src/jsr166y/ForkJoinPool.java

/*
 * 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.
 *
 * <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 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 <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 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
 * <em>NOT</em> use these pool execution methods, but instead use the
 * within-computation forms listed in the table. 
 *
 * <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>Arange 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>
 * 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 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<ForkJoinTask<?>> 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() {
        for (;;) {
            int pc = parallelism;
            int wc = workerCounts;
            int rc = wc & RUNNING_COUNT_MASK;
            int tc = wc >>> TOTAL_COUNT_SHIFT;
            if (tc < pc) {
                if (runState == TERMINATING ||
                    (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 ||
                            runState == TERMINATING)
                            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 to prime loop
            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 <code>null</code>.
     * @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</code>.
     * @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<ForkJoinTask<?>>();
        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 <T> void doSubmit(ForkJoinTask<T> 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> T invoke(ForkJoinTask<T> 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 <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) {
        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) {
        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) {
        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.  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<? super ForkJoinTask<?>> 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<Runnable> 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.
     *
     * <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).
     *
     * <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>
     */
    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();
        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 <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 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<sun.misc.Unsafe>() {
                        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());
            }
        }
    }
}
Revision:	1.58
Committed:	Fri Jul 23 13:07:43 2010 UTC (13 years, 9 months ago) by dl
Branch:	MAIN
Changes since 1.57:	+385 -272 lines
Log Message:	Better integration of helping and compensation