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pekko/akka-actor/src/main/java/akka/jsr166y/ForkJoinPool.java
Viktor Klang a7e5da6819 Packaging the new FJ pool into Akka
2012-01-27 12:44:40 +01:00

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/*
* 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 akka.jsr166y;
import akka.util.Unsafe;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.Random;
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.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.Condition;
/**
* An {@link ExecutorService} for running {@link ForkJoinTask}s.
* A {@code ForkJoinPool} provides the entry point for submissions
* from non-{@code ForkJoinTask} clients, as well as management and
* monitoring operations.
*
* <p>A {@code ForkJoinPool} differs from other kinds of {@link
* ExecutorService} mainly by virtue of employing
* <em>work-stealing</em>: all threads in the pool attempt to find and
* execute tasks submitted to the pool and/or created by other active
* tasks (eventually blocking waiting for work if none exist). This
* enables efficient processing when most tasks spawn other subtasks
* (as do most {@code ForkJoinTask}s), as well as when many small
* tasks are submitted to the pool from external clients. Especially
* when setting <em>asyncMode</em> to true in constructors, {@code
* ForkJoinPool}s may also be appropriate for use with event-style
* tasks that are never joined.
*
* <p>A {@code ForkJoinPool} is 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 IO or other unmanaged synchronization. The nested
* {@link ManagedBlocker} interface enables extension of the kinds of
* synchronization accommodated.
*
* <p>In addition to execution and lifecycle control methods, this
* class provides status check methods (for example
* {@link #getStealCount}) that are intended to aid in developing,
* tuning, and monitoring fork/join applications. Also, method
* {@link #toString} returns indications of pool state in a
* convenient form for informal monitoring.
*
* <p> As is the case with other ExecutorServices, there are three
* main task execution methods summarized in the following
* table. These are designed to be used primarily by clients not
* already engaged in fork/join computations in the current pool. The
* main forms of these methods accept instances of {@code
* ForkJoinTask}, but overloaded forms also allow mixed execution of
* plain {@code Runnable}- or {@code Callable}- based activities as
* well. However, tasks that are already executing in a pool should
* normally instead use the within-computation forms listed in the
* table unless using async event-style tasks that are not usually
* joined, in which case there is little difference among choice of
* methods.
*
* <table BORDER CELLPADDING=3 CELLSPACING=1>
* <tr>
* <td></td>
* <td ALIGN=CENTER> <b>Call from non-fork/join clients</b></td>
* <td ALIGN=CENTER> <b>Call from within fork/join computations</b></td>
* </tr>
* <tr>
* <td> <b>Arrange async execution</td>
* <td> {@link #execute(ForkJoinTask)}</td>
* <td> {@link ForkJoinTask#fork}</td>
* </tr>
* <tr>
* <td> <b>Await and obtain result</td>
* <td> {@link #invoke(ForkJoinTask)}</td>
* <td> {@link ForkJoinTask#invoke}</td>
* </tr>
* <tr>
* <td> <b>Arrange exec and obtain Future</td>
* <td> {@link #submit(ForkJoinTask)}</td>
* <td> {@link ForkJoinTask#fork} (ForkJoinTasks <em>are</em> Futures)</td>
* </tr>
* </table>
*
* <p><b>Sample Usage.</b> Normally a single {@code ForkJoinPool} is
* used for all parallel task execution in a program or subsystem.
* Otherwise, use would not usually outweigh the construction and
* bookkeeping overhead of creating a large set of threads. For
* example, a common pool could be used for the {@code SortTasks}
* illustrated in {@link RecursiveAction}. Because {@code
* ForkJoinPool} uses threads in {@linkplain java.lang.Thread#isDaemon
* daemon} mode, there is typically no need to explicitly {@link
* #shutdown} such a pool upon program exit.
*
* <pre> {@code
* static final ForkJoinPool mainPool = new ForkJoinPool();
* ...
* public void sort(long[] array) {
* mainPool.invoke(new SortTask(array, 0, array.length));
* }}</pre>
*
* <p><b>Implementation notes</b>: This implementation restricts the
* maximum number of running threads to 32767. Attempts to create
* pools with greater than the maximum number result in
* {@code IllegalArgumentException}.
*
* <p>This implementation rejects submitted tasks (that is, by throwing
* {@link RejectedExecutionException}) only when the pool is shut down
* or internal resources have been exhausted.
*
* @since 1.7
* @author Doug Lea
*/
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://research.sun.com/scalable/pubs/index.html) and
* "Idempotent work stealing" by Michael, Saraswat, and Vechev,
* PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186).
* The main differences ultimately stem from gc requirements that
* we null out taken slots as soon as we can, to maintain as small
* a footprint as possible even in programs generating huge
* numbers of tasks. To accomplish this, we shift the CAS
* arbitrating pop vs poll (steal) from being on the indices
* ("base" and "top") to the slots themselves. 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 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 loosely associate (via hashing)
* submission queues with submitting threads, and randomly scan
* these queues as well when looking for work. In essence,
* submitters act like workers except that they never take tasks,
* and they are multiplexed on to a finite number of shared work
* queues. However, classes are set up so that future extensions
* could allow submitters to optionally help perform tasks as
* well. Pool submissions from internal workers are also allowed,
* but use randomized rather than thread-hashed queue indices to
* avoid imbalance. 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 runState), because
* submitters encountering a busy queue try or create others so
* never block.
*
* 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 "runState" contains 32 bits needed to register and
* deregister WorkQueues, as well as to enable shutdown. It is
* only modified under a lock (normally briefly held, but
* occasionally protecting allocations and resizings) but even
* when locked remains available to check consistency.
*
* Recording WorkQueues. WorkQueues are recorded in the
* "workQueues" array that is created upon pool construction 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. Shared (submission) queues
* are at even indices, worker queues at odd indices. Grouping
* them together in this way simplifies and speeds up task
* scanning. To avoid flailing during start-up, the array is
* presized to hold twice #parallelism workers (which is unlikely
* to need further resizing during execution). But to avoid
* dealing with so many null slots, variable runState includes a
* mask for the nearest power of two that contains all current
* workers. 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 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. During a rescan, the worker might release
* some other queued worker rather than itself, which has the same
* net effect. 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 previously had fewer than two tasks, they
* signal waiting workers (or trigger creation of new ones if
* fewer than the given parallelism level -- see signalWork).
* These primary signals are buttressed by signals during rescans;
* together these cover the signals needed in cases when more
* tasks are pushed but untaken, and improve performance compared
* to having one thread wake up all workers.
*
* 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
* SHRINK_RATE nanosecs. This will slowly propagate, eventually
* terminating all workers after long periods of non-use.
*
* Shutdown and Termination. A call to shutdownNow atomically sets
* a runState bit and then (non-atomically) sets each workers
* runState 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 and
* tryPollForAndExec) 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 workers array to locate stealers, but
* often doesn't because stealers leave hints (that may become
* stale/wrong) of where to locate them. A stealHint 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_DEPTH) and fall back to suspending
* the worker and if necessary replacing it with another.
*
* 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. Currently, in keeping with on-demand
* signalling policy, we compensate only if blocking would leave
* less than one active (non-waiting, non-blocked) worker.
* Additionally, to avoid some false alarms due to GC, lagging
* counters, system activity, etc, compensated blocking for joins
* is only attempted after rechecks stabilize in
* ForkJoinTask.awaitJoin. (Retries are interspersed with
* Thread.yield, for good citizenship.)
*
* 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. All together, these low-level
* implementation choices produce as much as a factor of 4
* performance improvement compared to naive implementations, and
* enable the processing of billions of tasks per second, at the
* expense of some ugliness.
*
* 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 that help some methods perform reasonably even
* when interpreted (not compiled).
*
* The order of declarations in this file is: (1) declarations of
* statics (2) fields (along with constants used when unpacking
* some of them), listed in an order that tends to reduce
* contention among them a bit under most JVMs; (3) nested
* classes; (4) internal control methods; (5) callbacks and other
* support for ForkJoinTask methods; (6) exported methods (plus a
* few little helpers); (7) static block initializing all statics
* in a minimally dependent order.
*/
/**
* 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
* @throws NullPointerException if the pool is null
*/
public ForkJoinWorkerThread newThread(ForkJoinPool pool);
}
/**
* Default ForkJoinWorkerThreadFactory implementation; creates a
* new ForkJoinWorkerThread.
*/
static class DefaultForkJoinWorkerThreadFactory
implements ForkJoinWorkerThreadFactory {
public ForkJoinWorkerThread newThread(ForkJoinPool pool) {
return new ForkJoinWorkerThread(pool);
}
}
/**
* 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;
/**
* 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);
}
/**
* Generator for assigning sequence numbers as pool names.
*/
private static final AtomicInteger poolNumberGenerator;
/**
* 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 >>> 1) 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, when id is
* negative, there is at least one waiting worker, 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
* 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.
*
* Field runState is an int packed with:
* SHUTDOWN: true if shutdown is enabled (1 bit)
* SEQ: a sequence number updated upon (de)registering workers (15 bits)
* MASK: mask (power of 2 - 1) covering all registered poolIndexes (16 bits)
*
* The combination of mask and sequence number enables simple
* consistency checks: Staleness of read-only operations on the
* workers and queues arrays can be checked by comparing runState
* before vs after the reads. The low 16 bits (i.e, anding with
* SMASK) hold (the smallest power of two covering all worker
* indices, minus one. The mask for queues (vs workers) is twice
* this value plus 1.
*/
// 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 MAX_ID = 0x7fff; // max poolIndex
private static final int SMASK = 0xffff; // mask short bits
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;
// runState bits
private static final int SHUTDOWN = 1 << 31;
private static final int RS_SEQ = 1 << 16;
private static final int RS_SEQ_MASK = 0x7fff0000;
// access mode for WorkQueue
static final int LIFO_QUEUE = 0;
static final int FIFO_QUEUE = 1;
static final int SHARED_QUEUE = -1;
/**
* The wakeup interval (in nanoseconds) for a worker waiting for a
* task when the pool is quiescent to instead try to shrink the
* number of workers. The exact value does not matter too
* much. It must be short enough to release resources during
* sustained periods of idleness, but not so short that threads
* are continually re-created.
*/
private static final long SHRINK_RATE =
4L * 1000L * 1000L * 1000L; // 4 seconds
/**
* The timeout value for attempted shrinkage, includes
* some slop to cope with system timer imprecision.
*/
private static final long SHRINK_TIMEOUT = SHRINK_RATE - (SHRINK_RATE / 10);
/**
* The maximum stolen->joining link depth allowed in tryHelpStealer.
* 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_DEPTH = 16;
/*
* Field layout order in this class tends to matter more than one
* would like. Runtime layout order is only loosely related to
* declaration order and may differ across JVMs, but the following
* empirically works OK on current JVMs.
*/
volatile long ctl; // main pool control
final int parallelism; // parallelism level
final int localMode; // per-worker scheduling mode
int nextPoolIndex; // hint used in registerWorker
volatile int runState; // shutdown status, seq, and mask
WorkQueue[] workQueues; // main registry
final ReentrantLock lock; // for registration
final Condition termination; // for awaitTermination
final ForkJoinWorkerThreadFactory factory; // factory for new workers
final Thread.UncaughtExceptionHandler ueh; // per-worker UEH
final AtomicLong stealCount; // collect counts when terminated
final AtomicInteger nextWorkerNumber; // to create worker name string
final String workerNamePrefix; // Prefix for assigning worker names
/**
* 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 trySharedPush, 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 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. (Although we can avoid one case
* of this when locked in trySharedPush.) 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). Unfortunately, because they are recorded
* in a common array, WorkQueue instances are often moved to be
* adjacent by garbage collectors. To reduce impact, we use field
* padding that works OK on common platforms; this effectively
* trades off slightly slower average field access for the sake of
* avoiding really bad worst-case access. (Until better JVM
* support is in place, this padding is dependent on transient
* properties of JVM field layout rules.) We also take care in
* allocating and sizing and resizing the array. Non-shared queue
* arrays are initialized (via method growArray) by workers before
* use. Others are allocated on first use.
*/
static final class WorkQueue {
/**
* Capacity of work-stealing queue array upon initialization.
* Must be a power of two; at least 4, but set larger to
* reduce cacheline sharing among queues.
*/
static final int INITIAL_QUEUE_CAPACITY = 1 << 8;
/**
* 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 long totalSteals; // cumulative number of steals
int seed; // for random scanning; initialize nonzero
volatile int eventCount; // encoded inactivation count; < 0 if inactive
int nextWait; // encoded record of next event waiter
int rescans; // remaining scans until block
int nsteals; // top-level task executions since last idle
final int mode; // lifo, fifo, or shared
int poolIndex; // index of this queue in pool (or 0)
int stealHint; // index of most recent known stealer
volatile int runState; // 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 ForkJoinWorkerThread owner; // owning thread or null if shared
volatile Thread parker; // == owner during call to park; else null
ForkJoinTask<?> currentJoin; // task being joined in awaitJoin
ForkJoinTask<?> currentSteal; // current non-local task being executed
// Heuristic padding to ameliorate unfortunate memory placements
Object p00, p01, p02, p03, p04, p05, p06, p07, p08, p09, p0a;
WorkQueue(ForkJoinWorkerThread owner, int mode) {
this.owner = owner;
this.mode = mode;
// Place indices in the center of array (that is not yet allocated)
base = top = INITIAL_QUEUE_CAPACITY >>> 1;
}
/**
* Returns 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;
}
/**
* Pushes a task. Call only by owner in unshared queues.
*
* @param task the task. Caller must ensure non-null.
* @param p, if non-null, pool to signal if necessary
* @throw RejectedExecutionException if array cannot
* be resized
*/
final void push(ForkJoinTask<?> task, ForkJoinPool p) {
ForkJoinTask<?>[] a;
int s = top, m, n;
if ((a = array) != null) { // ignore if queue removed
U.putOrderedObject
(a, (((m = a.length - 1) & s) << ASHIFT) + ABASE, task);
if ((n = (top = s + 1) - base) <= 2) {
if (p != null)
p.signalWork();
}
else if (n >= m)
growArray(true);
}
}
/**
* Pushes a task if lock is free and array is either big
* enough or can be resized to be big enough.
*
* @param task the task. Caller must ensure non-null.
* @return true if submitted
*/
final boolean trySharedPush(ForkJoinTask<?> task) {
boolean submitted = false;
if (runState == 0 && U.compareAndSwapInt(this, RUNSTATE, 0, 1)) {
ForkJoinTask<?>[] a = array;
int s = top, n = s - base;
try {
if ((a != null && n < a.length - 1) ||
(a = growArray(false)) != null) { // must presize
int j = (((a.length - 1) & s) << ASHIFT) + ABASE;
U.putObject(a, (long)j, task); // don't need "ordered"
top = s + 1;
submitted = true;
}
} finally {
runState = 0; // unlock
}
}
return submitted;
}
/**
* Takes next task, if one exists, in FIFO order.
*/
final ForkJoinTask<?> poll() {
ForkJoinTask<?>[] a; int b, i;
while ((b = base) - top < 0 && (a = array) != null &&
(i = (a.length - 1) & b) >= 0) {
int j = (i << ASHIFT) + ABASE;
ForkJoinTask<?> t = (ForkJoinTask<?>)U.getObjectVolatile(a, j);
if (t != null && base == b &&
U.compareAndSwapObject(a, j, t, null)) {
base = b + 1;
return t;
}
}
return null;
}
/**
* Takes next task, if one exists, in LIFO order.
* Call only by owner in unshared queues.
*/
final ForkJoinTask<?> pop() {
ForkJoinTask<?> t; int m;
ForkJoinTask<?>[] a = array;
if (a != null && (m = a.length - 1) >= 0) {
for (int s; (s = top - 1) - base >= 0;) {
int j = ((m & s) << ASHIFT) + ABASE;
if ((t = (ForkJoinTask<?>)U.getObjectVolatile(a, j)) == null)
break;
if (U.compareAndSwapObject(a, j, t, null)) {
top = s;
return t;
}
}
}
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);
}
/**
* Returns task at index b if b is current base of queue.
*/
final ForkJoinTask<?> pollAt(int b) {
ForkJoinTask<?>[] a; int i;
ForkJoinTask<?> task = null;
if ((a = array) != null && (i = ((a.length - 1) & b)) >= 0) {
int j = (i << ASHIFT) + ABASE;
ForkJoinTask<?> t = (ForkJoinTask<?>)U.getObjectVolatile(a, j);
if (t != null && base == b &&
U.compareAndSwapObject(a, j, t, null)) {
base = b + 1;
task = t;
}
}
return task;
}
/**
* Pops the given task only if it is at the current top.
*/
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;
}
/**
* Polls the given task only if it is at the current base.
*/
final boolean pollFor(ForkJoinTask<?> task) {
ForkJoinTask<?>[] a; int b, i;
if ((b = base) - top < 0 && (a = array) != null &&
(i = (a.length - 1) & b) >= 0) {
int j = (i << ASHIFT) + ABASE;
if (U.getObjectVolatile(a, j) == task && base == b &&
U.compareAndSwapObject(a, j, task, null)) {
base = b + 1;
return true;
}
}
return false;
}
/**
* If present, removes from queue and executes the given task, or
* any other cancelled task. Returns (true) immediately on any CAS
* or consistency check failure so caller can retry.
*
* @return false if no progress can be made
*/
final boolean tryRemoveAndExec(ForkJoinTask<?> task) {
boolean removed = false, empty = true, progress = true;
ForkJoinTask<?>[] a; int m, s, b, n;
if ((a = array) != null && (m = a.length - 1) >= 0 &&
(n = (s = top) - (b = base)) > 0) {
for (ForkJoinTask<?> t;;) { // traverse from s to b
int j = ((--s & m) << ASHIFT) + ABASE;
t = (ForkJoinTask<?>)U.getObjectVolatile(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)
progress = false;
break;
}
}
}
if (removed)
task.doExec();
return progress;
}
/**
* 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.
*
* @param rejectOnFailure if true, throw exception if capacity
* exceeded (relayed ultimately to user); else return null.
*/
final ForkJoinTask<?>[] growArray(boolean rejectOnFailure) {
ForkJoinTask<?>[] oldA = array;
int size = oldA != null ? oldA.length << 1 : INITIAL_QUEUE_CAPACITY;
if (size <= MAXIMUM_QUEUE_CAPACITY) {
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;
}
else if (!rejectOnFailure)
return null;
else
throw new RejectedExecutionException("Queue capacity exceeded");
}
/**
* 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);
}
// Execution methods
/**
* Removes and runs tasks until empty, using local mode
* ordering.
*/
final void runLocalTasks() {
if (base - top < 0) {
for (ForkJoinTask<?> t; (t = nextLocalTask()) != null; )
t.doExec();
}
}
/**
* Executes a top-level task and any local tasks remaining
* after execution.
*
* @return true unless terminating
*/
final boolean runTask(ForkJoinTask<?> t) {
boolean alive = true;
if (t != null) {
currentSteal = t;
t.doExec();
runLocalTasks();
++nsteals;
currentSteal = null;
}
else if (runState < 0) // terminating
alive = false;
return alive;
}
/**
* Executes a non-top-level (stolen) task
*/
final void runSubtask(ForkJoinTask<?> t) {
if (t != null) {
ForkJoinTask<?> ps = currentSteal;
currentSteal = t;
t.doExec();
currentSteal = ps;
}
}
/**
* Computes next value for random probes. Scans don't require
* a very high quality generator, but also not a crummy one.
* Marsaglia xor-shift is cheap and works well enough. Note:
* This is manually inlined in several usages in ForkJoinPool
* to avoid writes inside busy scan loops.
*/
final int nextSeed() {
int r = seed;
r ^= r << 13;
r ^= r >>> 17;
r ^= r << 5;
return seed = r;
}
// Unsafe mechanics
private static final sun.misc.Unsafe U;
private static final long RUNSTATE;
private static final int ABASE;
private static final int ASHIFT;
static {
int s;
try {
U = getUnsafe();
Class<?> k = WorkQueue.class;
Class<?> ak = ForkJoinTask[].class;
RUNSTATE = U.objectFieldOffset
(k.getDeclaredField("runState"));
ABASE = U.arrayBaseOffset(ak);
s = U.arrayIndexScale(ak);
} catch (Exception e) {
throw new Error(e);
}
if ((s & (s-1)) != 0)
throw new Error("data type scale not a power of two");
ASHIFT = 31 - Integer.numberOfLeadingZeros(s);
}
}
/**
* 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<Void> {
EmptyTask() { status = ForkJoinTask.NORMAL; } // force done
public Void getRawResult() { return null; }
public void setRawResult(Void x) {}
public boolean exec() { return true; }
}
/**
* Computes a hash code for the given thread. This method is
* expected to provide higher-quality hash codes than those using
* method hashCode().
*/
static final int hashThread(Thread t) {
long id = (t == null) ? 0L : t.getId(); // Use MurmurHash of thread id
int h = (int)id ^ (int)(id >>> 32);
h ^= h >>> 16;
h *= 0x85ebca6b;
h ^= h >>> 13;
h *= 0xc2b2ae35;
return h ^ (h >>> 16);
}
/**
* Top-level runloop for workers
*/
final void runWorker(ForkJoinWorkerThread wt) {
WorkQueue w = wt.workQueue;
w.growArray(false); // Initialize queue array and seed in this thread
w.seed = hashThread(Thread.currentThread()) | (1 << 31); // force < 0
do {} while (w.runTask(scan(w)));
}
// Creating, registering and deregistering workers
/**
* Tries to create and start a worker
*/
private void addWorker() {
Throwable ex = null;
ForkJoinWorkerThread w = null;
try {
if ((w = factory.newThread(this)) != null) {
w.start();
return;
}
} catch (Throwable e) {
ex = e;
}
deregisterWorker(w, ex);
}
/**
* Callback from ForkJoinWorkerThread constructor to assign a
* public name. This must be separate from registerWorker because
* it is called during the "super" constructor call in
* ForkJoinWorkerThread.
*/
final String nextWorkerName() {
return workerNamePrefix.concat
(Integer.toString(nextWorkerNumber.addAndGet(1)));
}
/**
* Callback from ForkJoinWorkerThread constructor to establish and
* record its WorkQueue
*
* @param wt the worker thread
*/
final void registerWorker(ForkJoinWorkerThread wt) {
WorkQueue w = wt.workQueue;
ReentrantLock lock = this.lock;
lock.lock();
try {
int k = nextPoolIndex;
WorkQueue[] ws = workQueues;
if (ws != null) { // ignore on shutdown
int n = ws.length;
if (k < 0 || (k & 1) == 0 || k >= n || ws[k] != null) {
for (k = 1; k < n && ws[k] != null; k += 2)
; // workers are at odd indices
if (k >= n) // resize
workQueues = ws = Arrays.copyOf(ws, n << 1);
}
w.poolIndex = k;
w.eventCount = ~(k >>> 1) & SMASK; // Set up wait count
ws[k] = w; // record worker
nextPoolIndex = k + 2;
int rs = runState;
int m = rs & SMASK; // recalculate runState mask
if (k > m)
m = (m << 1) + 1;
runState = (rs & SHUTDOWN) | ((rs + RS_SEQ) & RS_SEQ_MASK) | m;
}
} finally {
lock.unlock();
}
}
/**
* Final callback from terminating worker, as well as failure to
* construct or start a worker in addWorker. 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 addWorker 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) {
w.runState = -1; // ensure runState is set
stealCount.getAndAdd(w.totalSteals + w.nsteals);
int idx = w.poolIndex;
ReentrantLock lock = this.lock;
lock.lock();
try { // remove record from array
WorkQueue[] ws = workQueues;
if (ws != null && idx >= 0 && idx < ws.length && ws[idx] == w)
ws[nextPoolIndex = idx] = null;
} finally {
lock.unlock();
}
}
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) && w != null) {
w.cancelAll(); // cancel remaining tasks
if (w.array != null) // suppress signal if never ran
signalWork(); // wake up or create replacement
}
if (ex != null) // rethrow
U.throwException(ex);
}
// 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_UNIT));
}
/**
* Activates or creates a worker
*/
final void signalWork() {
/*
* The while condition is true if: (there is are too few total
* workers OR there is at least one waiter) AND (there are too
* few active workers OR the pool is terminating). The value
* of e distinguishes the remaining cases: zero (no waiters)
* for create, negative if terminating (in which case do
* nothing), else release a waiter. The secondary checks for
* release (non-null array etc) can fail if the pool begins
* terminating after the test, and don't impose any added cost
* because JVMs must perform null and bounds checks anyway.
*/
long c; int e, u;
while ((((e = (int)(c = ctl)) | (u = (int)(c >>> 32))) &
(INT_SIGN|SHORT_SIGN)) == (INT_SIGN|SHORT_SIGN)) {
WorkQueue[] ws = workQueues; int i; WorkQueue w; Thread p;
if (e == 0) { // add a new worker
if (U.compareAndSwapLong
(this, CTL, c, (long)(((u + UTC_UNIT) & UTC_MASK) |
((u + UAC_UNIT) & UAC_MASK)) << 32)) {
addWorker();
break;
}
}
else if (e > 0 && ws != null &&
(i = ((~e << 1) | 1) & SMASK) < ws.length &&
(w = ws[i]) != null &&
w.eventCount == (e | INT_SIGN)) {
if (U.compareAndSwapLong
(this, CTL, c, (((long)(w.nextWait & E_MASK)) |
((long)(u + UAC_UNIT) << 32)))) {
w.eventCount = (e + E_SEQ) & E_MASK;
if ((p = w.parker) != null)
U.unpark(p); // release a waiting worker
break;
}
}
else
break;
}
}
/**
* Tries to decrement active count (sometimes implicitly) and
* possibly release or create a compensating worker in preparation
* for blocking. Fails on contention or termination.
*
* @return true if the caller can block, else should recheck and retry
*/
final boolean tryCompensate() {
WorkQueue[] ws; WorkQueue w; Thread p;
int pc = parallelism, e, u, ac, tc, i;
long c = ctl;
if ((e = (int)c) >= 0) {
if ((ac = ((u = (int)(c >>> 32)) >> UAC_SHIFT)) <= 0 &&
e != 0 && (ws = workQueues) != null &&
(i = ((~e << 1) | 1) & SMASK) < ws.length &&
(w = ws[i]) != null) {
if (w.eventCount == (e | INT_SIGN) &&
U.compareAndSwapLong
(this, CTL, c, ((long)(w.nextWait & E_MASK) |
(c & (AC_MASK|TC_MASK))))) {
w.eventCount = (e + E_SEQ) & E_MASK;
if ((p = w.parker) != null)
U.unpark(p);
return true; // release an idle worker
}
}
else if ((tc = (short)(u >>> UTC_SHIFT)) >= 0 && ac + pc > 1) {
long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK);
if (U.compareAndSwapLong(this, CTL, c, nc))
return true; // no compensation needed
}
else if (tc + pc < MAX_ID) {
long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK);
if (U.compareAndSwapLong(this, CTL, c, nc)) {
addWorker();
return true; // create replacement
}
}
}
return false;
}
// Submissions
/**
* Unless shutting down, adds the given task to some submission
* queue; using a randomly chosen queue index if the caller is a
* ForkJoinWorkerThread, else one based on caller thread's hash
* code. If no queue exists at the index, one is created. If the
* queue is busy, another is chosen by sweeping through the queues
* array.
*/
private void doSubmit(ForkJoinTask<?> task) {
if (task == null)
throw new NullPointerException();
Thread t = Thread.currentThread();
int r = ((t instanceof ForkJoinWorkerThread) ?
((ForkJoinWorkerThread)t).workQueue.nextSeed() : hashThread(t));
for (;;) {
int rs = runState, m = rs & SMASK;
int j = r &= (m & ~1); // even numbered queues
WorkQueue[] ws = workQueues;
if (rs < 0 || ws == null)
throw new RejectedExecutionException(); // shutting down
if (ws.length > m) { // consistency check
for (WorkQueue q;;) { // circular sweep
if (((q = ws[j]) != null ||
(q = tryAddSharedQueue(j)) != null) &&
q.trySharedPush(task)) {
signalWork();
return;
}
if ((j = (j + 2) & m) == r) {
Thread.yield(); // all queues busy
break;
}
}
}
}
}
/**
* Tries to add and register a new queue at the given index.
*
* @param idx the workQueues array index to register the queue
* @return the queue, or null if could not add because could
* not acquire lock or idx is unusable
*/
private WorkQueue tryAddSharedQueue(int idx) {
WorkQueue q = null;
ReentrantLock lock = this.lock;
if (idx >= 0 && (idx & 1) == 0 && !lock.isLocked()) {
// create queue outside of lock but only if apparently free
WorkQueue nq = new WorkQueue(null, SHARED_QUEUE);
if (lock.tryLock()) {
try {
WorkQueue[] ws = workQueues;
if (ws != null && idx < ws.length) {
if ((q = ws[idx]) == null) {
int rs; // update runState seq
ws[idx] = q = nq;
runState = (((rs = runState) & SHUTDOWN) |
((rs + RS_SEQ) & ~SHUTDOWN));
}
}
} finally {
lock.unlock();
}
}
}
return q;
}
// Scanning for tasks
/**
* Scans for and, if found, returns 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, randomly selecting
* the first #queues probes, favoring steals 2:1 over submissions
* (by exploiting even/odd indexing), and then performing a
* circular sweep of all queues. The scan terminates upon either
* finding a non-empty queue, or completing a full sweep. If the
* worker is not inactivated, it takes and returns a task from
* this queue. On failure to find a task, we take one of the
* following actions, after which the caller will retry calling
* this method unless terminated.
*
* * If not a complete sweep, try to release a waiting worker. If
* the scan terminated because the worker is inactivated, then the
* released worker will often be the calling worker, and it can
* succeed obtaining a task on the next call. Or maybe it is
* another worker, but with same net effect. Releasing in other
* cases as well ensures that we have enough workers running.
*
* * If the caller has run a task since the the last empty scan,
* return (to allow rescan) if other workers are not also yet
* enqueued. Field WorkQueue.rescans counts down on each scan to
* ensure eventual inactivation, and occasional calls to
* Thread.yield to help avoid interference with more useful
* activities on the system.
*
* * If pool is terminating, terminate the worker
*
* * If not already enqueued, try to inactivate and enqueue the
* worker on wait queue.
*
* * If already enqueued and none of the above apply, either park
* awaiting signal, or if this is the most recent waiter and pool
* is quiescent, relay to idleAwaitWork to check for termination
* and possibly shrink pool.
*
* @param w the worker (via its WorkQueue)
* @return a task or null of none found
*/
private final ForkJoinTask<?> scan(WorkQueue w) {
boolean swept = false; // true after full empty scan
WorkQueue[] ws; // volatile read order matters
int r = w.seed, ec = w.eventCount; // ec is negative if inactive
int rs = runState, m = rs & SMASK;
if ((ws = workQueues) != null && ws.length > m) {
ForkJoinTask<?> task = null;
for (int k = 0, j = -2 - m; ; ++j) {
WorkQueue q; int b;
if (j < 0) { // random probes while j negative
r ^= r << 13; r ^= r >>> 17; k = (r ^= r << 5) | (j & 1);
} // worker (not submit) for odd j
else // cyclic scan when j >= 0
k += (m >>> 1) | 1; // step by half to reduce bias
if ((q = ws[k & m]) != null && (b = q.base) - q.top < 0) {
if (ec >= 0)
task = q.pollAt(b); // steal
break;
}
else if (j > m) {
if (rs == runState) // staleness check
swept = true;
break;
}
}
w.seed = r; // save seed for next scan
if (task != null)
return task;
}
// Decode ctl on empty scan
long c = ctl; int e = (int)c, a = (int)(c >> AC_SHIFT), nr, ns;
if (!swept) { // try to release a waiter
WorkQueue v; Thread p;
if (e > 0 && a < 0 && ws != null &&
(v = ws[((~e << 1) | 1) & m]) != null &&
v.eventCount == (e | INT_SIGN) && U.compareAndSwapLong
(this, CTL, c, ((long)(v.nextWait & E_MASK) |
((c + AC_UNIT) & (AC_MASK|TC_MASK))))) {
v.eventCount = (e + E_SEQ) & E_MASK;
if ((p = v.parker) != null)
U.unpark(p);
}
}
else if ((nr = w.rescans) > 0) { // continue rescanning
int ac = a + parallelism;
if ((w.rescans = (ac < nr) ? ac : nr - 1) > 0 && w.seed < 0 &&
w.eventCount == ec)
Thread.yield(); // 1 bit randomness for yield call
}
else if (e < 0) // pool is terminating
w.runState = -1;
else if (ec >= 0) { // try to enqueue
long nc = (long)ec | ((c - AC_UNIT) & (AC_MASK|TC_MASK));
w.nextWait = e;
w.eventCount = ec | INT_SIGN; // mark as inactive
if (!U.compareAndSwapLong(this, CTL, c, nc))
w.eventCount = ec; // back out on CAS failure
else if ((ns = w.nsteals) != 0) { // set rescans if ran task
if (a <= 0) // ... unless too many active
w.rescans = a + parallelism;
w.nsteals = 0;
w.totalSteals += ns;
}
}
else{ // already queued
if (parallelism == -a)
idleAwaitWork(w); // quiescent
if (w.eventCount == ec) {
Thread.interrupted(); // clear status
ForkJoinWorkerThread wt = w.owner;
U.putObject(wt, PARKBLOCKER, this);
w.parker = wt; // emulate LockSupport.park
if (w.eventCount == ec) // recheck
U.park(false, 0L); // block
w.parker = null;
U.putObject(wt, PARKBLOCKER, null);
}
}
return null;
}
/**
* If inactivating worker w has caused pool to become quiescent,
* check for pool termination, and, so long as this is not the
* only worker, wait for event for up to SHRINK_RATE nanosecs On
* timeout, if ctl has not changed, terminate the worker, which
* will in turn wake up another worker to possibly repeat this
* process.
*
* @param w the calling worker
*/
private void idleAwaitWork(WorkQueue w) {
long c; int nw, ec;
if (!tryTerminate(false) &&
(int)((c = ctl) >> AC_SHIFT) + parallelism == 0 &&
(ec = w.eventCount) == ((int)c | INT_SIGN) &&
(nw = w.nextWait) != 0) {
long nc = ((long)(nw & E_MASK) | // ctl to restore on timeout
((c + AC_UNIT) & AC_MASK) | (c & TC_MASK));
ForkJoinTask.helpExpungeStaleExceptions(); // help clean
ForkJoinWorkerThread wt = w.owner;
while (ctl == c) {
long startTime = System.nanoTime();
Thread.interrupted(); // timed variant of version in scan()
U.putObject(wt, PARKBLOCKER, this);
w.parker = wt;
if (ctl == c)
U.park(false, SHRINK_RATE);
w.parker = null;
U.putObject(wt, PARKBLOCKER, null);
if (ctl != c)
break;
if (System.nanoTime() - startTime >= SHRINK_TIMEOUT &&
U.compareAndSwapLong(this, CTL, c, nc)) {
w.runState = -1; // shrink
w.eventCount = (ec + E_SEQ) | E_MASK;
break;
}
}
}
}
/**
* 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 of length greater than MAX_HELP_DEPTH links. All
* of these cases are dealt with by just retrying by caller.
*
* @param joiner the joining worker
* @param task the task to join
* @return true if found or ran a task (and so is immediately retryable)
*/
final boolean tryHelpStealer(WorkQueue joiner, ForkJoinTask<?> task) {
ForkJoinTask<?> subtask; // current target
boolean progress = false;
int depth = 0; // current chain depth
int m = runState & SMASK;
WorkQueue[] ws = workQueues;
if (ws != null && ws.length > m && (subtask = task).status >= 0) {
outer:for (WorkQueue j = joiner;;) {
// Try to find the stealer of subtask, by first using hint
WorkQueue stealer = null;
WorkQueue v = ws[j.stealHint & m];
if (v != null && v.currentSteal == subtask)
stealer = v;
else {
for (int i = 1; i <= m; i += 2) {
if ((v = ws[i]) != null && v.currentSteal == subtask) {
stealer = v;
j.stealHint = i; // save hint
break;
}
}
if (stealer == null)
break;
}
for (WorkQueue q = stealer;;) { // Try to help stealer
ForkJoinTask<?> t; int b;
if (task.status < 0)
break outer;
if ((b = q.base) - q.top < 0) {
progress = true;
if (subtask.status < 0)
break outer; // stale
if ((t = q.pollAt(b)) != null) {
stealer.stealHint = joiner.poolIndex;
joiner.runSubtask(t);
}
}
else { // empty - try to descend to find stealer's stealer
ForkJoinTask<?> next = stealer.currentJoin;
if (++depth == MAX_HELP_DEPTH || subtask.status < 0 ||
next == null || next == subtask)
break outer; // max depth, stale, dead-end, cyclic
subtask = next;
j = stealer;
break;
}
}
}
}
return progress;
}
/**
* If task is at base of some steal queue, steals and executes it.
*
* @param joiner the joining worker
* @param task the task
*/
final void tryPollForAndExec(WorkQueue joiner, ForkJoinTask<?> task) {
WorkQueue[] ws;
int m = runState & SMASK;
if ((ws = workQueues) != null && ws.length > m) {
for (int j = 1; j <= m && task.status >= 0; j += 2) {
WorkQueue q = ws[j];
if (q != null && q.pollFor(task)) {
joiner.runSubtask(task);
break;
}
}
}
}
/**
* Returns a non-empty steal queue, if one is found during a random,
* then cyclic scan, else null. This method must be retried by
* caller if, by the time it tries to use the queue, it is empty.
*/
private WorkQueue findNonEmptyStealQueue(WorkQueue w) {
int r = w.seed; // Same idea as scan(), but ignoring submissions
for (WorkQueue[] ws;;) {
int m = runState & SMASK;
if ((ws = workQueues) == null)
return null;
if (ws.length > m) {
WorkQueue q;
for (int n = m << 2, k = r, j = -n;;) {
r ^= r << 13; r ^= r >>> 17; r ^= r << 5;
if ((q = ws[(k | 1) & m]) != null && q.base - q.top < 0) {
w.seed = r;
return q;
}
else if (j > n)
return null;
else
k = (j++ < 0) ? r : k + ((m >>> 1) | 1);
}
}
}
}
/**
* 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) {
for (boolean active = true;;) {
w.runLocalTasks(); // exhaust local queue
WorkQueue q = findNonEmptyStealQueue(w);
if (q != null) {
ForkJoinTask<?> t;
if (!active) { // re-establish active count
long c;
active = true;
do {} while (!U.compareAndSwapLong
(this, CTL, c = ctl, c + AC_UNIT));
}
if ((t = q.poll()) != null)
w.runSubtask(t);
}
else {
long c;
if (active) { // decrement active count without queuing
active = false;
do {} while (!U.compareAndSwapLong
(this, CTL, c = ctl, c -= AC_UNIT));
}
else
c = ctl; // re-increment on exit
if ((int)(c >> AC_SHIFT) + parallelism == 0) {
do {} while (!U.compareAndSwapLong
(this, CTL, c = ctl, c + AC_UNIT));
break;
}
}
}
}
/**
* Gets and removes a local or stolen task for the given worker
*
* @return a task, if available
*/
final ForkJoinTask<?> nextTaskFor(WorkQueue w) {
for (ForkJoinTask<?> t;;) {
WorkQueue q;
if ((t = w.nextLocalTask()) != null)
return t;
if ((q = findNonEmptyStealQueue(w)) == null)
return null;
if ((t = q.poll()) != null)
return t;
}
}
/**
* Returns the approximate (non-atomic) number of idle threads per
* active thread to offset steal queue size for method
* ForkJoinTask.getSurplusQueuedTaskCount().
*/
final int idlePerActive() {
// Approximate at powers of two for small values, saturate past 4
int p = parallelism;
int a = p + (int)(ctl >> AC_SHIFT);
return (a > (p >>>= 1) ? 0 :
a > (p >>>= 1) ? 1 :
a > (p >>>= 1) ? 2 :
a > (p >>>= 1) ? 4 :
8);
}
// Termination
/**
* Sets SHUTDOWN bit of runState under lock
*/
private void enableShutdown() {
ReentrantLock lock = this.lock;
if (runState >= 0) {
lock.lock(); // don't need try/finally
runState |= SHUTDOWN;
lock.unlock();
}
}
/**
* Possibly initiates and/or completes termination. Upon
* termination, cancels all queued tasks and then
*
* @param now if true, unconditionally terminate, else only
* if no work and no active workers
* @return true if now terminating or terminated
*/
private boolean tryTerminate(boolean now) {
for (long c;;) {
if (((c = ctl) & STOP_BIT) != 0) { // already terminating
if ((short)(c >>> TC_SHIFT) == -parallelism) {
ReentrantLock lock = this.lock; // signal when no workers
lock.lock(); // don't need try/finally
termination.signalAll(); // signal when 0 workers
lock.unlock();
}
return true;
}
if (!now) {
if ((int)(c >> AC_SHIFT) != -parallelism || runState >= 0 ||
hasQueuedSubmissions())
return false;
// Check for unqueued inactive workers. One pass suffices.
WorkQueue[] ws = workQueues; WorkQueue w;
if (ws != null) {
int n = ws.length;
for (int i = 1; i < n; i += 2) {
if ((w = ws[i]) != null && w.eventCount >= 0)
return false;
}
}
}
if (U.compareAndSwapLong(this, CTL, c, c | STOP_BIT))
startTerminating();
}
}
/**
* Initiates 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.
*/
private void startTerminating() {
for (int pass = 0; pass < 3; ++pass) {
WorkQueue[] ws = workQueues;
if (ws != null) {
WorkQueue w; Thread wt;
int n = ws.length;
for (int j = 0; j < n; ++j) {
if ((w = ws[j]) != null) {
w.runState = -1;
if (pass > 0) {
w.cancelAll();
if (pass > 1 && (wt = w.owner) != null &&
!wt.isInterrupted()) {
try {
wt.interrupt();
} catch (SecurityException ignore) {
}
}
}
}
}
// Wake up workers parked on event queue
int i, e; long c; Thread p;
while ((i = ((~(e = (int)(c = ctl)) << 1) | 1) & SMASK) < n &&
(w = ws[i]) != null &&
w.eventCount == (e | INT_SIGN)) {
long nc = ((long)(w.nextWait & E_MASK) |
((c + AC_UNIT) & AC_MASK) |
(c & (TC_MASK|STOP_BIT)));
if (U.compareAndSwapLong(this, CTL, c, nc)) {
w.eventCount = (e + E_SEQ) & E_MASK;
if ((p = w.parker) != null)
U.unpark(p);
}
}
}
}
}
// 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(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,
Thread.UncaughtExceptionHandler handler,
boolean asyncMode) {
checkPermission();
if (factory == null)
throw new NullPointerException();
if (parallelism <= 0 || parallelism > MAX_ID)
throw new IllegalArgumentException();
this.parallelism = parallelism;
this.factory = factory;
this.ueh = handler;
this.localMode = asyncMode ? FIFO_QUEUE : LIFO_QUEUE;
this.nextPoolIndex = 1;
long np = (long)(-parallelism); // offset ctl counts
this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
// initialize workQueues array with room for 2*parallelism if possible
int n = parallelism << 1;
if (n >= MAX_ID)
n = MAX_ID;
else { // See Hackers Delight, sec 3.2, where n < (1 << 16)
n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8;
}
this.workQueues = new WorkQueue[(n + 1) << 1];
ReentrantLock lck = this.lock = new ReentrantLock();
this.termination = lck.newCondition();
this.stealCount = new AtomicLong();
this.nextWorkerNumber = new AtomicInteger();
StringBuilder sb = new StringBuilder("ForkJoinPool-");
sb.append(poolNumberGenerator.incrementAndGet());
sb.append("-worker-");
this.workerNamePrefix = sb.toString();
// Create initial submission queue
WorkQueue sq = tryAddSharedQueue(0);
if (sq != null)
sq.growArray(false);
}
// 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
* @return the task's result
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public <T> T invoke(ForkJoinTask<T> task) {
doSubmit(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) {
doSubmit(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 = ForkJoinTask.adapt(task, null);
doSubmit(job);
}
/**
* Submits a ForkJoinTask for execution.
*
* @param task the task to submit
* @return the task
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
doSubmit(task);
return task;
}
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public <T> ForkJoinTask<T> submit(Callable<T> task) {
if (task == null)
throw new NullPointerException();
ForkJoinTask<T> job = ForkJoinTask.adapt(task);
doSubmit(job);
return job;
}
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
public <T> ForkJoinTask<T> submit(Runnable task, T result) {
if (task == null)
throw new NullPointerException();
ForkJoinTask<T> job = ForkJoinTask.adapt(task, result);
doSubmit(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 = ForkJoinTask.adapt(task, null);
doSubmit(job);
return job;
}
/**
* @throws NullPointerException {@inheritDoc}
* @throws RejectedExecutionException {@inheritDoc}
*/
public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
ArrayList<ForkJoinTask<T>> forkJoinTasks =
new ArrayList<ForkJoinTask<T>>(tasks.size());
for (Callable<T> task : tasks)
forkJoinTasks.add(ForkJoinTask.adapt(task));
invoke(new InvokeAll<T>(forkJoinTasks));
@SuppressWarnings({"unchecked", "rawtypes"})
List<Future<T>> futures = (List<Future<T>>) (List) forkJoinTasks;
return futures;
}
static final class InvokeAll<T> extends RecursiveAction {
final ArrayList<ForkJoinTask<T>> tasks;
InvokeAll(ArrayList<ForkJoinTask<T>> tasks) { this.tasks = tasks; }
public void compute() {
try { invokeAll(tasks); }
catch (Exception ignore) {}
}
private static final long serialVersionUID = -7914297376763021607L;
}
/**
* 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 Thread.UncaughtExceptionHandler getUncaughtExceptionHandler() {
return ueh;
}
/**
* Returns the targeted parallelism level of this pool.
*
* @return the targeted parallelism level of this pool
*/
public int getParallelism() {
return parallelism;
}
/**
* Returns the number of worker threads that have started but not
* yet terminated. The result returned by this method may differ
* from {@link #getParallelism} when threads are created to
* maintain parallelism when others are cooperatively blocked.
*
* @return the number of worker threads
*/
public int getPoolSize() {
return 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 localMode != 0;
}
/**
* Returns an estimate of the number of worker threads that are
* not blocked waiting to join tasks or for other managed
* synchronization. This method may overestimate the
* number of running threads.
*
* @return the number of worker threads
*/
public int getRunningThreadCount() {
int rc = 0;
WorkQueue[] ws; WorkQueue w;
if ((ws = workQueues) != null) {
int n = ws.length;
for (int i = 1; i < n; i += 2) {
Thread.State s; ForkJoinWorkerThread wt;
if ((w = ws[i]) != null && (wt = w.owner) != null &&
w.eventCount >= 0 &&
(s = wt.getState()) != Thread.State.BLOCKED &&
s != Thread.State.WAITING &&
s != Thread.State.TIMED_WAITING)
++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 (int)(ctl >> AC_SHIFT) + parallelism == 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.get();
WorkQueue[] ws; WorkQueue w;
if ((ws = workQueues) != null) {
int n = ws.length;
for (int i = 1; i < n; i += 2) {
if ((w = ws[i]) != null)
count += w.totalSteals;
}
}
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) {
int n = ws.length;
for (int i = 1; i < n; 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) {
int n = ws.length;
for (int i = 0; i < n; 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) {
int n = ws.length;
for (int i = 0; i < n; i += 2) {
if ((w = ws[i]) != null && w.queueSize() != 0)
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) {
int n = ws.length;
for (int i = 0; i < n; 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<? super ForkJoinTask<?>> c) {
int count = 0;
WorkQueue[] ws; WorkQueue w; ForkJoinTask<?> t;
if ((ws = workQueues) != null) {
int n = ws.length;
for (int i = 0; i < n; ++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() {
long st = getStealCount();
long qt = getQueuedTaskCount();
long qs = getQueuedSubmissionCount();
int rc = getRunningThreadCount();
int pc = parallelism;
long c = ctl;
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 = runState < 0 ? "Shutting down" : "Running";
return super.toString() +
"[" + level +
", parallelism = " + pc +
", size = " + tc +
", active = " + ac +
", running = " + rc +
", steals = " + st +
", tasks = " + qt +
", submissions = " + qs +
"]";
}
/**
* Initiates an orderly shutdown in which previously submitted
* tasks are executed, but no new tasks will be accepted.
* Invocation has 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();
enableShutdown();
tryTerminate(false);
}
/**
* Attempts to cancel and/or stop all tasks, and reject all
* subsequently submitted tasks. Tasks that are in the process of
* being submitted or executed concurrently during the course of
* this method may or may not be rejected. This method cancels
* both existing and unexecuted tasks, in order to permit
* termination in the presence of task dependencies. So the method
* always returns an empty list (unlike the case for some other
* Executors).
*
* @return an empty list
* @throws SecurityException if a security manager exists and
* the caller is not permitted to modify threads
* because it does not hold {@link
* java.lang.RuntimePermission}{@code ("modifyThread")}
*/
public List<Runnable> shutdownNow() {
checkPermission();
enableShutdown();
tryTerminate(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);
}
/**
* 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 IO,
* 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);
}
/**
* Returns {@code true} if this pool has been shut down.
*
* @return {@code true} if this pool has been shut down
*/
public boolean isShutdown() {
return runState < 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.
*
* @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 {
long nanos = unit.toNanos(timeout);
final ReentrantLock lock = this.lock;
lock.lock();
try {
for (;;) {
if (isTerminated())
return true;
if (nanos <= 0)
return false;
nanos = termination.awaitNanos(nanos);
}
} finally {
lock.unlock();
}
}
/**
* Interface for extending managed parallelism for tasks running
* in {@link ForkJoinPool}s.
*
* <p>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}. The
* unusual methods in this API accommodate synchronizers that may,
* but don't usually, block for long periods. Similarly, they
* allow more efficient internal handling of cases in which
* additional workers may be, but usually are not, needed to
* ensure sufficient parallelism. Toward this end,
* implementations of method {@code isReleasable} must be amenable
* to repeated invocation.
*
* <p>For example, here is a ManagedBlocker based on a
* ReentrantLock:
* <pre> {@code
* class ManagedLocker implements ManagedBlocker {
* final ReentrantLock lock;
* boolean hasLock = false;
* ManagedLocker(ReentrantLock lock) { this.lock = lock; }
* public boolean block() {
* if (!hasLock)
* lock.lock();
* return true;
* }
* public boolean isReleasable() {
* return hasLock || (hasLock = lock.tryLock());
* }
* }}</pre>
*
* <p>Here is a class that possibly blocks waiting for an
* item on a given queue:
* <pre> {@code
* class QueueTaker<E> implements ManagedBlocker {
* final BlockingQueue<E> queue;
* volatile E item = null;
* QueueTaker(BlockingQueue<E> q) { this.queue = q; }
* public boolean block() throws InterruptedException {
* if (item == null)
* item = queue.take();
* return true;
* }
* public boolean isReleasable() {
* return item != null || (item = queue.poll()) != null;
* }
* public E getItem() { // call after pool.managedBlock completes
* return item;
* }
* }}</pre>
*/
public static interface ManagedBlocker {
/**
* Possibly blocks the current thread, for example waiting for
* a lock or condition.
*
* @return {@code true} if no additional blocking is necessary
* (i.e., if isReleasable would return true)
* @throws InterruptedException if interrupted while waiting
* (the method is not required to do so, but is allowed to)
*/
boolean block() throws InterruptedException;
/**
* Returns {@code true} if blocking is unnecessary.
*/
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.
*
* <p>If the caller is not a {@link ForkJoinTask}, this method is
* behaviorally equivalent to
* <pre> {@code
* while (!blocker.isReleasable())
* if (blocker.block())
* return;
* }</pre>
*
* 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();
ForkJoinPool p = ((t instanceof ForkJoinWorkerThread) ?
((ForkJoinWorkerThread)t).pool : null);
while (!blocker.isReleasable()) {
if (p == null || p.tryCompensate()) {
try {
do {} while (!blocker.isReleasable() && !blocker.block());
} finally {
if (p != null)
p.incrementActiveCount();
}
break;
}
}
}
// AbstractExecutorService overrides. These rely on undocumented
// fact that ForkJoinTask.adapt returns ForkJoinTasks that also
// implement RunnableFuture.
protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
return (RunnableFuture<T>) ForkJoinTask.adapt(runnable, value);
}
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
return (RunnableFuture<T>) ForkJoinTask.adapt(callable);
}
// Unsafe mechanics
private static final sun.misc.Unsafe U;
private static final long CTL;
private static final long RUNSTATE;
private static final long PARKBLOCKER;
static {
poolNumberGenerator = new AtomicInteger();
modifyThreadPermission = new RuntimePermission("modifyThread");
defaultForkJoinWorkerThreadFactory =
new DefaultForkJoinWorkerThreadFactory();
int s;
try {
U = getUnsafe();
Class<?> k = ForkJoinPool.class;
Class<?> tk = Thread.class;
CTL = U.objectFieldOffset
(k.getDeclaredField("ctl"));
RUNSTATE = U.objectFieldOffset
(k.getDeclaredField("runState"));
PARKBLOCKER = U.objectFieldOffset
(tk.getDeclaredField("parkBlocker"));
} catch (Exception e) {
throw new Error(e);
}
}
private static sun.misc.Unsafe getUnsafe() {
return Unsafe.instance;
}
}
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