--- jsr166/src/jsr166y/ForkJoinTask.java 2010/04/18 12:51:18 1.47
+++ jsr166/src/jsr166y/ForkJoinTask.java 2011/07/01 18:30:14 1.80
@@ -1,20 +1,27 @@
/*
* 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
+ * http://creativecommons.org/publicdomain/zero/1.0/
*/
package jsr166y;
-import java.util.concurrent.*;
-
import java.io.Serializable;
import java.util.Collection;
-import java.util.Collections;
import java.util.List;
import java.util.RandomAccess;
-import java.util.Map;
-import java.util.WeakHashMap;
+import java.lang.ref.WeakReference;
+import java.lang.ref.ReferenceQueue;
+import java.util.concurrent.Callable;
+import java.util.concurrent.CancellationException;
+import java.util.concurrent.ExecutionException;
+import java.util.concurrent.Future;
+import java.util.concurrent.RejectedExecutionException;
+import java.util.concurrent.RunnableFuture;
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.TimeoutException;
+import java.util.concurrent.locks.ReentrantLock;
+import java.lang.reflect.Constructor;
/**
* Abstract base class for tasks that run within a {@link ForkJoinPool}.
@@ -28,10 +35,10 @@ import java.util.WeakHashMap;
* start other subtasks. As indicated by the name of this class,
* many programs using {@code ForkJoinTask} employ only methods
* {@link #fork} and {@link #join}, or derivatives such as {@link
- * #invokeAll}. However, this class also provides a number of other
- * methods that can come into play in advanced usages, as well as
- * extension mechanics that allow support of new forms of fork/join
- * processing.
+ * #invokeAll(ForkJoinTask...) invokeAll}. However, this class also
+ * provides a number of other methods that can come into play in
+ * advanced usages, as well as extension mechanics that allow
+ * support of new forms of fork/join processing.
*
*
A {@code ForkJoinTask} is a lightweight form of {@link Future}.
* The efficiency of {@code ForkJoinTask}s stems from a set of
@@ -58,16 +65,17 @@ import java.util.WeakHashMap;
* rethrown to callers attempting to join them. These exceptions may
* additionally include {@link RejectedExecutionException} stemming
* from internal resource exhaustion, such as failure to allocate
- * internal task queues.
+ * internal task queues. Rethrown exceptions behave in the same way as
+ * regular exceptions, but, when possible, contain stack traces (as
+ * displayed for example using {@code ex.printStackTrace()}) of both
+ * the thread that initiated the computation as well as the thread
+ * actually encountering the exception; minimally only the latter.
*
*
The primary method for awaiting completion and extracting
* results of a task is {@link #join}, but there are several variants:
* The {@link Future#get} methods support interruptible and/or timed
* waits for completion and report results using {@code Future}
- * conventions. Method {@link #helpJoin} enables callers to actively
- * execute other tasks while awaiting joins, which is sometimes more
- * efficient but only applies when all subtasks are known to be
- * strictly tree-structured. Method {@link #invoke} is semantically
+ * conventions. Method {@link #invoke} is semantically
* equivalent to {@code fork(); join()} but always attempts to begin
* execution in the current thread. The "quiet" forms of
* these methods do not extract results or report exceptions. These
@@ -103,7 +111,17 @@ import java.util.WeakHashMap;
* ForkJoinTasks (as may be determined using method {@link
* #inForkJoinPool}). Attempts to invoke them in other contexts
* result in exceptions or errors, possibly including
- * ClassCastException.
+ * {@code ClassCastException}.
+ *
+ *
Method {@link #join} and its variants are appropriate for use
+ * only when completion dependencies are acyclic; that is, the
+ * parallel computation can be described as a directed acyclic graph
+ * (DAG). Otherwise, executions may encounter a form of deadlock as
+ * tasks cyclically wait for each other. However, this framework
+ * supports other methods and techniques (for example the use of
+ * {@link Phaser}, {@link #helpQuiesce}, and {@link #complete}) that
+ * may be of use in constructing custom subclasses for problems that
+ * are not statically structured as DAGs.
*
*
Most base support methods are {@code final}, to prevent
* overriding of implementations that are intrinsically tied to the
@@ -119,15 +137,15 @@ import java.util.WeakHashMap;
* computation. Large tasks should be split into smaller subtasks,
* usually via recursive decomposition. As a very rough rule of thumb,
* a task should perform more than 100 and less than 10000 basic
- * computational steps. If tasks are too big, then parallelism cannot
- * improve throughput. If too small, then memory and internal task
- * maintenance overhead may overwhelm processing.
+ * computational steps, and should avoid indefinite looping. If tasks
+ * are too big, then parallelism cannot improve throughput. If too
+ * small, then memory and internal task maintenance overhead may
+ * overwhelm processing.
*
*
This class provides {@code adapt} methods for {@link Runnable}
* and {@link Callable}, that may be of use when mixing execution of
- * {@code ForkJoinTasks} with other kinds of tasks. When all tasks
- * are of this form, consider using a pool in
- * {@linkplain ForkJoinPool#setAsyncMode async mode}.
+ * {@code ForkJoinTasks} with other kinds of tasks. When all tasks are
+ * of this form, consider using a pool constructed in asyncMode.
*
*
ForkJoinTasks are {@code Serializable}, which enables them to be
* used in extensions such as remote execution frameworks. It is
@@ -143,307 +161,410 @@ public abstract class ForkJoinTask im
* See the internal documentation of class ForkJoinPool for a
* general implementation overview. ForkJoinTasks are mainly
* responsible for maintaining their "status" field amidst relays
- * to methods in ForkJoinWorkerThread and ForkJoinPool. The
- * methods of this class are more-or-less layered into (1) basic
- * status maintenance (2) execution and awaiting completion (3)
- * user-level methods that additionally report results. This is
- * sometimes hard to see because this file orders exported methods
- * in a way that flows well in javadocs.
- */
-
- /**
- * Run control status bits packed into a single int to minimize
- * footprint and to ensure atomicity (via CAS). Status is
- * initially zero, and takes on nonnegative values until
- * completed, upon which status holds COMPLETED. CANCELLED, or
- * EXCEPTIONAL, which use the top 3 bits. Tasks undergoing
- * blocking waits by other threads have SIGNAL_MASK bits set --
- * bit 15 for external (nonFJ) waits, and the rest a count of
- * waiting FJ threads. (This representation relies on
- * ForkJoinPool max thread limits). Signal counts are not directly
- * incremented by ForkJoinTask methods, but instead via a call to
- * requestSignal within ForkJoinPool.preJoin, once their need is
- * established.
- *
- * Completion of a stolen task with SIGNAL_MASK bits set awakens
- * any waiters via notifyAll. Even though suboptimal for some
- * purposes, we use basic builtin wait/notify to take advantage of
- * "monitor inflation" in JVMs that we would otherwise need to
- * emulate to avoid adding further per-task bookkeeping overhead.
- * We want these monitors to be "fat", i.e., not use biasing or
- * thin-lock techniques, so use some odd coding idioms that tend
- * to avoid them.
+ * to methods in ForkJoinWorkerThread and ForkJoinPool.
*
- * Note that bits 16-28 are currently unused. Also value
- * 0x80000000 is available as spare completion value.
+ * The methods of this class are more-or-less layered into
+ * (1) basic status maintenance
+ * (2) execution and awaiting completion
+ * (3) user-level methods that additionally report results.
+ * This is sometimes hard to see because this file orders exported
+ * methods in a way that flows well in javadocs.
*/
- volatile int status; // accessed directly by pool and workers
-
- private static final int COMPLETION_MASK = 0xe0000000;
- private static final int NORMAL = 0xe0000000; // == mask
- private static final int CANCELLED = 0xc0000000;
- private static final int EXCEPTIONAL = 0xa0000000;
- private static final int SIGNAL_MASK = 0x0000ffff;
- private static final int INTERNAL_SIGNAL_MASK = 0x00007fff;
- private static final int EXTERNAL_SIGNAL = 0x00008000;
- /**
- * Table of exceptions thrown by tasks, to enable reporting by
- * callers. Because exceptions are rare, we don't directly keep
- * them with task objects, but instead use a weak ref table. Note
- * that cancellation exceptions don't appear in the table, but are
- * instead recorded as status values.
- * TODO: Use ConcurrentReferenceHashMap
+ /*
+ * The status field holds run control status bits packed into a
+ * single int to minimize footprint and to ensure atomicity (via
+ * CAS). Status is initially zero, and takes on nonnegative
+ * values until completed, upon which status holds value
+ * NORMAL, CANCELLED, or EXCEPTIONAL. Tasks undergoing blocking
+ * waits by other threads have the SIGNAL bit set. Completion of
+ * a stolen task with SIGNAL set awakens any waiters via
+ * notifyAll. Even though suboptimal for some purposes, we use
+ * basic builtin wait/notify to take advantage of "monitor
+ * inflation" in JVMs that we would otherwise need to emulate to
+ * avoid adding further per-task bookkeeping overhead. We want
+ * these monitors to be "fat", i.e., not use biasing or thin-lock
+ * techniques, so use some odd coding idioms that tend to avoid
+ * them.
*/
- static final Map, Throwable> exceptionMap =
- Collections.synchronizedMap
- (new WeakHashMap, Throwable>());
- // Maintaining completion status
+ /** The run status of this task */
+ volatile int status; // accessed directly by pool and workers
+ private static final int NORMAL = -1;
+ private static final int CANCELLED = -2;
+ private static final int EXCEPTIONAL = -3;
+ private static final int SIGNAL = 1;
/**
* Marks completion and wakes up threads waiting to join this task,
* also clearing signal request bits.
*
* @param completion one of NORMAL, CANCELLED, EXCEPTIONAL
+ * @return completion status on exit
*/
- private void setCompletion(int completion) {
- int s;
- while ((s = status) >= 0) {
+ private int setCompletion(int completion) {
+ for (int s;;) {
+ if ((s = status) < 0)
+ return s;
if (UNSAFE.compareAndSwapInt(this, statusOffset, s, completion)) {
- if ((s & SIGNAL_MASK) != 0) {
- Thread t = Thread.currentThread();
- if (t instanceof ForkJoinWorkerThread)
- ((ForkJoinWorkerThread) t).pool.updateRunningCount
- (s & INTERNAL_SIGNAL_MASK);
+ if (s != 0)
synchronized (this) { notifyAll(); }
- }
- return;
+ return completion;
}
}
}
/**
- * Record exception and set exceptional completion
- */
- private void setDoneExceptionally(Throwable rex) {
- exceptionMap.put(this, rex);
- setCompletion(EXCEPTIONAL);
- }
-
- /**
- * Main internal execution method: Unless done, calls exec and
- * records completion.
+ * Tries to block a worker thread until completed or timed out.
+ * Uses Object.wait time argument conventions.
+ * May fail on contention or interrupt.
*
- * @return true if ran and completed normally
+ * @param millis if > 0, wait time.
*/
- final boolean tryExec() {
+ final void tryAwaitDone(long millis) {
+ int s;
try {
- if (status < 0 || !exec())
- return false;
- } catch (Throwable rex) {
- setDoneExceptionally(rex);
- return false;
+ if (((s = status) > 0 ||
+ (s == 0 &&
+ UNSAFE.compareAndSwapInt(this, statusOffset, 0, SIGNAL))) &&
+ status > 0) {
+ synchronized (this) {
+ if (status > 0)
+ wait(millis);
+ }
+ }
+ } catch (InterruptedException ie) {
+ // caller must check termination
}
- setCompletion(NORMAL); // must be outside try block
- return true;
}
/**
- * Increments internal signal count (thus requesting signal upon
- * completion) unless already done. Call only once per join.
- * Used by ForkJoinPool.preJoin.
- *
- * @return status
+ * Blocks a non-worker-thread until completion.
+ * @return status upon completion
*/
- final int requestSignal() {
+ private int externalAwaitDone() {
int s;
- do {} while ((s = status) >= 0 &&
- !UNSAFE.compareAndSwapInt(this, statusOffset, s, s + 1));
+ if ((s = status) >= 0) {
+ boolean interrupted = false;
+ synchronized (this) {
+ while ((s = status) >= 0) {
+ if (s == 0)
+ UNSAFE.compareAndSwapInt(this, statusOffset,
+ 0, SIGNAL);
+ else {
+ try {
+ wait();
+ } catch (InterruptedException ie) {
+ interrupted = true;
+ }
+ }
+ }
+ }
+ if (interrupted)
+ Thread.currentThread().interrupt();
+ }
return s;
}
/**
- * Sets external signal request unless already done.
- *
- * @return status
+ * Blocks a non-worker-thread until completion or interruption or timeout.
*/
- private int requestExternalSignal() {
+ private int externalInterruptibleAwaitDone(long millis)
+ throws InterruptedException {
int s;
- do {} while ((s = status) >= 0 &&
- !UNSAFE.compareAndSwapInt(this, statusOffset,
- s, s | EXTERNAL_SIGNAL));
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ if ((s = status) >= 0) {
+ synchronized (this) {
+ while ((s = status) >= 0) {
+ if (s == 0)
+ UNSAFE.compareAndSwapInt(this, statusOffset,
+ 0, SIGNAL);
+ else {
+ wait(millis);
+ if (millis > 0L)
+ break;
+ }
+ }
+ }
+ }
return s;
}
- /*
- * Awaiting completion. The four versions, internal vs external X
- * untimed vs timed, have the same overall structure but differ
- * from each other enough to defy simple integration.
- */
-
/**
- * Blocks a worker until this task is done, also maintaining pool
- * and signal counts
+ * Primary execution method for stolen tasks. Unless done, calls
+ * exec and records status if completed, but doesn't wait for
+ * completion otherwise.
*/
- private void awaitDone(ForkJoinWorkerThread w) {
+ final void doExec() {
if (status >= 0) {
- w.pool.preJoin(this);
- while (status >= 0) {
- try { // minimize lock scope
- synchronized(this) {
- if (status >= 0)
- wait();
- else { // help release; also helps avoid lock-biasing
- notifyAll();
- break;
- }
- }
- } catch (InterruptedException ie) {
- cancelIfTerminating();
- }
+ boolean completed;
+ try {
+ completed = exec();
+ } catch (Throwable rex) {
+ setExceptionalCompletion(rex);
+ return;
}
+ if (completed)
+ setCompletion(NORMAL); // must be outside try block
}
}
/**
- * Blocks a non-ForkJoin thread until this task is done.
+ * Primary mechanics for join, get, quietlyJoin.
+ * @return status upon completion
*/
- private void externalAwaitDone() {
- if (requestExternalSignal() >= 0) {
- boolean interrupted = false;
- while (status >= 0) {
+ private int doJoin() {
+ Thread t; ForkJoinWorkerThread w; int s; boolean completed;
+ if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
+ if ((s = status) < 0)
+ return s;
+ if ((w = (ForkJoinWorkerThread)t).unpushTask(this)) {
try {
- synchronized(this) {
- if (status >= 0)
- wait();
- else {
- notifyAll();
- break;
- }
- }
- } catch (InterruptedException ie) {
- interrupted = true;
+ completed = exec();
+ } catch (Throwable rex) {
+ return setExceptionalCompletion(rex);
}
+ if (completed)
+ return setCompletion(NORMAL);
}
- if (interrupted)
- Thread.currentThread().interrupt();
+ return w.joinTask(this);
}
+ else
+ return externalAwaitDone();
}
/**
- * Blocks a worker until this task is done or timeout elapses
+ * Primary mechanics for invoke, quietlyInvoke.
+ * @return status upon completion
*/
- private void timedAwaitDone(ForkJoinWorkerThread w, long nanos) {
- if (status >= 0) {
- long startTime = System.nanoTime();
- ForkJoinPool pool = w.pool;
- pool.preJoin(this);
- while (status >= 0) {
- long nt = nanos - (System.nanoTime() - startTime);
- if (nt > 0) {
- long ms = nt / 1000000;
- int ns = (int) (nt % 1000000);
- try {
- synchronized(this) { if (status >= 0) wait(ms, ns); }
- } catch (InterruptedException ie) {
- cancelIfTerminating();
- }
- }
- else {
- int s; // adjust running count on timeout
- while ((s = status) >= 0 &&
- (s & INTERNAL_SIGNAL_MASK) != 0) {
- if (UNSAFE.compareAndSwapInt(this, statusOffset,
- s, s - 1)) {
- pool.updateRunningCount(1);
- break;
- }
- }
+ private int doInvoke() {
+ int s; boolean completed;
+ if ((s = status) < 0)
+ return s;
+ try {
+ completed = exec();
+ } catch (Throwable rex) {
+ return setExceptionalCompletion(rex);
+ }
+ if (completed)
+ return setCompletion(NORMAL);
+ else
+ return doJoin();
+ }
+
+ // Exception table support
+
+ /**
+ * Table of exceptions thrown by tasks, to enable reporting by
+ * callers. Because exceptions are rare, we don't directly keep
+ * them with task objects, but instead use a weak ref table. Note
+ * that cancellation exceptions don't appear in the table, but are
+ * instead recorded as status values.
+ *
+ * Note: These statics are initialized below in static block.
+ */
+ private static final ExceptionNode[] exceptionTable;
+ private static final ReentrantLock exceptionTableLock;
+ private static final ReferenceQueue