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package jsr166y; |
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import java.util.concurrent.*; |
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– |
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import java.util.ArrayList; |
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import java.util.Arrays; |
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import java.util.Collection; |
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* <td ALIGN=CENTER> <b>Call from within fork/join computations</b></td> |
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* </tr> |
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* <tr> |
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* <td> <b>Arange async execution</td> |
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* <td> <b>Arrange async execution</td> |
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* <td> {@link #execute(ForkJoinTask)}</td> |
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* <td> {@link ForkJoinTask#fork}</td> |
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* </tr> |
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* Beyond work-stealing support and essential bookkeeping, the |
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* main responsibility of this framework is to take actions when |
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* one worker is waiting to join a task stolen (or always held by) |
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* another. Becauae we are multiplexing many tasks on to a pool |
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* another. Because we are multiplexing many tasks on to a pool |
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* of workers, we can't just let them block (as in Thread.join). |
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* We also cannot just reassign the joiner's run-time stack with |
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* another and replace it later, which would be a form of |
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* links to try to find such a task. |
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* |
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* Compensating: Unless there are already enough live threads, |
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* method helpMaintainParallelism() may create or or |
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* method helpMaintainParallelism() may create or |
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* re-activate a spare thread to compensate for blocked |
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* joiners until they unblock. |
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* |
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* ManagedBlocker), we may create or resume others to take their |
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* place until they unblock (see below). Implementing this |
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* requires counts of the number of "running" threads (i.e., those |
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* that are neither blocked nor artifically suspended) as well as |
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* that are neither blocked nor artificially suspended) as well as |
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* the total number. These two values are packed into one field, |
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* "workerCounts" because we need accurate snapshots when deciding |
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* to create, resume or suspend. Note however that the |
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* correspondance of these counts to reality is not guaranteed. In |
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* correspondence of these counts to reality is not guaranteed. In |
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* particular updates for unblocked threads may lag until they |
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* actually wake up. |
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* |
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* about the same time as another is needlessly being created. We |
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* counteract this and related slop in part by requiring resumed |
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* spares to immediately recheck (in preStep) to see whether they |
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* they should re-suspend. |
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* should re-suspend. |
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* |
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* 6. Killing off unneeded workers. A timeout mechanism is used to |
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* shed unused workers: The oldest (first) event queue waiter uses |
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* 7. Deciding when to create new workers. The main dynamic |
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* control in this class is deciding when to create extra threads |
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* in method helpMaintainParallelism. We would like to keep |
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* exactly #parallelism threads running, which is an impossble |
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* exactly #parallelism threads running, which is an impossible |
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* task. We always need to create one when the number of running |
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* threads would become zero and all workers are busy. Beyond |
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* this, we must rely on heuristics that work well in the the |
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* presence of transients phenomena such as GC stalls, dynamic |
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* this, we must rely on heuristics that work well in the |
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* presence of transient phenomena such as GC stalls, dynamic |
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* compilation, and wake-up lags. These transients are extremely |
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* common -- we are normally trying to fully saturate the CPUs on |
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* a machine, so almost any activity other than running tasks |
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* "while ((local = field) != 0)") which are usually the simplest |
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* way to ensure the required read orderings (which are sometimes |
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* critical). Also several occurrences of the unusual "do {} |
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* while(!cas...)" which is the simplest way to force an update of |
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* while (!cas...)" which is the simplest way to force an update of |
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* a CAS'ed variable. There are also other coding oddities that |
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* help some methods perform reasonably even when interpreted (not |
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* compiled), at the expense of some messy constructions that |
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/** |
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* The time to block in a join (see awaitJoin) before checking if |
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* a new worker should be (re)started to maintain parallelism |
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* level. The value should be short enough to maintain gloabal |
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* level. The value should be short enough to maintain global |
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* responsiveness and progress but long enough to avoid |
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* counterproductive firings during GC stalls or unrelated system |
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* activity, and to not bog down systems with continual re-firings |
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/** |
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* The wakeup interval (in nanoseconds) for the oldest worker |
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* worker waiting for an event invokes tryShutdownUnusedWorker to shrink |
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* waiting for an event invokes tryShutdownUnusedWorker to shrink |
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* the number of workers. The exact value does not matter too |
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* much, but should be long enough to slowly release resources |
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* during long periods without use without disrupting normal use. |
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private volatile long stealCount; |
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/** |
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* Encoded record of top of treiber stack of threads waiting for |
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* Encoded record of top of Treiber stack of threads waiting for |
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* events. The top 32 bits contain the count being waited for. The |
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* bottom 16 bits contains one plus the pool index of waiting |
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* worker thread. (Bits 16-31 are unused.) |
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private volatile int eventCount; |
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/** |
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* Encoded record of top of treiber stack of spare threads waiting |
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* Encoded record of top of Treiber stack of spare threads waiting |
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* for resumption. The top 16 bits contain an arbitrary count to |
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* avoid ABA effects. The bottom 16bits contains one plus the pool |
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* index of waiting worker thread. |
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} |
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/** |
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* Nulls out record of worker in workers array |
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* Nulls out record of worker in workers array. |
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*/ |
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private void forgetWorker(ForkJoinWorkerThread w) { |
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int idx = w.poolIndex; |
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// Locking helps method recordWorker avoid unecessary expansion |
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// Locking helps method recordWorker avoid unnecessary expansion |
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final ReentrantLock lock = this.workerLock; |
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lock.lock(); |
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try { |
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/** |
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* Final callback from terminating worker. Removes record of |
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* worker from array, and adjusts counts. If pool is shutting |
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* down, tries to complete terminatation. |
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* down, tries to complete termination. |
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* |
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* @param w the worker |
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*/ |
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// Maintaining parallelism |
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/** |
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* Pushes worker onto the spare stack |
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* Pushes worker onto the spare stack. |
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*/ |
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final void pushSpare(ForkJoinWorkerThread w) { |
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int ns = (++w.spareCount << SPARE_COUNT_SHIFT) | (w.poolIndex + 1); |
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UNSAFE.compareAndSwapInt(this, spareWaitersOffset, |
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sw, w.nextSpare)) { |
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int c; // increment running count before resume |
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do {} while(!UNSAFE.compareAndSwapInt |
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(this, workerCountsOffset, |
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c = workerCounts, c + ONE_RUNNING)); |
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do {} while (!UNSAFE.compareAndSwapInt |
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(this, workerCountsOffset, |
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c = workerCounts, c + ONE_RUNNING)); |
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if (w.tryUnsuspend()) |
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LockSupport.unpark(w); |
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else // back out if w was shutdown |
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* Tries to increase the number of running workers if below target |
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* parallelism: If a spare exists tries to resume it via |
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* tryResumeSpare. Otherwise, if not enough total workers or all |
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* existing workers are busy, adds a new worker. In all casses also |
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* existing workers are busy, adds a new worker. In all cases also |
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* helps wake up releasable workers waiting for work. |
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*/ |
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private void helpMaintainParallelism() { |
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w.lastEventCount = ec; // no need to wait |
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break; |
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} |
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else if (!(inactivate |= active)) |
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else if (!(inactivate |= active)) |
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eventSync(w, wec); // must inactivate before sync |
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} |
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else |
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c = eventCount, c+1); |
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eventWaiters = 0L; // clobber lists |
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spareWaiters = 0; |
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ForkJoinWorkerThread[] ws = workers; |
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int n = ws.length; |
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for (int i = 0; i < n; ++i) { |
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ForkJoinWorkerThread w = ws[i]; |
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for (ForkJoinWorkerThread w : workers) { |
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if (w != null) { |
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w.shutdown(); |
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if (passes > 0 && !w.isTerminated()) { |
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} |
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/** |
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* Clear out and cancel submissions, ignoring exceptions |
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* Clears out and cancels submissions, ignoring exceptions. |
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*/ |
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private void cancelSubmissions() { |
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ForkJoinTask<?> task; |
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// misc support for ForkJoinWorkerThread |
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/** |
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* Returns pool number |
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* Returns pool number. |
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*/ |
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final int getPoolNumber() { |
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return poolNumber; |
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} |
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/** |
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* Tries to accumulates steal count from a worker, clearing |
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* the worker's value. |
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* Tries to accumulate steal count from a worker, clearing |
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* the worker's value if successful. |
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* |
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* @return true if worker steal count now zero |
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*/ |
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*/ |
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final int idlePerActive() { |
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int pc = parallelism; // use parallelism, not rc |
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int ac = runState; // no mask -- artifically boosts during shutdown |
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int ac = runState; // no mask -- artificially boosts during shutdown |
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// Use exact results for small values, saturate past 4 |
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return pc <= ac? 0 : pc >>> 1 <= ac? 1 : pc >>> 2 <= ac? 3 : pc >>> 3; |
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return ((pc <= ac) ? 0 : |
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(pc >>> 1 <= ac) ? 1 : |
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(pc >>> 2 <= ac) ? 3 : |
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pc >>> 3); |
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} |
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// Public and protected methods |
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* use {@link #defaultForkJoinWorkerThreadFactory}. |
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* @param handler the handler for internal worker threads that |
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* terminate due to unrecoverable errors encountered while executing |
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* tasks. For default value, use <code>null</code>. |
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* tasks. For default value, use {@code null}. |
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* @param asyncMode if true, |
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* establishes local first-in-first-out scheduling mode for forked |
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* tasks that are never joined. This mode may be more appropriate |
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* than default locally stack-based mode in applications in which |
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* worker threads only process event-style asynchronous tasks. |
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* For default value, use <code>false</code>. |
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* For default value, use {@code false}. |
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* @throws IllegalArgumentException if parallelism less than or |
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* equal to zero, or greater than implementation limit |
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* @throws NullPointerException if the factory is null |
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*/ |
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public long getQueuedTaskCount() { |
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long count = 0; |
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< |
ForkJoinWorkerThread[] ws = workers; |
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< |
int n = ws.length; |
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< |
for (int i = 0; i < n; ++i) { |
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< |
ForkJoinWorkerThread w = ws[i]; |
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> |
for (ForkJoinWorkerThread w : workers) |
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if (w != null) |
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count += w.getQueueSize(); |
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– |
} |
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return count; |
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} |
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*/ |
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protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c) { |
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int count = submissionQueue.drainTo(c); |
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ForkJoinWorkerThread[] ws = workers; |
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int n = ws.length; |
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< |
for (int i = 0; i < n; ++i) { |
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< |
ForkJoinWorkerThread w = ws[i]; |
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> |
for (ForkJoinWorkerThread w : workers) |
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if (w != null) |
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count += w.drainTasksTo(c); |
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– |
} |
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return count; |
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} |
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throws InterruptedException { |
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try { |
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return termination.awaitAdvanceInterruptibly(0, timeout, unit) > 0; |
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< |
} catch(TimeoutException ex) { |
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> |
} catch (TimeoutException ex) { |
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return false; |
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} |
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} |
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private static final long eventCountOffset = |
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objectFieldOffset("eventCount", ForkJoinPool.class); |
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private static final long eventWaitersOffset = |
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< |
objectFieldOffset("eventWaiters",ForkJoinPool.class); |
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> |
objectFieldOffset("eventWaiters", ForkJoinPool.class); |
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private static final long stealCountOffset = |
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< |
objectFieldOffset("stealCount",ForkJoinPool.class); |
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> |
objectFieldOffset("stealCount", ForkJoinPool.class); |
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private static final long spareWaitersOffset = |
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< |
objectFieldOffset("spareWaiters",ForkJoinPool.class); |
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> |
objectFieldOffset("spareWaiters", ForkJoinPool.class); |
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private static long objectFieldOffset(String field, Class<?> klazz) { |
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try { |
