/*
* 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/licenses/publicdomain
*/
package jsr166y;
import java.util.concurrent.*;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.locks.LockSupport;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.CountDownLatch;
/**
* 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 subtasks 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). 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 {@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.
*
*
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 follwoing
* table. These are designed to be used 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
* NOT use these pool execution methods, but instead use the
* within-computation forms listed in the table.
*
*
*
* |
* Call from non-fork/join clients |
* Call from within fork/join computations |
*
*
* | Arange 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) |
*
*
*
* Sample Usage. 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.
*
*
* static final ForkJoinPool mainPool = new ForkJoinPool();
* ...
* public void sort(long[] array) {
* mainPool.invoke(new SortTask(array, 0, array.length));
* }
*
*
* 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 exhuasted.
*
* @since 1.7
* @author Doug Lea
*/
public class ForkJoinPool extends AbstractExecutorService {
/*
* Implementation Overview
*
* This class provides the central bookkeeping and control for a
* set of worker threads: Submissions from non-FJ threads enter
* into a submission queue. Workers take these tasks and typically
* split them into subtasks that may be stolen by other workers.
* The main work-stealing mechanics implemented in class
* ForkJoinWorkerThread 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 worker queues, and
* lastly to new submissions. These mechanics do not consider
* 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.)
*
* Beyond work-stealing support and essential bookkeeping, the
* main responsibility of this framework is to arrange tactics for
* when one worker is waiting to join a task stolen (or always
* held by) another. Becauae 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. Given that the creation costs of most
* threads on most systems mainly surrounds setting up runtime
* stacks, thread creation and switching is usually not much more
* expensive than stack creation and switching, and is more
* flexible). Instead we combine two tactics:
*
* 1. Arranging for the joiner to execute some task that it
* would be running if the steal had not occurred. Method
* ForkJoinWorkerThread.helpJoinTask tracks joining->stealing
* links to try to find such a task.
*
* 2. Unless there are already enough live threads, creating or
* or re-activating a spare thread to compensate for the
* (blocked) joiner until it unblocks. Spares then suspend
* at their next opportunity or eventually die if unused for
* too long. See below and the internal documentation
* for tryAwaitJoin for more details about compensation
* rules.
*
* Because the determining existence of conservatively safe
* helping targets, the availability of already-created spares,
* and the apparent need to create new spares are all racy and
* require heuristic guidance, joins (in
* ForkJoinWorkerThread.joinTask) interleave these options until
* successful. Creating a new spare always succeeds, but also
* increases application footprint, so we try to avoid it, within
* reason.
*
* The ManagedBlocker extension API can't use option (1) so uses a
* special version of (2) in method awaitBlocker.
*
* The main throughput advantages of work-stealing stem from
* decentralized control -- workers mostly steal tasks from each
* other. We do not want to negate this by creating bottlenecks
* implementing other management responsibilities. So we use a
* collection of techniques that avoid, reduce, or cope well with
* contention. These entail several instances of bit-packing into
* CASable fields to maintain only the minimally required
* atomicity. To enable such packing, we restrict maximum
* parallelism to (1<<15)-1 (enabling twice this (to accommodate
* unbalanced increments and decrements) to fit into a 16 bit
* field, which is far in excess of normal operating range. Even
* though updates to some of these bookkeeping fields do sometimes
* contend with each other, they don't normally cache-contend with
* updates to others enough to warrant memory padding or
* isolation. So they are all held as fields of ForkJoinPool
* objects. The main capabilities are as follows:
*
* 1. Creating and removing workers. Workers are recorded in the
* "workers" array. This is an array as opposed to some other data
* structure to support index-based random steals by workers.
* Updates to the array recording new workers and unrecording
* terminated ones are protected from each other by a lock
* (workerLock) but the array is otherwise concurrently readable,
* and accessed directly by workers. To simplify index-based
* operations, the array size is always a power of two, and all
* readers must tolerate null slots. Currently, 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.
*
* 2. Bookkeeping for dynamically adding and removing workers. We
* aim to approximately maintain the given level of parallelism.
* When some workers are known to be blocked (on joins or via
* ManagedBlocker), we may create or resume others to take their
* place until they unblock (see below). Implementing this
* requires counts of the number of "running" threads (i.e., those
* that are neither blocked nor artifically suspended) as well as
* the total number. These two values are packed into one field,
* "workerCounts" because we need accurate snapshots when deciding
* to create, resume or suspend. Note however that the
* correspondance of these counts to reality is not guaranteed. In
* particular updates for unblocked threads may lag until they
* actually wake up.
*
* 3. Maintaining global run state. The run state of the pool
* consists of a runLevel (SHUTDOWN, TERMINATING, etc) similar to
* those in other Executor implementations, as well as a count of
* "active" workers -- those that are, or soon will be, or
* recently were executing tasks. The runLevel and active count
* are packed together in order to correctly trigger shutdown and
* termination. Without care, active counts can be subject to very
* high contention. We substantially reduce this contention by
* relaxing update rules. A worker must claim active status
* prospectively, by activating if it sees that a submitted or
* stealable task exists (it may find after activating that the
* task no longer exists). It stays active while processing this
* task (if it exists) and any other local subtasks it produces,
* until it cannot find any other tasks. It then tries
* inactivating (see method preStep), but upon update contention
* instead scans for more tasks, later retrying inactivation if it
* doesn't find any.
*
* 4. Managing idle workers waiting for tasks. We cannot let
* workers spin indefinitely scanning for tasks when none are
* available. On the other hand, we must quickly prod them into
* action when new tasks are submitted or generated. We
* park/unpark these idle workers using an event-count scheme.
* Field eventCount is incremented upon events that may enable
* workers that previously could not find a task to now find one:
* Submission of a new task to the pool, or another worker pushing
* a task onto a previously empty queue. (We also use this
* mechanism for termination and reconfiguration actions that
* require wakeups of idle workers). Each worker maintains its
* last known event count, and blocks when a scan for work did not
* find a task AND its lastEventCount matches the current
* eventCount. Waiting idle workers are recorded in a variant of
* Treiber stack headed by field eventWaiters which, when nonzero,
* encodes the thread index and count awaited for by the worker
* thread most recently calling eventSync. This thread in turn has
* a record (field nextEventWaiter) for the next waiting worker.
* In addition to allowing simpler decisions about need for
* wakeup, the event count bits in eventWaiters serve the role of
* tags to avoid ABA errors in Treiber stacks. To reduce delays
* in task diffusion, workers not otherwise occupied may invoke
* method releaseWaiters, that removes and signals (unparks)
* workers not waiting on current count. To minimize task
* production stalls associate with signalling, any worker pushing
* a task on an empty queue invokes the weaker method signalWork,
* that only releases idle workers until it detects interference
* by other threads trying to release, and lets them take
* over. The net effect is a tree-like diffusion of signals, where
* released threads (and possibly others) help with unparks. To
* further reduce contention effects a bit, failed CASes to
* increment field eventCount are tolerated without retries.
* Conceptually they are merged into the same event, which is OK
* when their only purpose is to enable workers to scan for work.
*
* 5. Managing suspension of extra workers. When a worker is about
* to block waiting for a join (or via ManagedBlockers), we may
* create a new thread to maintain parallelism level, or at least
* avoid starvation. Usually, extra threads are needed for only
* very short periods, yet join dependencies are such that we
* sometimes need them in bursts. Rather than create new threads
* each time this happens, we suspend no-longer-needed extra ones
* as "spares". For most purposes, we don't distinguish "extra"
* spare threads from normal "core" threads: On each call to
* preStep (the only point at which we can do this) a worker
* checks to see if there are now too many running workers, and if
* so, suspends itself. Methods tryAwaitJoin and awaitBlocker
* look for suspended threads to resume before considering
* creating a new replacement. We don't need a special data
* structure to maintain spares; simply scanning the workers array
* looking for worker.isSuspended() is fine because the calling
* thread is otherwise not doing anything useful anyway; we are at
* least as happy if after locating a spare, the caller doesn't
* actually block because the join is ready before we try to
* adjust and compensate. Note that this is intrinsically racy.
* One thread may become a spare at about the same time as another
* is needlessly being created. We counteract this and related
* slop in part by requiring resumed spares to immediately recheck
* (in preStep) to see whether they they should re-suspend. The
* only effective difference between "extra" and "core" threads is
* that we allow the "extra" ones to time out and die if they are
* not resumed within a keep-alive interval of a few seconds. This
* is implemented mainly within ForkJoinWorkerThread, but requires
* some coordination (isTrimmed() -- meaning killed while
* suspended) to correctly maintain pool counts.
*
* 6. Deciding when to create new workers. The main dynamic
* control in this class is deciding when to create extra threads,
* in methods awaitJoin and awaitBlocker. We always need to create
* one when the number of running threads would become zero and
* all workers are busy. However, this is not easy to detect
* reliably in the presence of transients so we use retries and
* allow slack (in tryAwaitJoin) to reduce false alarms. These
* effectively reduce churn at the price of systematically
* undershooting target parallelism when many threads are blocked.
* However, biasing toward undeshooting partially compensates for
* the above mechanics to suspend extra threads, that normally
* lead to overshoot because we can only suspend workers
* in-between top-level actions. It also better copes with the
* fact that some of the methods in this class tend to never
* become compiled (but are interpreted), so some components of
* the entire set of controls might execute many times faster than
* others. And similarly for cases where the apparent lack of work
* is just due to GC stalls and other transient system activity.
*
* Beware that there is a lot of representation-level coupling
* among classes ForkJoinPool, ForkJoinWorkerThread, and
* ForkJoinTask. For example, direct access to "workers" array by
* workers, and direct access to ForkJoinTask.status by both
* ForkJoinPool and ForkJoinWorkerThread. There is little point
* trying to reduce this, since any associated future changes in
* representations will need to be accompanied by algorithmic
* changes anyway.
*
* Style notes: There are lots of inline assignments (of form
* "while ((local = field) != 0)") which are usually the simplest
* way to ensure read orderings. Also 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), at the expense of messiness.
*
* The order of declarations in this file is: (1) statics (2)
* fields (along with constants used when unpacking some of them)
* (3) internal control methods (4) callbacks and other support
* for ForkJoinTask and ForkJoinWorkerThread classes, (5) exported
* methods (plus a few little helpers).
*/
/**
* 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 =
new DefaultForkJoinWorkerThreadFactory();
/**
* Permission required for callers of methods that may start or
* kill threads.
*/
private static final RuntimePermission modifyThreadPermission =
new RuntimePermission("modifyThread");
/**
* 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 =
new AtomicInteger();
/**
* Absolute bound for parallelism level. Twice this number must
* fit into a 16bit field to enable word-packing for some counts.
*/
private static final int MAX_THREADS = 0x7fff;
/**
* Array holding all worker threads in the pool. Array size must
* be a power of two. Updates and replacements are protected by
* workerLock, but the array is always kept in a consistent enough
* state to be randomly accessed without locking by workers
* performing work-stealing, as well as other traversal-based
* methods in this class. All readers must tolerate that some
* array slots may be null.
*/
volatile ForkJoinWorkerThread[] workers;
/**
* Queue for external submissions.
*/
private final LinkedTransferQueue> submissionQueue;
/**
* Lock protecting updates to workers array.
*/
private final ReentrantLock workerLock;
/**
* Latch released upon termination.
*/
private final Phaser termination;
/**
* Creation factory for worker threads.
*/
private final ForkJoinWorkerThreadFactory factory;
/**
* Sum of per-thread steal counts, updated only when threads are
* idle or terminating.
*/
private volatile long stealCount;
/**
* Encoded record of top of treiber stack of threads waiting for
* events. The top 32 bits contain the count being waited for. The
* bottom word contains one plus the pool index of waiting worker
* thread.
*/
private volatile long eventWaiters;
private static final int EVENT_COUNT_SHIFT = 32;
private static final long WAITER_ID_MASK = (1L << EVENT_COUNT_SHIFT)-1L;
/**
* A counter for events that may wake up worker threads:
* - Submission of a new task to the pool
* - A worker pushing a task on an empty queue
* - termination and reconfiguration
*/
private volatile int eventCount;
/**
* Lifecycle control. The low word contains the number of workers
* that are (probably) executing tasks. This value is atomically
* incremented before a worker gets a task to run, and decremented
* when worker has no tasks and cannot find any. Bits 16-18
* contain runLevel value. When all are zero, the pool is
* running. Level transitions are monotonic (running -> shutdown
* -> terminating -> terminated) so each transition adds a bit.
* These are bundled together to ensure consistent read for
* termination checks (i.e., that runLevel is at least SHUTDOWN
* and active threads is zero).
*/
private volatile int runState;
// Note: The order among run level values matters.
private static final int RUNLEVEL_SHIFT = 16;
private static final int SHUTDOWN = 1 << RUNLEVEL_SHIFT;
private static final int TERMINATING = 1 << (RUNLEVEL_SHIFT + 1);
private static final int TERMINATED = 1 << (RUNLEVEL_SHIFT + 2);
private static final int ACTIVE_COUNT_MASK = (1 << RUNLEVEL_SHIFT) - 1;
private static final int ONE_ACTIVE = 1; // active update delta
/**
* Holds number of total (i.e., created and not yet terminated)
* and running (i.e., not blocked on joins or other managed sync)
* threads, packed together to ensure consistent snapshot when
* making decisions about creating and suspending spare
* threads. Updated only by CAS. Note that adding a new worker
* requires incrementing both counts, since workers start off in
* running state. This field is also used for memory-fencing
* configuration parameters.
*/
private volatile int workerCounts;
private static final int TOTAL_COUNT_SHIFT = 16;
private static final int RUNNING_COUNT_MASK = (1 << TOTAL_COUNT_SHIFT) - 1;
private static final int ONE_RUNNING = 1;
private static final int ONE_TOTAL = 1 << TOTAL_COUNT_SHIFT;
/**
* The target parallelism level.
* Accessed directly by ForkJoinWorkerThreads.
*/
final int parallelism;
/**
* True if use local fifo, not default lifo, for local polling
* Read by, and replicated by ForkJoinWorkerThreads
*/
final boolean locallyFifo;
/**
* The uncaught exception handler used when any worker abruptly
* terminates.
*/
private final Thread.UncaughtExceptionHandler ueh;
/**
* Pool number, just for assigning useful names to worker threads
*/
private final int poolNumber;
// Utilities for CASing fields. Note that several of these
// are manually inlined by callers
/**
* Increments running count. Also used by ForkJoinTask.
*/
final void incrementRunningCount() {
int c;
do {} while (!UNSAFE.compareAndSwapInt(this, workerCountsOffset,
c = workerCounts,
c + ONE_RUNNING));
}
/**
* Tries to decrement running count unless already zero
*/
final boolean tryDecrementRunningCount() {
int wc = workerCounts;
if ((wc & RUNNING_COUNT_MASK) == 0)
return false;
return UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - ONE_RUNNING);
}
/**
* Tries to increment running count
*/
final boolean tryIncrementRunningCount() {
int wc;
return UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc = workerCounts, wc + ONE_RUNNING);
}
/**
* Tries incrementing active count; fails on contention.
* Called by workers before executing tasks.
*
* @return true on success
*/
final boolean tryIncrementActiveCount() {
int c;
return UNSAFE.compareAndSwapInt(this, runStateOffset,
c = runState, c + ONE_ACTIVE);
}
/**
* Tries decrementing active count; fails on contention.
* Called when workers cannot find tasks to run.
*/
final boolean tryDecrementActiveCount() {
int c;
return UNSAFE.compareAndSwapInt(this, runStateOffset,
c = runState, c - ONE_ACTIVE);
}
/**
* Advances to at least the given level. Returns true if not
* already in at least the given level.
*/
private boolean advanceRunLevel(int level) {
for (;;) {
int s = runState;
if ((s & level) != 0)
return false;
if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, s | level))
return true;
}
}
// workers array maintenance
/**
* Records and returns a workers array index for new worker.
*/
private int recordWorker(ForkJoinWorkerThread w) {
// Try using slot totalCount-1. If not available, scan and/or resize
int k = (workerCounts >>> TOTAL_COUNT_SHIFT) - 1;
final ReentrantLock lock = this.workerLock;
lock.lock();
try {
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
if (k < 0 || k >= nws || ws[k] != null) {
for (k = 0; k < nws && ws[k] != null; ++k)
;
if (k == nws)
ws = Arrays.copyOf(ws, nws << 1);
}
ws[k] = w;
workers = ws; // volatile array write ensures slot visibility
} finally {
lock.unlock();
}
return k;
}
/**
* Nulls out record of worker in workers array
*/
private void forgetWorker(ForkJoinWorkerThread w) {
int idx = w.poolIndex;
// Locking helps method recordWorker avoid unecessary expansion
final ReentrantLock lock = this.workerLock;
lock.lock();
try {
ForkJoinWorkerThread[] ws = workers;
if (idx >= 0 && idx < ws.length && ws[idx] == w) // verify
ws[idx] = null;
} finally {
lock.unlock();
}
}
// adding and removing workers
/**
* Tries to create and add new worker. Assumes that worker counts
* are already updated to accommodate the worker, so adjusts on
* failure.
*
* @return new worker or null if creation failed
*/
private ForkJoinWorkerThread addWorker() {
ForkJoinWorkerThread w = null;
try {
w = factory.newThread(this);
} finally { // Adjust on either null or exceptional factory return
if (w == null) {
onWorkerCreationFailure();
return null;
}
}
w.start(recordWorker(w), ueh);
return w;
}
/**
* Adjusts counts upon failure to create worker
*/
private void onWorkerCreationFailure() {
for (;;) {
int wc = workerCounts;
if ((wc >>> TOTAL_COUNT_SHIFT) == 0)
Thread.yield(); // wait for other counts to settle
else if (UNSAFE.compareAndSwapInt(this, workerCountsOffset, wc,
wc - (ONE_RUNNING|ONE_TOTAL)))
break;
}
tryTerminate(false); // in case of failure during shutdown
}
/**
* Creates and/or resumes enough workers to establish target
* parallelism, giving up if terminating or addWorker fails
*
* TODO: recast this to support lazier creation and automated
* parallelism maintenance
*/
private void ensureEnoughWorkers() {
while ((runState & TERMINATING) == 0) {
int pc = parallelism;
int wc = workerCounts;
int rc = wc & RUNNING_COUNT_MASK;
int tc = wc >>> TOTAL_COUNT_SHIFT;
if (tc < pc) {
if (UNSAFE.compareAndSwapInt
(this, workerCountsOffset,
wc, wc + (ONE_RUNNING|ONE_TOTAL)) &&
addWorker() == null)
break;
}
else if (tc > pc && rc < pc &&
tc > (runState & ACTIVE_COUNT_MASK)) {
ForkJoinWorkerThread spare = null;
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null && w.isSuspended()) {
if ((workerCounts & RUNNING_COUNT_MASK) > pc)
return;
if (w.tryResumeSpare())
incrementRunningCount();
break;
}
}
}
else
break;
}
}
/**
* Final callback from terminating worker. Removes record of
* worker from array, and adjusts counts. If pool is shutting
* down, tries to complete terminatation, else possibly replaces
* the worker.
*
* @param w the worker
*/
final void workerTerminated(ForkJoinWorkerThread w) {
if (w.active) { // force inactive
w.active = false;
do {} while (!tryDecrementActiveCount());
}
forgetWorker(w);
// Decrement total count, and if was running, running count
// Spin (waiting for other updates) if either would be negative
int nr = w.isTrimmed() ? 0 : ONE_RUNNING;
int unit = ONE_TOTAL + nr;
for (;;) {
int wc = workerCounts;
int rc = wc & RUNNING_COUNT_MASK;
if (rc - nr < 0 || (wc >>> TOTAL_COUNT_SHIFT) == 0)
Thread.yield(); // back off if waiting for other updates
else if (UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - unit))
break;
}
accumulateStealCount(w); // collect final count
if (!tryTerminate(false))
ensureEnoughWorkers();
}
// Waiting for and signalling events
/**
* Releases workers blocked on a count not equal to current count.
* @return true if any released
*/
private void releaseWaiters() {
long top;
while ((top = eventWaiters) != 0L) {
ForkJoinWorkerThread[] ws = workers;
int n = ws.length;
for (;;) {
int i = ((int)(top & WAITER_ID_MASK)) - 1;
if (i < 0 || (int)(top >>> EVENT_COUNT_SHIFT) == eventCount)
return;
ForkJoinWorkerThread w;
if (i < n && (w = ws[i]) != null &&
UNSAFE.compareAndSwapLong(this, eventWaitersOffset,
top, w.nextWaiter)) {
LockSupport.unpark(w);
top = eventWaiters;
}
else
break; // possibly stale; reread
}
}
}
/**
* Ensures eventCount on exit is different (mod 2^32) than on
* entry and wakes up all waiters
*/
private void signalEvent() {
int c;
do {} while (!UNSAFE.compareAndSwapInt(this, eventCountOffset,
c = eventCount, c+1));
releaseWaiters();
}
/**
* Advances eventCount and releases waiters until interference by
* other releasing threads is detected.
*/
final void signalWork() {
int c;
UNSAFE.compareAndSwapInt(this, eventCountOffset, c=eventCount, c+1);
long top;
while ((top = eventWaiters) != 0L) {
int ec = eventCount;
ForkJoinWorkerThread[] ws = workers;
int n = ws.length;
for (;;) {
int i = ((int)(top & WAITER_ID_MASK)) - 1;
if (i < 0 || (int)(top >>> EVENT_COUNT_SHIFT) == ec)
return;
ForkJoinWorkerThread w;
if (i < n && (w = ws[i]) != null &&
UNSAFE.compareAndSwapLong(this, eventWaitersOffset,
top, top = w.nextWaiter)) {
LockSupport.unpark(w);
if (top != eventWaiters) // let someone else take over
return;
}
else
break; // possibly stale; reread
}
}
}
/**
* If worker is inactive, blocks until terminating or event count
* advances from last value held by worker; in any case helps
* release others.
*
* @param w the calling worker thread
* @param retries the number of scans by caller failing to find work
* @return false if now too many threads running
*/
private boolean eventSync(ForkJoinWorkerThread w, int retries) {
int wec = w.lastEventCount;
if (retries > 1) { // can only block after 2nd miss
long nextTop = (((long)wec << EVENT_COUNT_SHIFT) |
((long)(w.poolIndex + 1)));
long top;
while ((runState < SHUTDOWN || !tryTerminate(false)) &&
(((int)(top = eventWaiters) & WAITER_ID_MASK) == 0 ||
(int)(top >>> EVENT_COUNT_SHIFT) == wec) &&
eventCount == wec) {
if (UNSAFE.compareAndSwapLong(this, eventWaitersOffset,
w.nextWaiter = top, nextTop)) {
accumulateStealCount(w); // transfer steals while idle
Thread.interrupted(); // clear/ignore interrupt
while (eventCount == wec)
w.doPark();
break;
}
}
wec = eventCount;
}
releaseWaiters();
int wc = workerCounts;
if ((wc & RUNNING_COUNT_MASK) <= parallelism) {
w.lastEventCount = wec;
return true;
}
if (wec != w.lastEventCount) // back up if may re-wait
w.lastEventCount = wec - (wc >>> TOTAL_COUNT_SHIFT);
return false;
}
/**
* Callback from workers invoked upon each top-level action (i.e.,
* stealing a task or taking a submission and running
* it). Performs one or both of the following:
*
* * If the worker cannot find work, updates its active status to
* inactive and updates activeCount unless there is contention, in
* which case it may try again (either in this or a subsequent
* call). Additionally, awaits the next task event and/or helps
* wake up other releasable waiters.
*
* * If there are too many running threads, suspends this worker
* (first forcing inactivation if necessary). If it is not
* resumed before a keepAlive elapses, the worker may be "trimmed"
* -- killed while suspended within suspendAsSpare. Otherwise,
* upon resume it rechecks to make sure that it is still needed.
*
* @param w the worker
* @param retries the number of scans by caller failing to find work
* find any (in which case it may block waiting for work).
*/
final void preStep(ForkJoinWorkerThread w, int retries) {
boolean active = w.active;
boolean inactivate = active && retries != 0;
for (;;) {
int rs, wc;
if (inactivate &&
UNSAFE.compareAndSwapInt(this, runStateOffset,
rs = runState, rs - ONE_ACTIVE))
inactivate = active = w.active = false;
if (((wc = workerCounts) & RUNNING_COUNT_MASK) <= parallelism) {
if (active || eventSync(w, retries))
break;
}
else if (!(inactivate |= active) && // must inactivate to suspend
UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - ONE_RUNNING) &&
!w.suspendAsSpare()) // false if trimmed
break;
}
}
/**
* Awaits join of the given task if enough threads, or can resume
* or create a spare. Fails (in which case the given task might
* not be done) upon contention or lack of decision about
* blocking. Returns void because caller must check
* task status on return anyway.
*
* We allow blocking if:
*
* 1. There would still be at least as many running threads as
* parallelism level if this thread blocks.
*
* 2. A spare is resumed to replace this worker. We tolerate
* slop in the decision to replace if a spare is found without
* first decrementing run count. This may release too many,
* but if so, the superfluous ones will re-suspend via
* preStep().
*
* 3. After #spares repeated checks, there are no fewer than #spare
* threads not running. We allow this slack to avoid hysteresis
* and as a hedge against lag/uncertainty of running count
* estimates when signalling or unblocking stalls.
*
* 4. All existing workers are busy (as rechecked via repeated
* retries by caller) and a new spare is created.
*
* If none of the above hold, we try to escape out by
* re-incrementing count and returning to caller, which can retry
* later.
*
* @param joinMe the task to join
* @param retries if negative, then serve only as a precheck
* that the thread can be replaced by a spare. Otherwise,
* the number of repeated calls to this method returning busy
* @return true if the call must be retried because there
* none of the blocking checks hold
*/
final boolean tryAwaitJoin(ForkJoinTask joinMe, int retries) {
if (joinMe.status < 0) // precheck for cancellation
return false;
if ((runState & TERMINATING) != 0) { // shutting down
joinMe.cancelIgnoringExceptions();
return false;
}
int pc = parallelism;
boolean running = true; // false when running count decremented
outer:for (;;) {
int wc = workerCounts;
int rc = wc & RUNNING_COUNT_MASK;
int tc = wc >>> TOTAL_COUNT_SHIFT;
if (running) { // replace with spare or decrement count
if (rc <= pc && tc > pc &&
(retries > 0 || tc > (runState & ACTIVE_COUNT_MASK))) {
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) { // search for spare
ForkJoinWorkerThread w = ws[i];
if (w != null) {
if (joinMe.status < 0)
return false;
if (w.isSuspended()) {
if ((workerCounts & RUNNING_COUNT_MASK)>=pc &&
w.tryResumeSpare()) {
running = false;
break outer;
}
continue outer; // rescan
}
}
}
}
if (retries < 0 || // < 0 means replacement check only
rc == 0 || joinMe.status < 0 || workerCounts != wc ||
!UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - ONE_RUNNING))
return false; // done or inconsistent or contended
running = false;
if (rc > pc)
break;
}
else { // allow blocking if enough threads
if (rc >= pc || joinMe.status < 0)
break;
int sc = tc - pc + 1; // = spare threads, plus the one to add
if (retries > sc) {
if (rc > 0 && rc >= pc - sc) // allow slack
break;
if (tc < MAX_THREADS &&
tc == (runState & ACTIVE_COUNT_MASK) &&
workerCounts == wc &&
UNSAFE.compareAndSwapInt(this, workerCountsOffset, wc,
wc+(ONE_RUNNING|ONE_TOTAL))) {
addWorker();
break;
}
}
if (workerCounts == wc && // back out to allow rescan
UNSAFE.compareAndSwapInt (this, workerCountsOffset,
wc, wc + ONE_RUNNING)) {
releaseWaiters(); // help others progress
return true; // let caller retry
}
}
}
// arrive here if can block
joinMe.internalAwaitDone();
int c; // to inline incrementRunningCount
do {} while (!UNSAFE.compareAndSwapInt
(this, workerCountsOffset,
c = workerCounts, c + ONE_RUNNING));
return false;
}
/**
* Same idea as (and shares many code snippets with) tryAwaitJoin,
* but self-contained because there are no caller retries.
* TODO: Rework to use simpler API.
*/
final void awaitBlocker(ManagedBlocker blocker)
throws InterruptedException {
boolean done;
if (done = blocker.isReleasable())
return;
int pc = parallelism;
int retries = 0;
boolean running = true; // false when running count decremented
outer:for (;;) {
int wc = workerCounts;
int rc = wc & RUNNING_COUNT_MASK;
int tc = wc >>> TOTAL_COUNT_SHIFT;
if (running) {
if (rc <= pc && tc > pc &&
(retries > 0 || tc > (runState & ACTIVE_COUNT_MASK))) {
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null) {
if (done = blocker.isReleasable())
return;
if (w.isSuspended()) {
if ((workerCounts & RUNNING_COUNT_MASK)>=pc &&
w.tryResumeSpare()) {
running = false;
break outer;
}
continue outer; // rescan
}
}
}
}
if (done = blocker.isReleasable())
return;
if (rc == 0 || workerCounts != wc ||
!UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - ONE_RUNNING))
continue;
running = false;
if (rc > pc)
break;
}
else {
if (rc >= pc || (done = blocker.isReleasable()))
break;
int sc = tc - pc + 1;
if (retries++ > sc) {
if (rc > 0 && rc >= pc - sc)
break;
if (tc < MAX_THREADS &&
tc == (runState & ACTIVE_COUNT_MASK) &&
workerCounts == wc &&
UNSAFE.compareAndSwapInt(this, workerCountsOffset, wc,
wc+(ONE_RUNNING|ONE_TOTAL))) {
addWorker();
break;
}
}
Thread.yield();
}
}
try {
if (!done)
do {} while (!blocker.isReleasable() && !blocker.block());
} finally {
if (!running) {
int c;
do {} while (!UNSAFE.compareAndSwapInt
(this, workerCountsOffset,
c = workerCounts, c + ONE_RUNNING));
}
}
}
/**
* Possibly initiates and/or completes termination.
*
* @param now if true, unconditionally terminate, else only
* if shutdown and empty queue and no active workers
* @return true if now terminating or terminated
*/
private boolean tryTerminate(boolean now) {
if (now)
advanceRunLevel(SHUTDOWN); // ensure at least SHUTDOWN
else if (runState < SHUTDOWN ||
!submissionQueue.isEmpty() ||
(runState & ACTIVE_COUNT_MASK) != 0)
return false;
if (advanceRunLevel(TERMINATING))
startTerminating();
// Finish now if all threads terminated; else in some subsequent call
if ((workerCounts >>> TOTAL_COUNT_SHIFT) == 0) {
advanceRunLevel(TERMINATED);
termination.arrive();
}
return true;
}
/**
* Actions on transition to TERMINATING
*/
private void startTerminating() {
for (int i = 0; i < 2; ++i) { // twice to mop up newly created workers
cancelSubmissions();
shutdownWorkers();
cancelWorkerTasks();
signalEvent();
interruptWorkers();
}
}
/**
* Clear out and cancel submissions, ignoring exceptions
*/
private void cancelSubmissions() {
ForkJoinTask task;
while ((task = submissionQueue.poll()) != null) {
try {
task.cancel(false);
} catch (Throwable ignore) {
}
}
}
/**
* Sets all worker run states to at least shutdown,
* also resuming suspended workers
*/
private void shutdownWorkers() {
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null)
w.shutdown();
}
}
/**
* Clears out and cancels all locally queued tasks
*/
private void cancelWorkerTasks() {
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null)
w.cancelTasks();
}
}
/**
* Unsticks all workers blocked on joins etc
*/
private void interruptWorkers() {
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null && !w.isTerminated()) {
try {
w.interrupt();
} catch (SecurityException ignore) {
}
}
}
}
// misc support for ForkJoinWorkerThread
/**
* Returns pool number
*/
final int getPoolNumber() {
return poolNumber;
}
/**
* Accumulates steal count from a worker, clearing
* the worker's value
*/
final void accumulateStealCount(ForkJoinWorkerThread w) {
int sc = w.stealCount;
if (sc != 0) {
long c;
w.stealCount = 0;
do {} while (!UNSAFE.compareAndSwapLong(this, stealCountOffset,
c = stealCount, c + sc));
}
}
/**
* Returns the approximate (non-atomic) number of idle threads per
* active thread.
*/
final int idlePerActive() {
int pc = parallelism; // use parallelism, not rc
int ac = runState; // no mask -- artifically boosts during shutdown
// Use exact results for small values, saturate past 4
return pc <= ac? 0 : pc >>> 1 <= ac? 1 : pc >>> 2 <= ac? 3 : pc >>> 3;
}
// Public and protected 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 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 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_THREADS)
throw new IllegalArgumentException();
this.parallelism = parallelism;
this.factory = factory;
this.ueh = handler;
this.locallyFifo = asyncMode;
int arraySize = initialArraySizeFor(parallelism);
this.workers = new ForkJoinWorkerThread[arraySize];
this.submissionQueue = new LinkedTransferQueue>();
this.workerLock = new ReentrantLock();
this.termination = new Phaser(1);
this.poolNumber = poolNumberGenerator.incrementAndGet();
}
/**
* Returns initial power of two size for workers array.
* @param pc the initial parallelism level
*/
private static int initialArraySizeFor(int pc) {
// See Hackers Delight, sec 3.2. We know MAX_THREADS < (1 >>> 16)
int size = pc < MAX_THREADS ? pc + 1 : MAX_THREADS;
size |= size >>> 1;
size |= size >>> 2;
size |= size >>> 4;
size |= size >>> 8;
return size + 1;
}
// Execution methods
/**
* Common code for execute, invoke and submit
*/
private void doSubmit(ForkJoinTask task) {
if (task == null)
throw new NullPointerException();
if (runState >= SHUTDOWN)
throw new RejectedExecutionException();
submissionQueue.offer(task);
signalEvent();
ensureEnoughWorkers();
}
/**
* Performs the given task, returning its result upon completion.
* If the caller is already engaged in a fork/join computation in
* the current pool, this method is equivalent in effect to
* {@link ForkJoinTask#invoke}.
*
* @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 invoke(ForkJoinTask task) {
doSubmit(task);
return task.join();
}
/**
* Arranges for (asynchronous) execution of the given task.
* If the caller is already engaged in a fork/join computation in
* the current pool, this method is equivalent in effect to
* {@link ForkJoinTask#fork}.
*
* @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) {
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.
* If the caller is already engaged in a fork/join computation in
* the current pool, this method is equivalent in effect to
* {@link ForkJoinTask#fork}.
*
* @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 ForkJoinTask submit(ForkJoinTask task) {
doSubmit(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 = 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 ForkJoinTask submit(Runnable task, T result) {
ForkJoinTask 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) {
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 List> invokeAll(Collection> tasks) {
ArrayList> forkJoinTasks =
new ArrayList>(tasks.size());
for (Callable task : tasks)
forkJoinTasks.add(ForkJoinTask.adapt(task));
invoke(new InvokeAll(forkJoinTasks));
@SuppressWarnings({"unchecked", "rawtypes"})
List> futures = (List>) (List) forkJoinTasks;
return futures;
}
static final class InvokeAll extends RecursiveAction {
final ArrayList> tasks;
InvokeAll(ArrayList> 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. This 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 workerCounts >>> TOTAL_COUNT_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 locallyFifo;
}
/**
* 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() {
return workerCounts & RUNNING_COUNT_MASK;
}
/**
* 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() {
return runState & ACTIVE_COUNT_MASK;
}
/**
* 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 (runState & ACTIVE_COUNT_MASK) == 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() {
return stealCount;
}
/**
* 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;
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null)
count += w.getQueueSize();
}
return count;
}
/**
* Returns an estimate of the number of tasks submitted to this
* pool that have not yet begun executing. This method takes time
* proportional to the number of submissions.
*
* @return the number of queued submissions
*/
public int getQueuedSubmissionCount() {
return submissionQueue.size();
}
/**
* 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() {
return !submissionQueue.isEmpty();
}
/**
* 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() {
return submissionQueue.poll();
}
/**
* 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 n = submissionQueue.drainTo(c);
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null)
n += w.drainTasksTo(c);
}
return n;
}
/**
* Returns count of total parks by existing workers.
* Used during development only since not meaningful to users.
*/
private int collectParkCount() {
int count = 0;
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null)
count += w.parkCount;
}
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 wc = workerCounts;
int tc = wc >>> TOTAL_COUNT_SHIFT;
int rc = wc & RUNNING_COUNT_MASK;
int pc = parallelism;
int rs = runState;
int ac = rs & ACTIVE_COUNT_MASK;
// int pk = collectParkCount();
return super.toString() +
"[" + runLevelToString(rs) +
", parallelism = " + pc +
", size = " + tc +
", active = " + ac +
", running = " + rc +
", steals = " + st +
", tasks = " + qt +
", submissions = " + qs +
// ", parks = " + pk +
"]";
}
private static String runLevelToString(int s) {
return ((s & TERMINATED) != 0 ? "Terminated" :
((s & TERMINATING) != 0 ? "Terminating" :
((s & SHUTDOWN) != 0 ? "Shutting down" :
"Running")));
}
/**
* 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();
advanceRunLevel(SHUTDOWN);
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 shutdownNow() {
checkPermission();
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() {
return runState >= TERMINATED;
}
/**
* 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, causing this executor not
* to properly terminate.
*
* @return {@code true} if terminating but not yet terminated
*/
public boolean isTerminating() {
return (runState & (TERMINATING|TERMINATED)) == TERMINATING;
}
/**
* Returns {@code true} if this pool has been shut down.
*
* @return {@code true} if this pool has been shut down
*/
public boolean isShutdown() {
return runState >= SHUTDOWN;
}
/**
* 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 {
try {
return termination.awaitAdvanceInterruptibly(0, timeout, unit) > 0;
} catch(TimeoutException ex) {
return false;
}
}
/**
* 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).
*
*
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());
* }
* }}
*/
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.
*
* 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)
((ForkJoinWorkerThread) t).pool.awaitBlocker(blocker);
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 (RunnableFuture) ForkJoinTask.adapt(runnable, value);
}
protected RunnableFuture newTaskFor(Callable callable) {
return (RunnableFuture) ForkJoinTask.adapt(callable);
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE = getUnsafe();
private static final long workerCountsOffset =
objectFieldOffset("workerCounts", ForkJoinPool.class);
private static final long runStateOffset =
objectFieldOffset("runState", ForkJoinPool.class);
private static final long eventCountOffset =
objectFieldOffset("eventCount", ForkJoinPool.class);
private static final long eventWaitersOffset =
objectFieldOffset("eventWaiters",ForkJoinPool.class);
private static final long stealCountOffset =
objectFieldOffset("stealCount",ForkJoinPool.class);
private static long objectFieldOffset(String field, Class klazz) {
try {
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
} catch (NoSuchFieldException e) {
// Convert Exception to corresponding Error
NoSuchFieldError error = new NoSuchFieldError(field);
error.initCause(e);
throw error;
}
}
/**
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
* Replace with a simple call to Unsafe.getUnsafe when integrating
* into a jdk.
*
* @return a sun.misc.Unsafe
*/
private static sun.misc.Unsafe getUnsafe() {
try {
return sun.misc.Unsafe.getUnsafe();
} catch (SecurityException se) {
try {
return java.security.AccessController.doPrivileged
(new java.security
.PrivilegedExceptionAction() {
public sun.misc.Unsafe run() throws Exception {
java.lang.reflect.Field f = sun.misc
.Unsafe.class.getDeclaredField("theUnsafe");
f.setAccessible(true);
return (sun.misc.Unsafe) f.get(null);
}});
} catch (java.security.PrivilegedActionException e) {
throw new RuntimeException("Could not initialize intrinsics",
e.getCause());
}
}
}
}