&
v.eventCount == (e | INT_SIGN) &&
U.compareAndSwapLong(this, CTL, c, nc)) {
v.eventCount = ne;
if ((p = v.parker) != null)
U.unpark(p);
}
}
}
/**
* 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. The
* implementation is very branchy to cope with potential
* inconsistencies or loops encountering chains that are stale,
* unknown, or so long that they are likely cyclic.
*
* @param joiner the joining worker
* @param task the task to join
* @return 0 if no progress can be made, negative if task
* known complete, else positive
*/
private int tryHelpStealer(WorkQueue joiner, ForkJoinTask task) {
int stat = 0, steps = 0; // bound to avoid cycles
if (task != null && joiner != null &&
joiner.base - joiner.top >= 0) { // hoist checks
restart: for (;;) {
ForkJoinTask subtask = task; // current target
for (WorkQueue j = joiner, v;;) { // v is stealer of subtask
WorkQueue[] ws; int m, s, h;
if ((s = task.status) < 0) {
stat = s;
break restart;
}
if ((ws = workQueues) == null || (m = ws.length - 1) <= 0)
break restart; // shutting down
if ((v = ws[h = (j.hint | 1) & m]) == null ||
v.currentSteal != subtask) {
for (int origin = h;;) { // find stealer
if (((h = (h + 2) & m) & 15) == 1 &&
(subtask.status < 0 || j.currentJoin != subtask))
continue restart; // occasional staleness check
if ((v = ws[h]) != null &&
v.currentSteal == subtask) {
j.hint = h; // save hint
break;
}
if (h == origin)
break restart; // cannot find stealer
}
}
for (;;) { // help stealer or descend to its stealer
ForkJoinTask[] a; int b;
if (subtask.status < 0) // surround probes with
continue restart; // consistency checks
if ((b = v.base) - v.top < 0 && (a = v.array) != null) {
int i = (((a.length - 1) & b) << ASHIFT) + ABASE;
ForkJoinTask t =
(ForkJoinTask)U.getObjectVolatile(a, i);
if (subtask.status < 0 || j.currentJoin != subtask ||
v.currentSteal != subtask)
continue restart; // stale
stat = 1; // apparent progress
if (v.base == b) {
if (t == null)
break restart;
if (U.compareAndSwapObject(a, i, t, null)) {
U.putOrderedInt(v, QBASE, b + 1);
ForkJoinTask ps = joiner.currentSteal;
int jt = joiner.top;
do {
joiner.currentSteal = t;
t.doExec(); // clear local tasks too
} while (task.status >= 0 &&
joiner.top != jt &&
(t = joiner.pop()) != null);
joiner.currentSteal = ps;
break restart;
}
}
}
else { // empty -- try to descend
ForkJoinTask next = v.currentJoin;
if (subtask.status < 0 || j.currentJoin != subtask ||
v.currentSteal != subtask)
continue restart; // stale
else if (next == null || ++steps == MAX_HELP)
break restart; // dead-end or maybe cyclic
else {
subtask = next;
j = v;
break;
}
}
}
}
}
}
return stat;
}
/**
* Analog of tryHelpStealer for CountedCompleters. Tries to steal
* and run tasks within the target's computation.
*
* @param task the task to join
* @param maxTasks the maximum number of other tasks to run
*/
final int helpComplete(WorkQueue joiner, CountedCompleter task,
int maxTasks) {
WorkQueue[] ws; int m;
int s = 0;
if ((ws = workQueues) != null && (m = ws.length - 1) >= 0 &&
joiner != null && task != null) {
int j = joiner.poolIndex;
int scans = m + m + 1;
long c = 0L; // for stability check
for (int k = scans; ; j += 2) {
WorkQueue q;
if ((s = task.status) < 0)
break;
else if (joiner.internalPopAndExecCC(task)) {
if (--maxTasks <= 0) {
s = task.status;
break;
}
k = scans;
}
else if ((s = task.status) < 0)
break;
else if ((q = ws[j & m]) != null && q.pollAndExecCC(task)) {
if (--maxTasks <= 0) {
s = task.status;
break;
}
k = scans;
}
else if (--k < 0) {
if (c == (c = ctl))
break;
k = scans;
}
}
}
return s;
}
/**
* Tries to decrement active count (sometimes implicitly) and
* possibly release or create a compensating worker in preparation
* for blocking. Fails on contention or termination. Otherwise,
* adds a new thread if no idle workers are available and pool
* may become starved.
*
* @param c the assumed ctl value
*/
final boolean tryCompensate(long c) {
WorkQueue[] ws = workQueues;
int pc = parallelism, e = (int)c, m, tc;
if (ws != null && (m = ws.length - 1) >= 0 && e >= 0 && ctl == c) {
WorkQueue w = ws[e & m];
if (e != 0 && w != null) {
Thread p;
long nc = ((long)(w.nextWait & E_MASK) |
(c & (AC_MASK|TC_MASK)));
int ne = (e + E_SEQ) & E_MASK;
if (w.eventCount == (e | INT_SIGN) &&
U.compareAndSwapLong(this, CTL, c, nc)) {
w.eventCount = ne;
if ((p = w.parker) != null)
U.unpark(p);
return true; // replace with idle worker
}
}
else if ((tc = (short)(c >>> TC_SHIFT)) >= 0 &&
(int)(c >> AC_SHIFT) + pc > 1) {
long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK);
if (U.compareAndSwapLong(this, CTL, c, nc))
return true; // no compensation
}
else if (tc + pc < MAX_CAP) {
long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK);
if (U.compareAndSwapLong(this, CTL, c, nc)) {
ForkJoinWorkerThreadFactory fac;
Throwable ex = null;
ForkJoinWorkerThread wt = null;
try {
if ((fac = factory) != null &&
(wt = fac.newThread(this)) != null) {
wt.start();
return true;
}
} catch (Throwable rex) {
ex = rex;
}
deregisterWorker(wt, ex); // clean up and return false
}
}
}
return false;
}
/**
* Helps and/or blocks until the given task is done.
*
* @param joiner the joining worker
* @param task the task
* @return task status on exit
*/
final int awaitJoin(WorkQueue joiner, ForkJoinTask task) {
int s = 0;
if (task != null && (s = task.status) >= 0 && joiner != null) {
ForkJoinTask prevJoin = joiner.currentJoin;
joiner.currentJoin = task;
do {} while (joiner.tryRemoveAndExec(task) && // process local tasks
(s = task.status) >= 0);
if (s >= 0 && (task instanceof CountedCompleter))
s = helpComplete(joiner, (CountedCompleter)task, Integer.MAX_VALUE);
long cc = 0; // for stability checks
while (s >= 0 && (s = task.status) >= 0) {
if ((s = tryHelpStealer(joiner, task)) == 0 &&
(s = task.status) >= 0) {
if (!tryCompensate(cc))
cc = ctl;
else {
if (task.trySetSignal() && (s = task.status) >= 0) {
synchronized (task) {
if (task.status >= 0) {
try { // see ForkJoinTask
task.wait(); // for explanation
} catch (InterruptedException ie) {
}
}
else
task.notifyAll();
}
}
long c; // reactivate
do {} while (!U.compareAndSwapLong
(this, CTL, c = ctl,
((c & ~AC_MASK) |
((c & AC_MASK) + AC_UNIT))));
}
}
}
joiner.currentJoin = prevJoin;
}
return s;
}
/**
* Stripped-down variant of awaitJoin used by timed joins. Tries
* to help join only while there is continuous progress. (Caller
* will then enter a timed wait.)
*
* @param joiner the joining worker
* @param task the task
*/
final void helpJoinOnce(WorkQueue joiner, ForkJoinTask task) {
int s;
if (joiner != null && task != null && (s = task.status) >= 0) {
ForkJoinTask prevJoin = joiner.currentJoin;
joiner.currentJoin = task;
do {} while (joiner.tryRemoveAndExec(task) && // process local tasks
(s = task.status) >= 0);
if (s >= 0) {
if (task instanceof CountedCompleter)
helpComplete(joiner, (CountedCompleter)task, Integer.MAX_VALUE);
do {} while (task.status >= 0 &&
tryHelpStealer(joiner, task) > 0);
}
joiner.currentJoin = prevJoin;
}
}
/**
* 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() {
int r = ThreadLocalRandom.nextSecondarySeed();
for (;;) {
int ps = plock, m; WorkQueue[] ws; WorkQueue q;
if ((ws = workQueues) != null && (m = ws.length - 1) >= 0) {
for (int j = (m + 1) << 2; j >= 0; --j) {
if ((q = ws[(((r - j) << 1) | 1) & m]) != null &&
q.base - q.top < 0)
return q;
}
}
if (plock == ps)
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;
for (boolean active = true;;) {
long c; WorkQueue q; ForkJoinTask t; int b;
while ((t = w.nextLocalTask()) != null)
t.doExec();
if ((q = findNonEmptyStealQueue()) != null) {
if (!active) { // re-establish active count
active = true;
do {} while (!U.compareAndSwapLong
(this, CTL, c = ctl,
((c & ~AC_MASK) |
((c & AC_MASK) + AC_UNIT))));
}
if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null) {
(w.currentSteal = t).doExec();
w.currentSteal = ps;
}
}
else if (active) { // decrement active count without queuing
long nc = ((c = ctl) & ~AC_MASK) | ((c & AC_MASK) - AC_UNIT);
if ((int)(nc >> AC_SHIFT) + parallelism == 0)
break; // bypass decrement-then-increment
if (U.compareAndSwapLong(this, CTL, c, nc))
active = false;
}
else if ((int)((c = ctl) >> AC_SHIFT) + parallelism <= 0 &&
U.compareAndSwapLong
(this, CTL, c, ((c & ~AC_MASK) |
((c & AC_MASK) + 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; int b;
if ((t = w.nextLocalTask()) != null)
return t;
if ((q = findNonEmptyStealQueue()) == null)
return null;
if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null)
return t;
}
}
/**
* Returns a cheap heuristic guide for task partitioning when
* programmers, frameworks, tools, or languages have little or no
* idea about task granularity. In essence by offering this
* method, we ask users only about tradeoffs in overhead vs
* expected throughput and its variance, rather than how finely to
* partition tasks.
*
* In a steady state strict (tree-structured) computation, each
* thread makes available for stealing enough tasks for other
* threads to remain active. Inductively, if all threads play by
* the same rules, each thread should make available only a
* constant number of tasks.
*
* The minimum useful constant is just 1. But using a value of 1
* would require immediate replenishment upon each steal to
* maintain enough tasks, which is infeasible. Further,
* partitionings/granularities of offered tasks should minimize
* steal rates, which in general means that threads nearer the top
* of computation tree should generate more than those nearer the
* bottom. In perfect steady state, each thread is at
* approximately the same level of computation tree. However,
* producing extra tasks amortizes the uncertainty of progress and
* diffusion assumptions.
*
* So, users will want to use values larger (but not much larger)
* than 1 to both smooth over transient shortages and hedge
* against uneven progress; as traded off against the cost of
* extra task overhead. We leave the user to pick a threshold
* value to compare with the results of this call to guide
* decisions, but recommend values such as 3.
*
* When all threads are active, it is on average OK to estimate
* surplus strictly locally. In steady-state, if one thread is
* maintaining say 2 surplus tasks, then so are others. So we can
* just use estimated queue length. However, this strategy alone
* leads to serious mis-estimates in some non-steady-state
* conditions (ramp-up, ramp-down, other stalls). We can detect
* many of these by further considering the number of "idle"
* threads, that are known to have zero queued tasks, so
* compensate by a factor of (#idle/#active) threads.
*
* Note: The approximation of #busy workers as #active workers is
* not very good under current signalling scheme, and should be
* improved.
*/
static int getSurplusQueuedTaskCount() {
Thread t; ForkJoinWorkerThread wt; ForkJoinPool pool; WorkQueue q;
if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)) {
int p = (pool = (wt = (ForkJoinWorkerThread)t).pool).parallelism;
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. The caller
* triggering termination runs three passes through workQueues:
* (0) Setting termination status, followed by wakeups of queued
* workers; (1) cancelling all tasks; (2) interrupting lagging
* threads (likely in external tasks, but possibly also blocked in
* joins). Each pass repeats previous steps because of potential
* lagging thread creation.
*
* @param now if true, unconditionally terminate, else only
* if no work and no active workers
* @param enable if true, enable shutdown when next possible
* @return true if now terminating or terminated
*/
private boolean tryTerminate(boolean now, boolean enable) {
int ps;
if (this == common) // cannot shut down
return false;
if ((ps = plock) >= 0) { // enable by setting plock
if (!enable)
return false;
if ((ps & PL_LOCK) != 0 ||
!U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
ps = acquirePlock();
int nps = ((ps + PL_LOCK) & ~SHUTDOWN) | SHUTDOWN;
if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
releasePlock(nps);
}
for (long c;;) {
if (((c = ctl) & STOP_BIT) != 0) { // already terminating
if ((short)(c >>> TC_SHIFT) + parallelism <= 0) {
synchronized (this) {
notifyAll(); // signal when 0 workers
}
}
return true;
}
if (!now) { // check if idle & no tasks
WorkQueue[] ws; WorkQueue w;
if ((int)(c >> AC_SHIFT) + parallelism > 0)
return false;
if ((ws = workQueues) != null) {
for (int i = 0; i < ws.length; ++i) {
if ((w = ws[i]) != null &&
(!w.isEmpty() ||
((i & 1) != 0 && w.eventCount >= 0))) {
signalWork(ws, w);
return false;
}
}
}
}
if (U.compareAndSwapLong(this, CTL, c, c | STOP_BIT)) {
for (int pass = 0; pass < 3; ++pass) {
WorkQueue[] ws; WorkQueue w; Thread wt;
if ((ws = workQueues) != null) {
int n = ws.length;
for (int i = 0; i < n; ++i) {
if ((w = ws[i]) != null) {
w.qlock = -1;
if (pass > 0) {
w.cancelAll();
if (pass > 1 && (wt = w.owner) != null) {
if (!wt.isInterrupted()) {
try {
wt.interrupt();
} catch (Throwable ignore) {
}
}
U.unpark(wt);
}
}
}
}
// Wake up workers parked on event queue
int i, e; long cc; Thread p;
while ((e = (int)(cc = ctl) & E_MASK) != 0 &&
(i = e & SMASK) < n && i >= 0 &&
(w = ws[i]) != null) {
long nc = ((long)(w.nextWait & E_MASK) |
((cc + AC_UNIT) & AC_MASK) |
(cc & (TC_MASK|STOP_BIT)));
if (w.eventCount == (e | INT_SIGN) &&
U.compareAndSwapLong(this, CTL, cc, nc)) {
w.eventCount = (e + E_SEQ) & E_MASK;
w.qlock = -1;
if ((p = w.parker) != null)
U.unpark(p);
}
}
}
}
}
}
}
// external operations on common pool
/**
* Returns common pool queue for a thread that has submitted at
* least one task.
*/
static WorkQueue commonSubmitterQueue() {
ForkJoinPool p; WorkQueue[] ws; int m, z;
return ((z = ThreadLocalRandom.getProbe()) != 0 &&
(p = common) != null &&
(ws = p.workQueues) != null &&
(m = ws.length - 1) >= 0) ?
ws[m & z & SQMASK] : null;
}
/**
* Tries to pop the given task from submitter's queue in common pool.
*/
final boolean tryExternalUnpush(ForkJoinTask task) {
WorkQueue joiner; ForkJoinTask[] a; int m, s;
WorkQueue[] ws = workQueues;
int z = ThreadLocalRandom.getProbe();
boolean popped = false;
if (ws != null && (m = ws.length - 1) >= 0 &&
(joiner = ws[z & m & SQMASK]) != null &&
joiner.base != (s = joiner.top) &&
(a = joiner.array) != null) {
long j = (((a.length - 1) & (s - 1)) << ASHIFT) + ABASE;
if (U.getObject(a, j) == task &&
U.compareAndSwapInt(joiner, QLOCK, 0, 1)) {
if (joiner.top == s && joiner.array == a &&
U.compareAndSwapObject(a, j, task, null)) {
joiner.top = s - 1;
popped = true;
}
joiner.qlock = 0;
}
}
return popped;
}
final int externalHelpComplete(CountedCompleter task, int maxTasks) {
WorkQueue joiner; int m;
WorkQueue[] ws = workQueues;
int j = ThreadLocalRandom.getProbe();
int s = 0;
if (ws != null && (m = ws.length - 1) >= 0 &&
(joiner = ws[j & m & SQMASK]) != null && task != null) {
int scans = m + m + 1;
long c = 0L; // for stability check
j |= 1; // poll odd queues
for (int k = scans; ; j += 2) {
WorkQueue q;
if ((s = task.status) < 0)
break;
else if (joiner.externalPopAndExecCC(task)) {
if (--maxTasks <= 0) {
s = task.status;
break;
}
k = scans;
}
else if ((s = task.status) < 0)
break;
else if ((q = ws[j & m]) != null && q.pollAndExecCC(task)) {
if (--maxTasks <= 0) {
s = task.status;
break;
}
k = scans;
}
else if (--k < 0) {
if (c == (c = ctl))
break;
k = scans;
}
}
}
return s;
}
// 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.mode = (short)mode;
this.parallelism = (short)parallelism;
long np = (long)(-parallelism); // offset ctl counts
this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
}
/**
* Returns the common pool instance. This pool is statically
* constructed; its run state is unaffected by attempts to {@link
* #shutdown} or {@link #shutdownNow}. However this pool and any
* ongoing processing are automatically terminated upon program
* {@link System#exit}. Any program that relies on asynchronous
* task processing to complete before program termination should
* invoke {@code commonPool().}{@link #awaitQuiescence awaitQuiescence},
* before exit.
*
* @return the common pool instance
* @since 1.8
*/
public static ForkJoinPool commonPool() {
// assert common != null : "static init error";
return common;
}
// Execution methods
/**
* Performs the given task, returning its result upon completion.
* If the computation encounters an unchecked Exception or Error,
* it is rethrown as the outcome of this invocation. Rethrown
* exceptions behave in the same way as regular exceptions, but,
* when possible, contain stack traces (as displayed for example
* using {@code ex.printStackTrace()}) of both the current thread
* as well as the thread actually encountering the exception;
* minimally only the latter.
*
* @param task the task
* @param the type of the task's result
* @return the task's result
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public T invoke(ForkJoinTask task) {
if (task == null)
throw new NullPointerException();
externalPush(task);
return task.join();
}
/**
* Arranges for (asynchronous) execution of the given task.
*
* @param task the task
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public void execute(ForkJoinTask task) {
if (task == null)
throw new NullPointerException();
externalPush(task);
}
// AbstractExecutorService methods
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public void execute(Runnable task) {
if (task == null)
throw new NullPointerException();
ForkJoinTask job;
if (task instanceof ForkJoinTask) // avoid re-wrap
job = (ForkJoinTask) task;
else
job = new ForkJoinTask.RunnableExecuteAction(task);
externalPush(job);
}
/**
* Submits a ForkJoinTask for execution.
*
* @param task the task to submit
* @param the type of the task's result
* @return the task
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public ForkJoinTask submit(ForkJoinTask task) {
if (task == null)
throw new NullPointerException();
externalPush(task);
return task;
}
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public ForkJoinTask submit(Callable task) {
ForkJoinTask job = new ForkJoinTask.AdaptedCallable(task);
externalPush(job);
return job;
}
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public ForkJoinTask submit(Runnable task, T result) {
ForkJoinTask job = new ForkJoinTask.AdaptedRunnable(task, result);
externalPush(job);
return job;
}
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public ForkJoinTask submit(Runnable task) {
if (task == null)
throw new NullPointerException();
ForkJoinTask job;
if (task instanceof ForkJoinTask) // avoid re-wrap
job = (ForkJoinTask) task;
else
job = new ForkJoinTask.AdaptedRunnableAction(task);
externalPush(job);
return job;
}
/**
* @throws NullPointerException {@inheritDoc}
* @throws RejectedExecutionException {@inheritDoc}
*/
public List> invokeAll(Collection> tasks) {
// In previous versions of this class, this method constructed
// a task to run ForkJoinTask.invokeAll, but now external
// invocation of multiple tasks is at least as efficient.
ArrayList> futures = new ArrayList>(tasks.size());
boolean done = false;
try {
for (Callable t : tasks) {
ForkJoinTask f = new ForkJoinTask.AdaptedCallable(t);
futures.add(f);
externalPush(f);
}
for (int i = 0, size = futures.size(); i < size; i++)
((ForkJoinTask)futures.get(i)).quietlyJoin();
done = true;
return futures;
} finally {
if (!done)
for (int i = 0, size = futures.size(); i < size; i++)
futures.get(i).cancel(false);
}
}
/**
* Returns the factory used for constructing new workers.
*
* @return the factory used for constructing new workers
*/
public ForkJoinWorkerThreadFactory getFactory() {
return factory;
}
/**
* Returns the handler for internal worker threads that terminate
* due to unrecoverable errors encountered while executing tasks.
*
* @return the handler, or {@code null} if none
*/
public UncaughtExceptionHandler getUncaughtExceptionHandler() {
return ueh;
}
/**
* Returns the targeted parallelism level of this pool.
*
* @return the targeted parallelism level of this pool
*/
public int getParallelism() {
int par;
return ((par = parallelism) > 0) ? par : 1;
}
/**
* Returns the targeted parallelism level of the common pool.
*
* @return the targeted parallelism level of the common pool
* @since 1.8
*/
public static int getCommonPoolParallelism() {
return commonParallelism;
}
/**
* Returns the number of worker threads that have started but not
* yet terminated. The result returned by this method may differ
* from {@link #getParallelism} when threads are created to
* maintain parallelism when others are cooperatively blocked.
*
* @return the number of worker threads
*/
public int getPoolSize() {
return parallelism + (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 mode == FIFO_QUEUE;
}
/**
* 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 = parallelism + (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 parallelism + (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() {
long count = 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; WorkQueue w; ForkJoinTask t;
if ((ws = workQueues) != null) {
for (int i = 0; i < ws.length; i += 2) {
if ((w = ws[i]) != null && (t = w.poll()) != null)
return t;
}
}
return null;
}
/**
* Removes all available unexecuted submitted and forked tasks
* from scheduling queues and adds them to the given collection,
* without altering their execution status. These may include
* artificially generated or wrapped tasks. This method is
* designed to be invoked only when the pool is known to be
* quiescent. Invocations at other times may not remove all
* tasks. A failure encountered while attempting to add elements
* to collection {@code c} may result in elements being in
* neither, either or both collections when the associated
* exception is thrown. The behavior of this operation is
* undefined if the specified collection is modified while the
* operation is in progress.
*
* @param c the collection to transfer elements into
* @return the number of elements transferred
*/
protected int drainTasksTo(Collection> c) {
int count = 0;
WorkQueue[] ws; WorkQueue w; ForkJoinTask t;
if ((ws = workQueues) != null) {
for (int i = 0; i < ws.length; ++i) {
if ((w = ws[i]) != null) {
while ((t = w.poll()) != null) {
c.add(t);
++count;
}
}
}
}
return count;
}
/**
* Returns a string identifying this pool, as well as its state,
* including indications of run state, parallelism level, and
* worker and task counts.
*
* @return a string identifying this pool, as well as its state
*/
public String toString() {
// Use a single pass through workQueues to collect counts
long qt = 0L, qs = 0L; int rc = 0;
long st = 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 = parallelism;
int tc = pc + (short)(c >>> TC_SHIFT);
int ac = pc + (int)(c >> AC_SHIFT);
if (ac < 0) // ignore transient negative
ac = 0;
String level;
if ((c & STOP_BIT) != 0)
level = (tc == 0) ? "Terminated" : "Terminating";
else
level = plock < 0 ? "Shutting down" : "Running";
return super.toString() +
"[" + level +
", parallelism = " + pc +
", size = " + tc +
", active = " + ac +
", running = " + rc +
", steals = " + st +
", tasks = " + qt +
", submissions = " + qs +
"]";
}
/**
* Possibly initiates an orderly shutdown in which previously
* submitted tasks are executed, but no new tasks will be
* accepted. Invocation has no effect on execution state if this
* is the {@link #commonPool()}, and no additional effect if
* already shut down. Tasks that are in the process of being
* submitted concurrently during the course of this method may or
* may not be rejected.
*
* @throws SecurityException if a security manager exists and
* the caller is not permitted to modify threads
* because it does not hold {@link
* java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public void shutdown() {
checkPermission();
tryTerminate(false, true);
}
/**
* Possibly attempts to cancel and/or stop all tasks, and reject
* all subsequently submitted tasks. Invocation has no effect on
* execution state if this is the {@link #commonPool()}, and no
* additional effect if already shut down. Otherwise, tasks that
* are in the process of being submitted or executed concurrently
* during the course of this method may or may not be
* rejected. This method cancels both existing and unexecuted
* tasks, in order to permit termination in the presence of task
* dependencies. So the method always returns an empty list
* (unlike the case for some other Executors).
*
* @return an empty list
* @throws SecurityException if a security manager exists and
* the caller is not permitted to modify threads
* because it does not hold {@link
* java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public List shutdownNow() {
checkPermission();
tryTerminate(true, true);
return Collections.emptyList();
}
/**
* Returns {@code true} if all tasks have completed following shut down.
*
* @return {@code true} if all tasks have completed following shut down
*/
public boolean isTerminated() {
long c = ctl;
return ((c & STOP_BIT) != 0L &&
(short)(c >>> TC_SHIFT) + parallelism <= 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() {
long c = ctl;
return ((c & STOP_BIT) != 0L &&
(short)(c >>> TC_SHIFT) + parallelism > 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 plock < 0;
}
/**
* Blocks until all tasks have completed execution after a
* shutdown request, or the timeout occurs, or the current thread
* is interrupted, whichever happens first. Because the {@link
* #commonPool()} never terminates until program shutdown, when
* applied to the common pool, this method is equivalent to {@link
* #awaitQuiescence(long, TimeUnit)} but always returns {@code false}.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return {@code true} if this executor terminated and
* {@code false} if the timeout elapsed before termination
* @throws InterruptedException if interrupted while waiting
*/
public boolean awaitTermination(long timeout, TimeUnit unit)
throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
if (this == common) {
awaitQuiescence(timeout, unit);
return false;
}
long nanos = unit.toNanos(timeout);
if (isTerminated())
return true;
if (nanos <= 0L)
return false;
long deadline = System.nanoTime() + nanos;
synchronized (this) {
for (;;) {
if (isTerminated())
return true;
if (nanos <= 0L)
return false;
long millis = TimeUnit.NANOSECONDS.toMillis(nanos);
wait(millis > 0L ? millis : 1L);
nanos = deadline - System.nanoTime();
}
}
}
/**
* If called by a ForkJoinTask operating in this pool, equivalent
* in effect to {@link ForkJoinTask#helpQuiesce}. Otherwise,
* waits and/or attempts to assist performing tasks until this
* pool {@link #isQuiescent} or the indicated timeout elapses.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return {@code true} if quiescent; {@code false} if the
* timeout elapsed.
*/
public boolean awaitQuiescence(long timeout, TimeUnit unit) {
long nanos = unit.toNanos(timeout);
ForkJoinWorkerThread wt;
Thread thread = Thread.currentThread();
if ((thread instanceof ForkJoinWorkerThread) &&
(wt = (ForkJoinWorkerThread)thread).pool == this) {
helpQuiescePool(wt.workQueue);
return true;
}
long startTime = System.nanoTime();
WorkQueue[] ws;
int r = 0, m;
boolean found = true;
while (!isQuiescent() && (ws = workQueues) != null &&
(m = ws.length - 1) >= 0) {
if (!found) {
if ((System.nanoTime() - startTime) > nanos)
return false;
Thread.yield(); // cannot block
}
found = false;
for (int j = (m + 1) << 2; j >= 0; --j) {
ForkJoinTask t; WorkQueue q; int b;
if ((q = ws[r++ & m]) != null && (b = q.base) - q.top < 0) {
found = true;
if ((t = q.pollAt(b)) != null)
t.doExec();
break;
}
}
}
return true;
}
/**
* Waits and/or attempts to assist performing tasks indefinitely
* until the {@link #commonPool()} {@link #isQuiescent}.
*/
static void quiesceCommonPool() {
common.awaitQuiescence(Long.MAX_VALUE, TimeUnit.NANOSECONDS);
}
/**
* Interface for extending managed parallelism for tasks running
* in {@link ForkJoinPool}s.
*
* A {@code ManagedBlocker} provides two methods. Method
* {@code isReleasable} must return {@code true} if blocking is
* not necessary. Method {@code block} blocks the current thread
* if necessary (perhaps internally invoking {@code isReleasable}
* before actually blocking). These actions are performed by any
* thread invoking {@link ForkJoinPool#managedBlock(ManagedBlocker)}.
* The unusual methods in this API accommodate synchronizers that
* may, but don't usually, block for long periods. Similarly, they
* allow more efficient internal handling of cases in which
* additional workers may be, but usually are not, needed to
* ensure sufficient parallelism. Toward this end,
* implementations of method {@code isReleasable} must be amenable
* to repeated invocation.
*
*
For example, here is a ManagedBlocker based on a
* ReentrantLock:
*
{@code
* class ManagedLocker implements ManagedBlocker {
* final ReentrantLock lock;
* boolean hasLock = false;
* ManagedLocker(ReentrantLock lock) { this.lock = lock; }
* public boolean block() {
* if (!hasLock)
* lock.lock();
* return true;
* }
* public boolean isReleasable() {
* return hasLock || (hasLock = lock.tryLock());
* }
* }}
*
* Here is a class that possibly blocks waiting for an
* item on a given queue:
*
{@code
* class QueueTaker implements ManagedBlocker {
* final BlockingQueue queue;
* volatile E item = null;
* QueueTaker(BlockingQueue q) { this.queue = q; }
* public boolean block() throws InterruptedException {
* if (item == null)
* item = queue.take();
* return true;
* }
* public boolean isReleasable() {
* return item != null || (item = queue.poll()) != null;
* }
* public E getItem() { // call after pool.managedBlock completes
* return item;
* }
* }}
*/
public static interface ManagedBlocker {
/**
* Possibly blocks the current thread, for example waiting for
* a lock or condition.
*
* @return {@code true} if no additional blocking is necessary
* (i.e., if isReleasable would return true)
* @throws InterruptedException if interrupted while waiting
* (the method is not required to do so, but is allowed to)
*/
boolean block() throws InterruptedException;
/**
* Returns {@code true} if blocking is unnecessary.
* @return {@code true} if blocking is unnecessary
*/
boolean isReleasable();
}
/**
* Blocks in accord with the given blocker. If the current thread
* is a {@link ForkJoinWorkerThread}, this method possibly
* arranges for a spare thread to be activated if necessary to
* ensure sufficient parallelism while the current thread is blocked.
*
* If the caller is not a {@link ForkJoinTask}, this method is
* behaviorally equivalent to
*
{@code
* while (!blocker.isReleasable())
* if (blocker.block())
* return;
* }
*
* If the caller is a {@code ForkJoinTask}, then the pool may
* first be expanded to ensure parallelism, and later adjusted.
*
* @param blocker the blocker
* @throws InterruptedException if blocker.block did so
*/
public static void managedBlock(ManagedBlocker blocker)
throws InterruptedException {
Thread t = Thread.currentThread();
if (t instanceof ForkJoinWorkerThread) {
ForkJoinPool p = ((ForkJoinWorkerThread)t).pool;
while (!blocker.isReleasable()) {
if (p.tryCompensate(p.ctl)) {
try {
do {} while (!blocker.isReleasable() &&
!blocker.block());
} finally {
p.incrementActiveCount();
}
break;
}
}
}
else {
do {} while (!blocker.isReleasable() &&
!blocker.block());
}
}
// AbstractExecutorService overrides. These rely on undocumented
// fact that ForkJoinTask.adapt returns ForkJoinTasks that also
// implement RunnableFuture.
protected RunnableFuture newTaskFor(Runnable runnable, T value) {
return new ForkJoinTask.AdaptedRunnable(runnable, value);
}
protected RunnableFuture newTaskFor(Callable callable) {
return new ForkJoinTask.AdaptedCallable(callable);
}
// Unsafe mechanics
private static final sun.misc.Unsafe U;
private static final long CTL;
private static final long PARKBLOCKER;
private static final int ABASE;
private static final int ASHIFT;
private static final long STEALCOUNT;
private static final long PLOCK;
private static final long INDEXSEED;
private static final long QBASE;
private static final long QLOCK;
static {
// initialize field offsets for CAS etc
try {
U = sun.misc.Unsafe.getUnsafe();
Class k = ForkJoinPool.class;
CTL = U.objectFieldOffset
(k.getDeclaredField("ctl"));
STEALCOUNT = U.objectFieldOffset
(k.getDeclaredField("stealCount"));
PLOCK = U.objectFieldOffset
(k.getDeclaredField("plock"));
INDEXSEED = U.objectFieldOffset
(k.getDeclaredField("indexSeed"));
Class tk = Thread.class;
PARKBLOCKER = U.objectFieldOffset
(tk.getDeclaredField("parkBlocker"));
Class wk = WorkQueue.class;
QBASE = U.objectFieldOffset
(wk.getDeclaredField("base"));
QLOCK = U.objectFieldOffset
(wk.getDeclaredField("qlock"));
Class ak = ForkJoinTask[].class;
ABASE = U.arrayBaseOffset(ak);
int scale = U.arrayIndexScale(ak);
if ((scale & (scale - 1)) != 0)
throw new Error("data type scale not a power of two");
ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
} catch (Exception e) {
throw new Error(e);
}
defaultForkJoinWorkerThreadFactory =
new DefaultForkJoinWorkerThreadFactory();
modifyThreadPermission = new RuntimePermission("modifyThread");
common = java.security.AccessController.doPrivileged
(new java.security.PrivilegedAction() {
public ForkJoinPool run() { return makeCommonPool(); }});
int par = common.parallelism; // report 1 even if threads disabled
commonParallelism = par > 0 ? par : 1;
}
/**
* Creates and returns the common pool, respecting user settings
* specified via system properties.
*/
private static ForkJoinPool makeCommonPool() {
int parallelism = -1;
ForkJoinWorkerThreadFactory factory
= defaultForkJoinWorkerThreadFactory;
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 (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-");
}
}