src/jsr166y/CountedCompleter.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/publicdomain/zero/1.0/
 */

package jsr166y;

/**
 * A {@link ForkJoinTask} with a completion action performed when
 * triggered and there are no remaining pending
 * actions. CountedCompleters are in general more robust in the
 * presence of subtask stalls and blockage than are other forms for
 * ForkJoinTasks, but are in general less intuitive to program.  Uses
 * of CountedCompleter are similar to those of other completion based
 * components (such as {@link java.nio.channels.CompletionHandler})
 * except that multiple <em>pending</em> completions may be necessary
 * to trigger the {@link #onCompletion} action, not just one. Unless
 * initialized otherwise, the {@link #getPendingCount pending count}
 * starts at zero, but may be (atomically) changed using methods
 * {@link #setPendingCount}, {@link #addToPendingCount}, and {@link
 * #compareAndSetPendingCount}. Upon invocation of {@link
 * #tryComplete}, if the pending action count is nonzero, it is
 * decremented; otherwise, the completion action is performed, and if
 * this completer itself has a completer, the process is continued
 * with its completer.  As is the case with related synchronization
 * components such as {@link java.util.concurrent.Phaser Phaser} and
 * {@link java.util.concurrent.Semaphore Semaphore}, these methods
 * affect only internal counts; they do not establish any further
 * internal bookkeeping. In particular, the identities of pending
 * tasks are not maintained. As illustrated below, you can create
 * subclasses that do record some or all pending tasks or their
 * results when needed. Because CountedCompleters provide only basic
 * synchronization mechanisms, it may be useful to create further
 * abstract subclasses that maintain linkages and fields and support
 * methods appropriate for a set of related usages.
 *
 * <p>A concrete CountedCompleter class must define method {@link
 * #compute}, that should in most cases (as illustrated below), invoke
 * {@code tryComplete()} once before returning. The class may also
 * optionally override method {@link #onCompletion} to perform an
 * action upon normal completion, and method {@link
 * #onExceptionalCompletion} to perform an action upon any exception.
 *
 * <p>CountedCompleters most often do not bear results, in which case
 * they are normally declared as {@code CountedCompleter<Void>}, and
 * will always return {@code null} as a result value.  In other cases,
 * you should override method {@link #getRawResult} to provide a
 * result from {@code join(), invoke()}, and related methods.  In
 * general, this method should return the value of a field (or a
 * function of one or more fields) of the CountedCompleter object that
 * holds the result upon completion. Method {@link #setRawResult} by
 * default plays no role in CountedCompleters.  It is possible, but
 * not usually applicable, to override this method to maintain other
 * objects or fields holding result data.
 *
 * <p>A CountedCompleter that does not itself have a completer (i.e.,
 * one for which {@link #getCompleter} returns {@code null}) can be
 * used as a regular ForkJoinTask with this added functionality.
 * However, any completer that in turn has another completer serves
 * only as an internal helper for other computations, so its own task
 * status (as reported in methods such as {@link ForkJoinTask#isDone})
 * is arbitrary; this status changes only upon explicit invocations of
 * {@link #complete}, {@link ForkJoinTask#cancel}, {@link
 * ForkJoinTask#completeExceptionally} or upon exceptional completion
 * of method {@code compute}. Upon any exceptional completion, the
 * exception may be relayed to a task's completer (and its completer,
 * and so on), if one exists and it has not otherwise already
 * completed. Similarly, cancelling an internal CountedCompleter has
 * only a local effect on that completer, so is not often useful.
 *
 * <p><b>Sample Usages.</b>
 *
 * <p><b>Parallel recursive decomposition.</b> CountedCompleters may
 * be arranged in trees similar to those often used with {@link
 * RecursiveAction}s, although the constructions involved in setting
 * them up typically vary. Here, the completer of each task is its
 * parent in the computation tree. Even though they entail a bit more
 * bookkeeping, CountedCompleters may be better choices when applying
 * a possibly time-consuming operation (that cannot be further
 * subdivided) to each element of an array or collection; especially
 * when the operation takes a significantly different amount of time
 * to complete for some elements than others, either because of
 * intrinsic variation (for example IO) or auxiliary effects such as
 * garbage collection.  Because CountedCompleters provide their own
 * continuations, other threads need not block waiting to perform
 * them.
 *
 * <p>For example, here is an initial version of a class that uses
 * divide-by-two recursive decomposition to divide work into single
 * pieces (leaf tasks). Even when work is split into individual calls,
 * tree-based techniques are usually preferable to directly forking
 * leaf tasks, because they reduce inter-thread communication and
 * improve load balancing. In the recursive case, the second of each
 * pair of subtasks to finish triggers completion of its parent
 * (because no result combination is performed, the default no-op
 * implementation of method {@code onCompletion} is not overridden). A
 * static utility method sets up the base task and invokes it
 * (here, implicitly using the {@link ForkJoinPool#commonPool()}).
 *
 * <pre> {@code
 * class MyOperation<E> { void apply(E e) { ... }  }
 *
 * class ForEach<E> extends CountedCompleter<Void> {
 *
 *     public static <E> void forEach(E[] array, MyOperation<E> op) {
 *         new ForEach<E>(null, array, op, 0, array.length).invoke();
 *     }
 *
 *     final E[] array; final MyOperation<E> op; final int lo, hi;
 *     ForEach(CountedCompleter<?> p, E[] array, MyOperation<E> op, int lo, int hi) {
 *         super(p);
 *         this.array = array; this.op = op; this.lo = lo; this.hi = hi;
 *     }
 *
 *     public void compute() { // version 1
 *         if (hi - lo >= 2) {
 *             int mid = (lo + hi) >>> 1;
 *             setPendingCount(2); // must set pending count before fork
 *             new ForEach(this, array, op, mid, hi).fork(); // right child
 *             new ForEach(this, array, op, lo, mid).fork(); // left child
 *         }
 *         else if (hi > lo)
 *             op.apply(array[lo]);
 *         tryComplete();
 *     }
 * } }</pre>
 *
 * This design can be improved by noticing that in the recursive case,
 * the task has nothing to do after forking its right task, so can
 * directly invoke its left task before returning. (This is an analog
 * of tail recursion removal.)  Also, because the task returns upon
 * executing its left task (rather than falling through to invoke
 * tryComplete) the pending count is set to one:
 *
 * <pre> {@code
 * class ForEach<E> ...
 *     public void compute() { // version 2
 *         if (hi - lo >= 2) {
 *             int mid = (lo + hi) >>> 1;
 *             setPendingCount(1); // only one pending
 *             new ForEach(this, array, op, mid, hi).fork(); // right child
 *             new ForEach(this, array, op, lo, mid).compute(); // direct invoke
 *         }
 *         else {
 *             if (hi > lo)
 *                 op.apply(array[lo]);
 *             tryComplete();
 *         }
 *     }
 * }</pre>
 *
 * As a further improvement, notice that the left task need not even
 * exist.  Instead of creating a new one, we can iterate using the
 * original task, and add a pending count for each fork.
 *
 * <pre> {@code
 * class ForEach<E> ...
 *     public void compute() { // version 3
 *         int l = lo,  h = hi;
 *         while (h - l >= 2) {
 *             int mid = (l + h) >>> 1;
 *             addToPendingCount(1);
 *             new ForEach(this, array, op, mid, h).fork(); // right child
 *             h = mid;
 *         }
 *         if (h > l)
 *             op.apply(array[l]);
 *         tryComplete();
 *     }
 * }</pre>
 *
 * Additional improvements of such classes might entail precomputing
 * pending counts so that they can be established in constructors,
 * specializing classes for leaf steps, subdividing by say, four,
 * instead of two per iteration, and using an adaptive threshold
 * instead of always subdividing down to single elements.
 *
 * <p><b>Recording subtasks.</b> CountedCompleter tasks that combine
 * results of multiple subtasks usually need to access these results
 * in method {@link #onCompletion}. As illustrated in the following
 * class (that performs a simplified form of map-reduce where mappings
 * and reductions are all of type {@code E}), one way to do this in
 * divide and conquer designs is to have each subtask record its
 * sibling, so that it can be accessed in method {@code onCompletion}.
 * This technique applies to reductions in which the order of
 * combining left and right results does not matter; ordered
 * reductions require explicit left/right designations.  Variants of
 * other streamlinings seen in the above examples may also apply.
 *
 * <pre> {@code
 * class MyMapper<E> { E apply(E v) {  ...  } }
 * class MyReducer<E> { E apply(E x, E y) {  ...  } }
 * class MapReducer<E> extends CountedCompleter<E> {
 *     final E[] array; final MyMapper<E> mapper;
 *     final MyReducer<E> reducer; final int lo, hi;
 *     MapReducer<E> sibling;
 *     E result;
 *     MapReducer(CountedCompleter p, E[] array, MyMapper<E> mapper,
 *                MyReducer<E> reducer, int lo, int hi) {
 *         super(p);
 *         this.array = array; this.mapper = mapper;
 *         this.reducer = reducer; this.lo = lo; this.hi = hi;
 *     }
 *     public void compute() {
 *         if (hi - lo >= 2) {
 *             int mid = (lo + hi) >>> 1;
 *             MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid);
 *             MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi);
 *             left.sibling = right;
 *             right.sibling = left;
 *             setPendingCount(1); // only right is pending
 *             right.fork();
 *             left.compute();     // directly execute left
 *         }
 *         else {
 *             if (hi > lo)
 *                 result = mapper.apply(array[lo]);
 *             tryComplete();
 *         }
 *     }
 *     public void onCompletion(CountedCompleter caller) {
 *         if (caller != this) {
 *            MapReducer<E> child = (MapReducer<E>)caller;
 *            MapReducer<E> sib = child.sibling;
 *            if (sib == null || sib.result == null)
 *                result = child.result;
 *            else
 *                result = reducer.apply(child.result, sib.result);
 *         }
 *     }
 *     public E getRawResult() { return result; }
 *
 *     public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) {
 *         return new MapReducer<E>(null, array, mapper, reducer,
 *                                  0, array.length).invoke();
 *     }
 * } }</pre>
 *
 * Here, method {@code onCompletion} takes a form common to many
 * completion designs that combine results. This callback-style method
 * is triggered once per task, in either of the two different contexts
 * in which the pending count is, or becomes, zero: (1) by a task
 * itself, if its pending count is zero upon invocation of {@code
 * tryComplete}, or (2) by any of its subtasks when they complete and
 * decrement the pending count to zero. The {@code caller} argument
 * distinguishes cases.  Most often, when the caller is {@code this},
 * no action is necessary. Otherwise the caller argument can be used
 * (usually via a cast) to supply a value (and/or links to other
 * values) to be combined.  Asuuming proper use of pending counts, the
 * actions inside {@code onCompletion} occur (once) upon completion of
 * a task and its subtasks. No additional synchronization is required
 * within this method to ensure thread safety of accesses to fields of
 * this task or other completed tasks.
 *
 * <p><b>Searching.</b> A tree of CountedCompleters can search for a
 * value or property in different parts of a data structure, and
 * report a result in an {@link java.util.concurrent.AtomicReference}
 * as soon as one is found. The others can poll the result to avoid
 * unnecessary work. (You could additionally {@link #cancel} other
 * tasks, but it is usually simpler and more efficient to just let
 * them notice that the result is set and if so skip further
 * processing.)  Illustrating again with an array using full
 * partitioning (again, in practice, leaf tasks will almost always
 * process more than one element):
 *
 * <pre> {@code
 * class Searcher<E> extends CountedCompleter<E> {
 *     final E[] array; final AtomicReference<E> result; final int lo, hi;
 *     Searcher(CountedCompleter<?> p, E[] array, AtomicReference<E> result, int lo, int hi) {
 *         super(p);
 *         this.array = array; this.result = result; this.lo = lo; this.hi = hi;
 *     }
 *     public E getRawResult() { return result.get(); }
 *     public void compute() { // similar to ForEach version 3
 *         int l = lo,  h = hi;
 *         while (h - l >= 2 && result.get() == null) {
 *             int mid = (l + h) >>> 1;
 *             addToPendingCount(1);
 *             new Searcher(this, array, result, mid, h).fork();
 *             h = mid;
 *         }
 *         if (h > l && result.get() == null && matches(array[l]) &&
 *             result.compareAndSet(null, array[l]))
 *             getRoot().quietlyComplete(); // root task is now joinable
 *
 *         tryComplete(); // normally complete whether or not found
 *     }
 *     boolean matches(E e) { ... } // return true if found
 *
 *     public static <E> E search(E[] array) {
 *         return new Searcher<E>(null, array, new AtomicReference<E>(), 0, array.length).invoke();
 *     }
 *}}</pre>
 *
 * In this example, as well as others in which tasks have no other
 * effects except to compareAndSet a common result, the trailing
 * unconditional invocation of {@code tryComplete} could be made
 * conditional ({@code if (result.get() == null) tryComplete();})
 * because no further bookkeeping is required to manage completions
 * once the root task completes.
 *
 * <p><b>Triggers.</b> Some CountedCompleters are themselves never
 * forked, but instead serve as bits of plumbing in other designs;
 * including those in which the completion of one of more async tasks
 * triggers another async task. For example:
 *
 * <pre> {@code
 * class HeaderBuilder extends CountedCompleter<...> { ... }
 * class BodyBuilder extends CountedCompleter<...> { ... }
 * class PacketSender extends CountedCompleter<...> {
 *     PacketSender(...) { super(null, 1); ... } // trigger on second completion
 *     public void compute() { } // never called
 *     public void onCompletion(CountedCompleter<?> caller) { sendPacket(); }
 * }
 * // sample use:
 * PacketSender p = new PacketSender();
 * new HeaderBuilder(p, ...).fork();
 * new BodyBuilder(p, ...).fork();
 * }</pre>
 *
 * @since 1.8
 * @author Doug Lea
 */
public abstract class CountedCompleter<T> extends ForkJoinTask<T> {
    private static final long serialVersionUID = 5232453752276485070L;

    /** This task's completer, or null if none */
    final CountedCompleter<?> completer;
    /** The number of pending tasks until completion */
    volatile int pending;

    /**
     * Creates a new CountedCompleter with the given completer
     * and initial pending count.
     *
     * @param completer this tasks completer, or {@code null} if none
     * @param initialPendingCount the initial pending count
     */
    protected CountedCompleter(CountedCompleter<?> completer,
                               int initialPendingCount) {
        this.completer = completer;
        this.pending = initialPendingCount;
    }

    /**
     * Creates a new CountedCompleter with the given completer
     * and an initial pending count of zero.
     *
     * @param completer this tasks completer, or {@code null} if none
     */
    protected CountedCompleter(CountedCompleter<?> completer) {
        this.completer = completer;
    }

    /**
     * Creates a new CountedCompleter with no completer
     * and an initial pending count of zero.
     */
    protected CountedCompleter() {
        this.completer = null;
    }

    /**
     * The main computation performed by this task.
     */
    public abstract void compute();

    /**
     * Performs an action when method {@link #tryComplete} is invoked
     * and there are no pending counts, or when the unconditional
     * method {@link #complete} is invoked.  By default, this method
     * does nothing. You can distinguish cases by checking the
     * identity of the given caller argument. If not equal to {@code
     * this}, then it is typically a subtask that may contain results
     * (and/or links to other results) to combine.
     *
     * @param caller the task invoking this method (which may
     * be this task itself).
     */
    public void onCompletion(CountedCompleter<?> caller) {
    }

    /**
     * Performs an action when method {@link #completeExceptionally}
     * is invoked or method {@link #compute} throws an exception, and
     * this task has not otherwise already completed normally. On
     * entry to this method, this task {@link
     * ForkJoinTask#isCompletedAbnormally}.  The return value of this
     * method controls further propagation: If {@code true} and this
     * task has a completer, then this completer is also completed
     * exceptionally.  The default implementation of this method does
     * nothing except return {@code true}.
     *
     * @param ex the exception
     * @param caller the task invoking this method (which may
     * be this task itself).
     * @return true if this exception should be propagated to this
     * tasks completer, if one exists.
     */
    public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) {
        return true;
    }

    /**
     * Returns the completer established in this task's constructor,
     * or {@code null} if none.
     *
     * @return the completer
     */
    public final CountedCompleter<?> getCompleter() {
        return completer;
    }

    /**
     * Returns the current pending count.
     *
     * @return the current pending count
     */
    public final int getPendingCount() {
        return pending;
    }

    /**
     * Sets the pending count to the given value.
     *
     * @param count the count
     */
    public final void setPendingCount(int count) {
        pending = count;
    }

    /**
     * Adds (atomically) the given value to the pending count.
     *
     * @param delta the value to add
     */
    public final void addToPendingCount(int delta) {
        int c; // note: can replace with intrinsic in jdk8
        do {} while (!U.compareAndSwapInt(this, PENDING, c = pending, c+delta));
    }

    /**
     * Sets (atomically) the pending count to the given count only if
     * it currently holds the given expected value.
     *
     * @param expected the expected value
     * @param count the new value
     * @return true is successful
     */
    public final boolean compareAndSetPendingCount(int expected, int count) {
        return U.compareAndSwapInt(this, PENDING, expected, count);
    }

    /**
     * Returns the root of the current computation; i.e., this
     * task if it has no completer, else its completer's root.
     *
     * @return the root of the current computation
     */
    public final CountedCompleter<?> getRoot() {
        CountedCompleter<?> a = this, p;
        while ((p = a.completer) != null)
            a = p;
        return a;
    }

    /**
     * If the pending count is nonzero, decrements the count;
     * otherwise invokes {@link #onCompletion} and then similarly
     * tries to complete this task's completer, if one exists,
     * else marks this task as complete.
     */
    public final void tryComplete() {
        CountedCompleter<?> a = this, s = a;
        for (int c;;) {
            if ((c = a.pending) == 0) {
                a.onCompletion(s);
                if ((a = (s = a).completer) == null) {
                    s.quietlyComplete();
                    return;
                }
            }
            else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
                return;
        }
    }

    /**
     * Regardless of pending count, invokes {@link #onCompletion},
     * marks this task as complete and further triggers {@link
     * #tryComplete} on this task's completer, if one exists.  The
     * given rawResult is used as an argument to {@link #setRawResult}
     * before invoking {@link #onCompletion} or marking this task as
     * complete; its value is meaningful only for classes overriding
     * {@code setRawResult}.
     *
     * <p>This method may be useful when forcing completion as soon as
     * any one (versus all) of several subtask results are obtained.
     * However, in the common (and recommended) case in which {@code
     * setRawResult} is not overridden, this effect can be obtained
     * more simply using {@code getRoot().quietlyComplete();}.
     *
     * @param rawResult the raw result
     */
    public void complete(T rawResult) {
        CountedCompleter<?> p;
        setRawResult(rawResult);
        onCompletion(this);
        quietlyComplete();
        if ((p = completer) != null)
            p.tryComplete();
    }

    /**
     * Support for FJT exception propagation
     */
    void internalPropagateException(Throwable ex) {
        CountedCompleter<?> a = this, s = a;
        while (a.onExceptionalCompletion(ex, s) &&
               (a = (s = a).completer) != null && a.status >= 0)
            a.recordExceptionalCompletion(ex);
    }

    /**
     * Implements execution conventions for CountedCompleters
     */
    protected final boolean exec() {
        compute();
        return false;
    }

    /**
     * Returns the result of the computation. By default
     * returns {@code null}, which is appropriate for {@code Void}
     * actions, but in other cases should be overridden.
     *
     * @return the result of the computation
     */
    public T getRawResult() { return null; }

    /**
     * A method that result-bearing CountedCompleters may optionally
     * use to help maintain result data.  By default, does nothing.
     * If this method is overridden to update existing objects or
     * fields, then it must in general be defined to be thread-safe.
     */
    protected void setRawResult(T t) { }

    // Unsafe mechanics
    private static final sun.misc.Unsafe U;
    private static final long PENDING;
    static {
        try {
            U = sun.misc.Unsafe.getUnsafe();
            PENDING = U.objectFieldOffset
                (CountedCompleter.class.getDeclaredField("pending"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }

    /**
     * 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:	1.14
Committed:	Mon Nov 19 18:12:42 2012 UTC (11 years, 5 months ago) by dl
Branch:	MAIN
Changes since 1.13:	+118 -36 lines
Log Message:	better memory positioning