| /* |
| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| * |
| * This code is free software; you can redistribute it and/or modify it |
| * under the terms of the GNU General Public License version 2 only, as |
| * published by the Free Software Foundation. Oracle designates this |
| * particular file as subject to the "Classpath" exception as provided |
| * by Oracle in the LICENSE file that accompanied this code. |
| * |
| * This code is distributed in the hope that it will be useful, but WITHOUT |
| * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
| * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
| * version 2 for more details (a copy is included in the LICENSE file that |
| * accompanied this code). |
| * |
| * You should have received a copy of the GNU General Public License version |
| * 2 along with this work; if not, write to the Free Software Foundation, |
| * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
| * |
| * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
| * or visit www.oracle.com if you need additional information or have any |
| * questions. |
| */ |
| |
| /* |
| * This file is available under and governed by the GNU General Public |
| * License version 2 only, as published by the Free Software Foundation. |
| * However, the following notice accompanied the original version of this |
| * file: |
| * |
| * 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 java.util.concurrent; |
| |
| import java.util.Collection; |
| import java.util.concurrent.RejectedExecutionException; |
| |
| /** |
| * A thread managed by a {@link ForkJoinPool}, which executes |
| * {@link ForkJoinTask}s. |
| * This class is subclassable solely for the sake of adding |
| * functionality -- there are no overridable methods dealing with |
| * scheduling or execution. However, you can override initialization |
| * and termination methods surrounding the main task processing loop. |
| * If you do create such a subclass, you will also need to supply a |
| * custom {@link ForkJoinPool.ForkJoinWorkerThreadFactory} to use it |
| * in a {@code ForkJoinPool}. |
| * |
| * @since 1.7 |
| * @author Doug Lea |
| */ |
| public class ForkJoinWorkerThread extends Thread { |
| /* |
| * Overview: |
| * |
| * ForkJoinWorkerThreads are managed by ForkJoinPools and perform |
| * ForkJoinTasks. This class includes bookkeeping in support of |
| * worker activation, suspension, and lifecycle control described |
| * in more detail in the internal documentation of class |
| * ForkJoinPool. And as described further below, this class also |
| * includes special-cased support for some ForkJoinTask |
| * methods. But the main mechanics involve work-stealing: |
| * |
| * Work-stealing queues are special forms of Deques that support |
| * only three of the four possible end-operations -- push, pop, |
| * and deq (aka steal), under the further constraints that push |
| * and pop are called only from the owning thread, while deq may |
| * be called from other threads. (If you are unfamiliar with |
| * them, you probably want to read Herlihy and Shavit's book "The |
| * Art of Multiprocessor programming", chapter 16 describing these |
| * in more detail before proceeding.) The main work-stealing |
| * queue design is roughly similar to those in the papers "Dynamic |
| * Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005 |
| * (http://research.sun.com/scalable/pubs/index.html) and |
| * "Idempotent work stealing" by Michael, Saraswat, and Vechev, |
| * PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186). |
| * The main differences ultimately stem from gc requirements that |
| * we null out taken slots as soon as we can, to maintain as small |
| * a footprint as possible even in programs generating huge |
| * numbers of tasks. To accomplish this, we shift the CAS |
| * arbitrating pop vs deq (steal) from being on the indices |
| * ("queueBase" and "queueTop") to the slots themselves (mainly |
| * via method "casSlotNull()"). So, both a successful pop and deq |
| * mainly entail a CAS of a slot from non-null to null. Because |
| * we rely on CASes of references, we do not need tag bits on |
| * queueBase or queueTop. They are simple ints as used in any |
| * circular array-based queue (see for example ArrayDeque). |
| * Updates to the indices must still be ordered in a way that |
| * guarantees that queueTop == queueBase means the queue is empty, |
| * but otherwise may err on the side of possibly making the queue |
| * appear nonempty when a push, pop, or deq have not fully |
| * committed. Note that this means that the deq operation, |
| * considered individually, is not wait-free. One thief cannot |
| * successfully continue until another in-progress one (or, if |
| * previously empty, a push) completes. However, in the |
| * aggregate, we ensure at least probabilistic non-blockingness. |
| * If an attempted steal fails, a thief always chooses a different |
| * random victim target to try next. So, in order for one thief to |
| * progress, it suffices for any in-progress deq or new push on |
| * any empty queue to complete. |
| * |
| * This approach also enables support for "async mode" where local |
| * task processing is in FIFO, not LIFO order; simply by using a |
| * version of deq rather than pop when locallyFifo is true (as set |
| * by the ForkJoinPool). This allows use in message-passing |
| * frameworks in which tasks are never joined. However neither |
| * mode considers affinities, loads, cache localities, etc, so |
| * rarely provide the best possible performance on a given |
| * machine, but portably provide good throughput by averaging over |
| * these factors. (Further, even if we did try to use such |
| * information, we do not usually have a basis for exploiting |
| * it. For example, some sets of tasks profit from cache |
| * affinities, but others are harmed by cache pollution effects.) |
| * |
| * When a worker would otherwise be blocked waiting to join a |
| * task, it first tries a form of linear helping: Each worker |
| * records (in field currentSteal) the most recent task it stole |
| * from some other worker. Plus, it records (in field currentJoin) |
| * the task it is currently actively joining. Method joinTask uses |
| * these markers to try to find a worker to help (i.e., steal back |
| * a task from and execute it) that could hasten completion of the |
| * actively joined task. In essence, the joiner executes a task |
| * that would be on its own local deque had the to-be-joined task |
| * not been stolen. This may be seen as a conservative variant of |
| * the approach in Wagner & Calder "Leapfrogging: a portable |
| * technique for implementing efficient futures" SIGPLAN Notices, |
| * 1993 (http://portal.acm.org/citation.cfm?id=155354). It differs |
| * in that: (1) We only maintain dependency links across workers |
| * upon steals, rather than use per-task bookkeeping. This may |
| * require a linear scan of workers array to locate stealers, but |
| * usually doesn't because stealers leave hints (that may become |
| * stale/wrong) of where to locate them. This isolates cost to |
| * when it is needed, rather than adding to per-task overhead. |
| * (2) It is "shallow", ignoring nesting and potentially cyclic |
| * mutual steals. (3) It is intentionally racy: field currentJoin |
| * is updated only while actively joining, which means that we |
| * miss links in the chain during long-lived tasks, GC stalls etc |
| * (which is OK since blocking in such cases is usually a good |
| * idea). (4) We bound the number of attempts to find work (see |
| * MAX_HELP) and fall back to suspending the worker and if |
| * necessary replacing it with another. |
| * |
| * Efficient implementation of these algorithms currently relies |
| * on an uncomfortable amount of "Unsafe" mechanics. To maintain |
| * correct orderings, reads and writes of variable queueBase |
| * require volatile ordering. Variable queueTop need not be |
| * volatile because non-local reads always follow those of |
| * queueBase. Similarly, because they are protected by volatile |
| * queueBase reads, reads of the queue array and its slots by |
| * other threads do not need volatile load semantics, but writes |
| * (in push) require store order and CASes (in pop and deq) |
| * require (volatile) CAS semantics. (Michael, Saraswat, and |
| * Vechev's algorithm has similar properties, but without support |
| * for nulling slots.) Since these combinations aren't supported |
| * using ordinary volatiles, the only way to accomplish these |
| * efficiently is to use direct Unsafe calls. (Using external |
| * AtomicIntegers and AtomicReferenceArrays for the indices and |
| * array is significantly slower because of memory locality and |
| * indirection effects.) |
| * |
| * Further, performance on most platforms is very sensitive to |
| * placement and sizing of the (resizable) queue array. Even |
| * though these queues don't usually become all that big, the |
| * initial size must be large enough to counteract cache |
| * contention effects across multiple queues (especially in the |
| * presence of GC cardmarking). Also, to improve thread-locality, |
| * queues are initialized after starting. |
| */ |
| |
| /** |
| * Mask for pool indices encoded as shorts |
| */ |
| private static final int SMASK = 0xffff; |
| |
| /** |
| * Capacity of work-stealing queue array upon initialization. |
| * Must be a power of two. Initial size must be at least 4, but is |
| * padded to minimize cache effects. |
| */ |
| private static final int INITIAL_QUEUE_CAPACITY = 1 << 13; |
| |
| /** |
| * Maximum size for queue array. Must be a power of two |
| * less than or equal to 1 << (31 - width of array entry) to |
| * ensure lack of index wraparound, but is capped at a lower |
| * value to help users trap runaway computations. |
| */ |
| private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 24; // 16M |
| |
| /** |
| * The work-stealing queue array. Size must be a power of two. |
| * Initialized when started (as oposed to when constructed), to |
| * improve memory locality. |
| */ |
| ForkJoinTask<?>[] queue; |
| |
| /** |
| * The pool this thread works in. Accessed directly by ForkJoinTask. |
| */ |
| final ForkJoinPool pool; |
| |
| /** |
| * Index (mod queue.length) of next queue slot to push to or pop |
| * from. It is written only by owner thread, and accessed by other |
| * threads only after reading (volatile) queueBase. Both queueTop |
| * and queueBase are allowed to wrap around on overflow, but |
| * (queueTop - queueBase) still estimates size. |
| */ |
| int queueTop; |
| |
| /** |
| * Index (mod queue.length) of least valid queue slot, which is |
| * always the next position to steal from if nonempty. |
| */ |
| volatile int queueBase; |
| |
| /** |
| * The index of most recent stealer, used as a hint to avoid |
| * traversal in method helpJoinTask. This is only a hint because a |
| * worker might have had multiple steals and this only holds one |
| * of them (usually the most current). Declared non-volatile, |
| * relying on other prevailing sync to keep reasonably current. |
| */ |
| int stealHint; |
| |
| /** |
| * Index of this worker in pool array. Set once by pool before |
| * running, and accessed directly by pool to locate this worker in |
| * its workers array. |
| */ |
| final int poolIndex; |
| |
| /** |
| * Encoded record for pool task waits. Usages are always |
| * surrounded by volatile reads/writes |
| */ |
| int nextWait; |
| |
| /** |
| * Complement of poolIndex, offset by count of entries of task |
| * waits. Accessed by ForkJoinPool to manage event waiters. |
| */ |
| volatile int eventCount; |
| |
| /** |
| * Seed for random number generator for choosing steal victims. |
| * Uses Marsaglia xorshift. Must be initialized as nonzero. |
| */ |
| int seed; |
| |
| /** |
| * Number of steals. Directly accessed (and reset) by pool when |
| * idle. |
| */ |
| int stealCount; |
| |
| /** |
| * True if this worker should or did terminate |
| */ |
| volatile boolean terminate; |
| |
| /** |
| * Set to true before LockSupport.park; false on return |
| */ |
| volatile boolean parked; |
| |
| /** |
| * True if use local fifo, not default lifo, for local polling. |
| * Shadows value from ForkJoinPool. |
| */ |
| final boolean locallyFifo; |
| |
| /** |
| * The task most recently stolen from another worker (or |
| * submission queue). All uses are surrounded by enough volatile |
| * reads/writes to maintain as non-volatile. |
| */ |
| ForkJoinTask<?> currentSteal; |
| |
| /** |
| * The task currently being joined, set only when actively trying |
| * to help other stealers in helpJoinTask. All uses are surrounded |
| * by enough volatile reads/writes to maintain as non-volatile. |
| */ |
| ForkJoinTask<?> currentJoin; |
| |
| /** |
| * Creates a ForkJoinWorkerThread operating in the given pool. |
| * |
| * @param pool the pool this thread works in |
| * @throws NullPointerException if pool is null |
| */ |
| protected ForkJoinWorkerThread(ForkJoinPool pool) { |
| super(pool.nextWorkerName()); |
| this.pool = pool; |
| int k = pool.registerWorker(this); |
| poolIndex = k; |
| eventCount = ~k & SMASK; // clear wait count |
| locallyFifo = pool.locallyFifo; |
| Thread.UncaughtExceptionHandler ueh = pool.ueh; |
| if (ueh != null) |
| setUncaughtExceptionHandler(ueh); |
| setDaemon(true); |
| } |
| |
| // Public methods |
| |
| /** |
| * Returns the pool hosting this thread. |
| * |
| * @return the pool |
| */ |
| public ForkJoinPool getPool() { |
| return pool; |
| } |
| |
| /** |
| * Returns the index number of this thread in its pool. The |
| * returned value ranges from zero to the maximum number of |
| * threads (minus one) that have ever been created in the pool. |
| * This method may be useful for applications that track status or |
| * collect results per-worker rather than per-task. |
| * |
| * @return the index number |
| */ |
| public int getPoolIndex() { |
| return poolIndex; |
| } |
| |
| // Randomization |
| |
| /** |
| * Computes next value for random victim probes and backoffs. |
| * Scans don't require a very high quality generator, but also not |
| * a crummy one. Marsaglia xor-shift is cheap and works well |
| * enough. Note: This is manually inlined in FJP.scan() to avoid |
| * writes inside busy loops. |
| */ |
| private int nextSeed() { |
| int r = seed; |
| r ^= r << 13; |
| r ^= r >>> 17; |
| r ^= r << 5; |
| return seed = r; |
| } |
| |
| // Run State management |
| |
| /** |
| * Initializes internal state after construction but before |
| * processing any tasks. If you override this method, you must |
| * invoke {@code super.onStart()} at the beginning of the method. |
| * Initialization requires care: Most fields must have legal |
| * default values, to ensure that attempted accesses from other |
| * threads work correctly even before this thread starts |
| * processing tasks. |
| */ |
| protected void onStart() { |
| queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY]; |
| int r = pool.workerSeedGenerator.nextInt(); |
| seed = (r == 0) ? 1 : r; // must be nonzero |
| } |
| |
| /** |
| * Performs cleanup associated with termination of this worker |
| * thread. If you override this method, you must invoke |
| * {@code super.onTermination} at the end of the overridden method. |
| * |
| * @param exception the exception causing this thread to abort due |
| * to an unrecoverable error, or {@code null} if completed normally |
| */ |
| protected void onTermination(Throwable exception) { |
| try { |
| terminate = true; |
| cancelTasks(); |
| pool.deregisterWorker(this, exception); |
| } catch (Throwable ex) { // Shouldn't ever happen |
| if (exception == null) // but if so, at least rethrown |
| exception = ex; |
| } finally { |
| if (exception != null) |
| UNSAFE.throwException(exception); |
| } |
| } |
| |
| /** |
| * This method is required to be public, but should never be |
| * called explicitly. It performs the main run loop to execute |
| * {@link ForkJoinTask}s. |
| */ |
| public void run() { |
| Throwable exception = null; |
| try { |
| onStart(); |
| pool.work(this); |
| } catch (Throwable ex) { |
| exception = ex; |
| } finally { |
| onTermination(exception); |
| } |
| } |
| |
| /* |
| * Intrinsics-based atomic writes for queue slots. These are |
| * basically the same as methods in AtomicReferenceArray, but |
| * specialized for (1) ForkJoinTask elements (2) requirement that |
| * nullness and bounds checks have already been performed by |
| * callers and (3) effective offsets are known not to overflow |
| * from int to long (because of MAXIMUM_QUEUE_CAPACITY). We don't |
| * need corresponding version for reads: plain array reads are OK |
| * because they are protected by other volatile reads and are |
| * confirmed by CASes. |
| * |
| * Most uses don't actually call these methods, but instead |
| * contain inlined forms that enable more predictable |
| * optimization. We don't define the version of write used in |
| * pushTask at all, but instead inline there a store-fenced array |
| * slot write. |
| * |
| * Also in most methods, as a performance (not correctness) issue, |
| * we'd like to encourage compilers not to arbitrarily postpone |
| * setting queueTop after writing slot. Currently there is no |
| * intrinsic for arranging this, but using Unsafe putOrderedInt |
| * may be a preferable strategy on some compilers even though its |
| * main effect is a pre-, not post- fence. To simplify possible |
| * changes, the option is left in comments next to the associated |
| * assignments. |
| */ |
| |
| /** |
| * CASes slot i of array q from t to null. Caller must ensure q is |
| * non-null and index is in range. |
| */ |
| private static final boolean casSlotNull(ForkJoinTask<?>[] q, int i, |
| ForkJoinTask<?> t) { |
| return UNSAFE.compareAndSwapObject(q, (i << ASHIFT) + ABASE, t, null); |
| } |
| |
| /** |
| * Performs a volatile write of the given task at given slot of |
| * array q. Caller must ensure q is non-null and index is in |
| * range. This method is used only during resets and backouts. |
| */ |
| private static final void writeSlot(ForkJoinTask<?>[] q, int i, |
| ForkJoinTask<?> t) { |
| UNSAFE.putObjectVolatile(q, (i << ASHIFT) + ABASE, t); |
| } |
| |
| // queue methods |
| |
| /** |
| * Pushes a task. Call only from this thread. |
| * |
| * @param t the task. Caller must ensure non-null. |
| */ |
| final void pushTask(ForkJoinTask<?> t) { |
| ForkJoinTask<?>[] q; int s, m; |
| if ((q = queue) != null) { // ignore if queue removed |
| long u = (((s = queueTop) & (m = q.length - 1)) << ASHIFT) + ABASE; |
| UNSAFE.putOrderedObject(q, u, t); |
| queueTop = s + 1; // or use putOrderedInt |
| if ((s -= queueBase) <= 2) |
| pool.signalWork(); |
| else if (s == m) |
| growQueue(); |
| } |
| } |
| |
| /** |
| * Creates or doubles queue array. Transfers elements by |
| * emulating steals (deqs) from old array and placing, oldest |
| * first, into new array. |
| */ |
| private void growQueue() { |
| ForkJoinTask<?>[] oldQ = queue; |
| int size = oldQ != null ? oldQ.length << 1 : INITIAL_QUEUE_CAPACITY; |
| if (size > MAXIMUM_QUEUE_CAPACITY) |
| throw new RejectedExecutionException("Queue capacity exceeded"); |
| if (size < INITIAL_QUEUE_CAPACITY) |
| size = INITIAL_QUEUE_CAPACITY; |
| ForkJoinTask<?>[] q = queue = new ForkJoinTask<?>[size]; |
| int mask = size - 1; |
| int top = queueTop; |
| int oldMask; |
| if (oldQ != null && (oldMask = oldQ.length - 1) >= 0) { |
| for (int b = queueBase; b != top; ++b) { |
| long u = ((b & oldMask) << ASHIFT) + ABASE; |
| Object x = UNSAFE.getObjectVolatile(oldQ, u); |
| if (x != null && UNSAFE.compareAndSwapObject(oldQ, u, x, null)) |
| UNSAFE.putObjectVolatile |
| (q, ((b & mask) << ASHIFT) + ABASE, x); |
| } |
| } |
| } |
| |
| /** |
| * Tries to take a task from the base of the queue, failing if |
| * empty or contended. Note: Specializations of this code appear |
| * in locallyDeqTask and elsewhere. |
| * |
| * @return a task, or null if none or contended |
| */ |
| final ForkJoinTask<?> deqTask() { |
| ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i; |
| if (queueTop != (b = queueBase) && |
| (q = queue) != null && // must read q after b |
| (i = (q.length - 1) & b) >= 0 && |
| (t = q[i]) != null && queueBase == b && |
| UNSAFE.compareAndSwapObject(q, (i << ASHIFT) + ABASE, t, null)) { |
| queueBase = b + 1; |
| return t; |
| } |
| return null; |
| } |
| |
| /** |
| * Tries to take a task from the base of own queue. Called only |
| * by this thread. |
| * |
| * @return a task, or null if none |
| */ |
| final ForkJoinTask<?> locallyDeqTask() { |
| ForkJoinTask<?> t; int m, b, i; |
| ForkJoinTask<?>[] q = queue; |
| if (q != null && (m = q.length - 1) >= 0) { |
| while (queueTop != (b = queueBase)) { |
| if ((t = q[i = m & b]) != null && |
| queueBase == b && |
| UNSAFE.compareAndSwapObject(q, (i << ASHIFT) + ABASE, |
| t, null)) { |
| queueBase = b + 1; |
| return t; |
| } |
| } |
| } |
| return null; |
| } |
| |
| /** |
| * Returns a popped task, or null if empty. |
| * Called only by this thread. |
| */ |
| private ForkJoinTask<?> popTask() { |
| int m; |
| ForkJoinTask<?>[] q = queue; |
| if (q != null && (m = q.length - 1) >= 0) { |
| for (int s; (s = queueTop) != queueBase;) { |
| int i = m & --s; |
| long u = (i << ASHIFT) + ABASE; // raw offset |
| ForkJoinTask<?> t = q[i]; |
| if (t == null) // lost to stealer |
| break; |
| if (UNSAFE.compareAndSwapObject(q, u, t, null)) { |
| queueTop = s; // or putOrderedInt |
| return t; |
| } |
| } |
| } |
| return null; |
| } |
| |
| /** |
| * Specialized version of popTask to pop only if topmost element |
| * is the given task. Called only by this thread. |
| * |
| * @param t the task. Caller must ensure non-null. |
| */ |
| final boolean unpushTask(ForkJoinTask<?> t) { |
| ForkJoinTask<?>[] q; |
| int s; |
| if ((q = queue) != null && (s = queueTop) != queueBase && |
| UNSAFE.compareAndSwapObject |
| (q, (((q.length - 1) & --s) << ASHIFT) + ABASE, t, null)) { |
| queueTop = s; // or putOrderedInt |
| return true; |
| } |
| return false; |
| } |
| |
| /** |
| * Returns next task, or null if empty or contended. |
| */ |
| final ForkJoinTask<?> peekTask() { |
| int m; |
| ForkJoinTask<?>[] q = queue; |
| if (q == null || (m = q.length - 1) < 0) |
| return null; |
| int i = locallyFifo ? queueBase : (queueTop - 1); |
| return q[i & m]; |
| } |
| |
| // Support methods for ForkJoinPool |
| |
| /** |
| * Runs the given task, plus any local tasks until queue is empty |
| */ |
| final void execTask(ForkJoinTask<?> t) { |
| currentSteal = t; |
| for (;;) { |
| if (t != null) |
| t.doExec(); |
| if (queueTop == queueBase) |
| break; |
| t = locallyFifo ? locallyDeqTask() : popTask(); |
| } |
| ++stealCount; |
| currentSteal = null; |
| } |
| |
| /** |
| * Removes and cancels all tasks in queue. Can be called from any |
| * thread. |
| */ |
| final void cancelTasks() { |
| ForkJoinTask<?> cj = currentJoin; // try to cancel ongoing tasks |
| if (cj != null && cj.status >= 0) |
| cj.cancelIgnoringExceptions(); |
| ForkJoinTask<?> cs = currentSteal; |
| if (cs != null && cs.status >= 0) |
| cs.cancelIgnoringExceptions(); |
| while (queueBase != queueTop) { |
| ForkJoinTask<?> t = deqTask(); |
| if (t != null) |
| t.cancelIgnoringExceptions(); |
| } |
| } |
| |
| /** |
| * Drains tasks to given collection c. |
| * |
| * @return the number of tasks drained |
| */ |
| final int drainTasksTo(Collection<? super ForkJoinTask<?>> c) { |
| int n = 0; |
| while (queueBase != queueTop) { |
| ForkJoinTask<?> t = deqTask(); |
| if (t != null) { |
| c.add(t); |
| ++n; |
| } |
| } |
| return n; |
| } |
| |
| // Support methods for ForkJoinTask |
| |
| /** |
| * Returns an estimate of the number of tasks in the queue. |
| */ |
| final int getQueueSize() { |
| return queueTop - queueBase; |
| } |
| |
| /** |
| * Gets and removes a local task. |
| * |
| * @return a task, if available |
| */ |
| final ForkJoinTask<?> pollLocalTask() { |
| return locallyFifo ? locallyDeqTask() : popTask(); |
| } |
| |
| /** |
| * Gets and removes a local or stolen task. |
| * |
| * @return a task, if available |
| */ |
| final ForkJoinTask<?> pollTask() { |
| ForkJoinWorkerThread[] ws; |
| ForkJoinTask<?> t = pollLocalTask(); |
| if (t != null || (ws = pool.workers) == null) |
| return t; |
| int n = ws.length; // cheap version of FJP.scan |
| int steps = n << 1; |
| int r = nextSeed(); |
| int i = 0; |
| while (i < steps) { |
| ForkJoinWorkerThread w = ws[(i++ + r) & (n - 1)]; |
| if (w != null && w.queueBase != w.queueTop && w.queue != null) { |
| if ((t = w.deqTask()) != null) |
| return t; |
| i = 0; |
| } |
| } |
| return null; |
| } |
| |
| /** |
| * The maximum stolen->joining link depth allowed in helpJoinTask, |
| * as well as the maximum number of retries (allowing on average |
| * one staleness retry per level) per attempt to instead try |
| * compensation. Depths for legitimate chains are unbounded, but |
| * we use a fixed constant to avoid (otherwise unchecked) cycles |
| * and bound staleness of traversal parameters at the expense of |
| * sometimes blocking when we could be helping. |
| */ |
| private static final int MAX_HELP = 16; |
| |
| /** |
| * Possibly runs some tasks and/or blocks, until joinMe is done. |
| * |
| * @param joinMe the task to join |
| * @return completion status on exit |
| */ |
| final int joinTask(ForkJoinTask<?> joinMe) { |
| ForkJoinTask<?> prevJoin = currentJoin; |
| currentJoin = joinMe; |
| for (int s, retries = MAX_HELP;;) { |
| if ((s = joinMe.status) < 0) { |
| currentJoin = prevJoin; |
| return s; |
| } |
| if (retries > 0) { |
| if (queueTop != queueBase) { |
| if (!localHelpJoinTask(joinMe)) |
| retries = 0; // cannot help |
| } |
| else if (retries == MAX_HELP >>> 1) { |
| --retries; // check uncommon case |
| if (tryDeqAndExec(joinMe) >= 0) |
| Thread.yield(); // for politeness |
| } |
| else |
| retries = helpJoinTask(joinMe) ? MAX_HELP : retries - 1; |
| } |
| else { |
| retries = MAX_HELP; // restart if not done |
| pool.tryAwaitJoin(joinMe); |
| } |
| } |
| } |
| |
| /** |
| * If present, pops and executes the given task, or any other |
| * cancelled task |
| * |
| * @return false if any other non-cancelled task exists in local queue |
| */ |
| private boolean localHelpJoinTask(ForkJoinTask<?> joinMe) { |
| int s, i; ForkJoinTask<?>[] q; ForkJoinTask<?> t; |
| if ((s = queueTop) != queueBase && (q = queue) != null && |
| (i = (q.length - 1) & --s) >= 0 && |
| (t = q[i]) != null) { |
| if (t != joinMe && t.status >= 0) |
| return false; |
| if (UNSAFE.compareAndSwapObject |
| (q, (i << ASHIFT) + ABASE, t, null)) { |
| queueTop = s; // or putOrderedInt |
| t.doExec(); |
| } |
| } |
| return true; |
| } |
| |
| /** |
| * Tries to locate and execute tasks for a stealer of the given |
| * task, or in turn one of its stealers, Traces |
| * currentSteal->currentJoin links looking for a thread working on |
| * a descendant of the given task and with a non-empty queue to |
| * steal back and execute tasks from. The implementation is very |
| * branchy to cope with potential inconsistencies or loops |
| * encountering chains that are stale, unknown, or of length |
| * greater than MAX_HELP links. All of these cases are dealt with |
| * by just retrying by caller. |
| * |
| * @param joinMe the task to join |
| * @param canSteal true if local queue is empty |
| * @return true if ran a task |
| */ |
| private boolean helpJoinTask(ForkJoinTask<?> joinMe) { |
| boolean helped = false; |
| int m = pool.scanGuard & SMASK; |
| ForkJoinWorkerThread[] ws = pool.workers; |
| if (ws != null && ws.length > m && joinMe.status >= 0) { |
| int levels = MAX_HELP; // remaining chain length |
| ForkJoinTask<?> task = joinMe; // base of chain |
| outer:for (ForkJoinWorkerThread thread = this;;) { |
| // Try to find v, the stealer of task, by first using hint |
| ForkJoinWorkerThread v = ws[thread.stealHint & m]; |
| if (v == null || v.currentSteal != task) { |
| for (int j = 0; ;) { // search array |
| if ((v = ws[j]) != null && v.currentSteal == task) { |
| thread.stealHint = j; |
| break; // save hint for next time |
| } |
| if (++j > m) |
| break outer; // can't find stealer |
| } |
| } |
| // Try to help v, using specialized form of deqTask |
| for (;;) { |
| ForkJoinTask<?>[] q; int b, i; |
| if (joinMe.status < 0) |
| break outer; |
| if ((b = v.queueBase) == v.queueTop || |
| (q = v.queue) == null || |
| (i = (q.length-1) & b) < 0) |
| break; // empty |
| long u = (i << ASHIFT) + ABASE; |
| ForkJoinTask<?> t = q[i]; |
| if (task.status < 0) |
| break outer; // stale |
| if (t != null && v.queueBase == b && |
| UNSAFE.compareAndSwapObject(q, u, t, null)) { |
| v.queueBase = b + 1; |
| v.stealHint = poolIndex; |
| ForkJoinTask<?> ps = currentSteal; |
| currentSteal = t; |
| t.doExec(); |
| currentSteal = ps; |
| helped = true; |
| } |
| } |
| // Try to descend to find v's stealer |
| ForkJoinTask<?> next = v.currentJoin; |
| if (--levels > 0 && task.status >= 0 && |
| next != null && next != task) { |
| task = next; |
| thread = v; |
| } |
| else |
| break; // max levels, stale, dead-end, or cyclic |
| } |
| } |
| return helped; |
| } |
| |
| /** |
| * Performs an uncommon case for joinTask: If task t is at base of |
| * some workers queue, steals and executes it. |
| * |
| * @param t the task |
| * @return t's status |
| */ |
| private int tryDeqAndExec(ForkJoinTask<?> t) { |
| int m = pool.scanGuard & SMASK; |
| ForkJoinWorkerThread[] ws = pool.workers; |
| if (ws != null && ws.length > m && t.status >= 0) { |
| for (int j = 0; j <= m; ++j) { |
| ForkJoinTask<?>[] q; int b, i; |
| ForkJoinWorkerThread v = ws[j]; |
| if (v != null && |
| (b = v.queueBase) != v.queueTop && |
| (q = v.queue) != null && |
| (i = (q.length - 1) & b) >= 0 && |
| q[i] == t) { |
| long u = (i << ASHIFT) + ABASE; |
| if (v.queueBase == b && |
| UNSAFE.compareAndSwapObject(q, u, t, null)) { |
| v.queueBase = b + 1; |
| v.stealHint = poolIndex; |
| ForkJoinTask<?> ps = currentSteal; |
| currentSteal = t; |
| t.doExec(); |
| currentSteal = ps; |
| } |
| break; |
| } |
| } |
| } |
| return t.status; |
| } |
| |
| /** |
| * Implements ForkJoinTask.getSurplusQueuedTaskCount(). Returns |
| * an estimate of the number of tasks, offset by a function of |
| * number of idle workers. |
| * |
| * This method provides a cheap heuristic guide for task |
| * partitioning when programmers, frameworks, tools, or languages |
| * have little or no idea about task granularity. In essence by |
| * offering this method, we ask users only about tradeoffs in |
| * overhead vs expected throughput and its variance, rather than |
| * how finely to partition tasks. |
| * |
| * In a steady state strict (tree-structured) computation, each |
| * thread makes available for stealing enough tasks for other |
| * threads to remain active. Inductively, if all threads play by |
| * the same rules, each thread should make available only a |
| * constant number of tasks. |
| * |
| * The minimum useful constant is just 1. But using a value of 1 |
| * would require immediate replenishment upon each steal to |
| * maintain enough tasks, which is infeasible. Further, |
| * partitionings/granularities of offered tasks should minimize |
| * steal rates, which in general means that threads nearer the top |
| * of computation tree should generate more than those nearer the |
| * bottom. In perfect steady state, each thread is at |
| * approximately the same level of computation tree. However, |
| * producing extra tasks amortizes the uncertainty of progress and |
| * diffusion assumptions. |
| * |
| * So, users will want to use values larger, but not much larger |
| * than 1 to both smooth over transient shortages and hedge |
| * against uneven progress; as traded off against the cost of |
| * extra task overhead. We leave the user to pick a threshold |
| * value to compare with the results of this call to guide |
| * decisions, but recommend values such as 3. |
| * |
| * When all threads are active, it is on average OK to estimate |
| * surplus strictly locally. In steady-state, if one thread is |
| * maintaining say 2 surplus tasks, then so are others. So we can |
| * just use estimated queue length (although note that (queueTop - |
| * queueBase) can be an overestimate because of stealers lagging |
| * increments of queueBase). However, this strategy alone leads |
| * to serious mis-estimates in some non-steady-state conditions |
| * (ramp-up, ramp-down, other stalls). We can detect many of these |
| * by further considering the number of "idle" threads, that are |
| * known to have zero queued tasks, so compensate by a factor of |
| * (#idle/#active) threads. |
| */ |
| final int getEstimatedSurplusTaskCount() { |
| return queueTop - queueBase - pool.idlePerActive(); |
| } |
| |
| /** |
| * Runs tasks until {@code pool.isQuiescent()}. We piggyback on |
| * pool's active count ctl maintenance, but rather than blocking |
| * when tasks cannot be found, we rescan until all others cannot |
| * find tasks either. The bracketing by pool quiescerCounts |
| * updates suppresses pool auto-shutdown mechanics that could |
| * otherwise prematurely terminate the pool because all threads |
| * appear to be inactive. |
| */ |
| final void helpQuiescePool() { |
| boolean active = true; |
| ForkJoinTask<?> ps = currentSteal; // to restore below |
| ForkJoinPool p = pool; |
| p.addQuiescerCount(1); |
| for (;;) { |
| ForkJoinWorkerThread[] ws = p.workers; |
| ForkJoinWorkerThread v = null; |
| int n; |
| if (queueTop != queueBase) |
| v = this; |
| else if (ws != null && (n = ws.length) > 1) { |
| ForkJoinWorkerThread w; |
| int r = nextSeed(); // cheap version of FJP.scan |
| int steps = n << 1; |
| for (int i = 0; i < steps; ++i) { |
| if ((w = ws[(i + r) & (n - 1)]) != null && |
| w.queueBase != w.queueTop) { |
| v = w; |
| break; |
| } |
| } |
| } |
| if (v != null) { |
| ForkJoinTask<?> t; |
| if (!active) { |
| active = true; |
| p.addActiveCount(1); |
| } |
| if ((t = (v != this) ? v.deqTask() : |
| locallyFifo ? locallyDeqTask() : popTask()) != null) { |
| currentSteal = t; |
| t.doExec(); |
| currentSteal = ps; |
| } |
| } |
| else { |
| if (active) { |
| active = false; |
| p.addActiveCount(-1); |
| } |
| if (p.isQuiescent()) { |
| p.addActiveCount(1); |
| p.addQuiescerCount(-1); |
| break; |
| } |
| } |
| } |
| } |
| |
| // Unsafe mechanics |
| private static final sun.misc.Unsafe UNSAFE; |
| private static final long ABASE; |
| private static final int ASHIFT; |
| |
| static { |
| int s; |
| try { |
| UNSAFE = sun.misc.Unsafe.getUnsafe(); |
| Class a = ForkJoinTask[].class; |
| ABASE = UNSAFE.arrayBaseOffset(a); |
| s = UNSAFE.arrayIndexScale(a); |
| } catch (Exception e) { |
| throw new Error(e); |
| } |
| if ((s & (s-1)) != 0) |
| throw new Error("data type scale not a power of two"); |
| ASHIFT = 31 - Integer.numberOfLeadingZeros(s); |
| } |
| |
| } |
