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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/licenses/publicdomain |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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package jsr166y; |
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* Updates tend not to contend with each other except during |
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* bursts while submitted tasks begin or end. In some cases when |
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* they do contend, threads can instead do something else |
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* (usually, scan for tesks) until contention subsides. |
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* (usually, scan for tasks) until contention subsides. |
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* |
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* To enable packing, we restrict maximum parallelism to (1<<15)-1 |
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* (which is far in excess of normal operating range) to allow |
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* shutdown schemes. |
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* |
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* Wait Queuing. Unlike HPC work-stealing frameworks, we cannot |
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* let workers spin indefinitely scanning for tasks when none are |
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* can be immediately found, and we cannot start/resume workers |
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* unless there appear to be tasks available. On the other hand, |
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* we must quickly prod them into action when new tasks are |
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* submitted or generated. We park/unpark workers after placing |
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* in an event wait queue when they cannot find work. This "queue" |
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* is actually a simple Treiber stack, headed by the "id" field of |
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* ctl, plus a 15bit counter value to both wake up waiters (by |
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* advancing their count) and avoid ABA effects. Successors are |
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* held in worker field "nextWait". Queuing deals with several |
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* intrinsic races, mainly that a task-producing thread can miss |
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* seeing (and signalling) another thread that gave up looking for |
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* work but has not yet entered the wait queue. We solve this by |
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* requiring a full sweep of all workers both before (in scan()) |
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* and after (in awaitWork()) a newly waiting worker is added to |
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* the wait queue. During a rescan, the worker might release some |
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* other queued worker rather than itself, which has the same net |
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* effect. |
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* let workers spin indefinitely scanning for tasks when none can |
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* be found immediately, and we cannot start/resume workers unless |
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* there appear to be tasks available. On the other hand, we must |
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* quickly prod them into action when new tasks are submitted or |
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* generated. We park/unpark workers after placing in an event |
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* wait queue when they cannot find work. This "queue" is actually |
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* a simple Treiber stack, headed by the "id" field of ctl, plus a |
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* 15bit counter value to both wake up waiters (by advancing their |
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* count) and avoid ABA effects. Successors are held in worker |
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* field "nextWait". Queuing deals with several intrinsic races, |
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* mainly that a task-producing thread can miss seeing (and |
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* signalling) another thread that gave up looking for work but |
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* has not yet entered the wait queue. We solve this by requiring |
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* a full sweep of all workers both before (in scan()) and after |
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* (in tryAwaitWork()) a newly waiting worker is added to the wait |
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* queue. During a rescan, the worker might release some other |
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* queued worker rather than itself, which has the same net |
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* effect. Because enqueued workers may actually be rescanning |
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* rather than waiting, we set and clear the "parked" field of |
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* ForkJoinWorkerThread to reduce unnecessary calls to unpark. |
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* (Use of the parked field requires a secondary recheck to avoid |
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* missed signals.) |
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* |
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* Signalling. We create or wake up workers only when there |
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* appears to be at least one task they might be able to find and |
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* Trimming workers. To release resources after periods of lack of |
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* use, a worker starting to wait when the pool is quiescent will |
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* time out and terminate if the pool has remained quiescent for |
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* SHRINK_RATE nanosecs. |
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* SHRINK_RATE nanosecs. This will slowly propagate, eventually |
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* terminating all workers after long periods of non-use. |
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* |
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* Submissions. External submissions are maintained in an |
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* array-based queue that is structured identically to |
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* ForkJoinWorkerThread queues (which see) except for the use of |
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* submissionLock in method addSubmission. Unlike worker queues, |
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* multiple external threads can add new submissions. |
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* ForkJoinWorkerThread queues except for the use of |
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* submissionLock in method addSubmission. Unlike the case for |
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* worker queues, multiple external threads can add new |
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* submissions, so adding requires a lock. |
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* |
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* Compensation. Beyond work-stealing support and lifecycle |
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* control, the main responsibility of this framework is to take |
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* if blocking would leave less than one active (non-waiting, |
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* non-blocked) worker. Additionally, to avoid some false alarms |
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* due to GC, lagging counters, system activity, etc, compensated |
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* blocking for joins is only attempted after a number of rechecks |
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* proportional to the current apparent deficit (where retries are |
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* interspersed with Thread.yield, for good citizenship). The |
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* variable blockedCount, incremented before blocking and |
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* decremented after, is sometimes needed to distinguish cases of |
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* waiting for work vs blocking on joins or other managed sync, |
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* but both the cases are equivalent for most pool control, so we |
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* can update non-atomically. (Additionally, contention on |
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* blockedCount alleviates some contention on ctl). |
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* blocking for joins is only attempted after rechecks stabilize |
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* (retries are interspersed with Thread.yield, for good |
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* citizenship). The variable blockedCount, incremented before |
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* blocking and decremented after, is sometimes needed to |
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* distinguish cases of waiting for work vs blocking on joins or |
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* other managed sync. Both cases are equivalent for most pool |
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* control, so we can update non-atomically. (Additionally, |
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* contention on blockedCount alleviates some contention on ctl). |
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* |
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* Shutdown and Termination. A call to shutdownNow atomically sets |
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* the ctl stop bit and then (non-atomically) sets each workers |
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* negative, there is at least one waiting worker, and when e is |
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* negative, the pool is terminating. To deal with these possibly |
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* negative fields, we use casts in and out of "short" and/or |
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* signed shifts to maintain signedness. Note: AC_SHIFT is |
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* redundantly declared in ForkJoinWorkerThread in order to |
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* integrate a surplus-threads check. |
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* signed shifts to maintain signedness. |
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*/ |
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volatile long ctl; |
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|
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/** |
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* Index (mod submission queue length) of next element to take |
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* from submission queue. |
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* from submission queue. Usage is identical to that for |
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* per-worker queues -- see ForkJoinWorkerThread internal |
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* documentation. |
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*/ |
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volatile int queueBase; |
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|
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/** |
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* Index (mod submission queue length) of next element to add |
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* in submission queue. |
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* in submission queue. Usage is identical to that for |
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* per-worker queues -- see ForkJoinWorkerThread internal |
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* documentation. |
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*/ |
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int queueTop; |
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|
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private int nextWorkerIndex; |
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|
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/** |
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* SeqLock and index masking for for updates to workers array. |
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* Locked when SG_UNIT is set. Unlocking clears bit by adding |
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* SeqLock and index masking for updates to workers array. Locked |
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> |
* when SG_UNIT is set. Unlocking clears bit by adding |
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* SG_UNIT. Staleness of read-only operations can be checked by |
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* comparing scanGuard to value before the reads. The low 16 bits |
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* (i.e, anding with SMASK) hold (the smallest power of two |
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*/ |
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private boolean scan(ForkJoinWorkerThread w, int a) { |
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int g = scanGuard; // mask 0 avoids useless scans if only one active |
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int m = parallelism == 1 - a? 0 : g & SMASK; |
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int m = (parallelism == 1 - a && blockedCount == 0) ? 0 : g & SMASK; |
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ForkJoinWorkerThread[] ws = workers; |
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if (ws == null || ws.length <= m) // staleness check |
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return false; |
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} |
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|
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/** |
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* Tries to enqueue worker in wait queue and await change in |
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* worker's eventCount. Before blocking, rescans queues to avoid |
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* missed signals. If the pool is quiescent, possibly terminates |
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* worker upon exit. |
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* Tries to enqueue worker w in wait queue and await change in |
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* worker's eventCount. If the pool is quiescent and there is |
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* more than one worker, possibly terminates worker upon exit. |
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* Otherwise, before blocking, rescans queues to avoid missed |
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* signals. Upon finding work, releases at least one worker |
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* (which may be the current worker). Rescans restart upon |
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* detected staleness or failure to release due to |
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* contention. Note the unusual conventions about Thread.interrupt |
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* here and elsewhere: Because interrupts are used solely to alert |
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* threads to check termination, which is checked here anyway, we |
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* clear status (using Thread.interrupted) before any call to |
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* park, so that park does not immediately return due to status |
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* being set via some other unrelated call to interrupt in user |
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* code. |
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* |
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* @param w the calling worker |
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* @param c the ctl value on entry |
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*/ |
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private boolean tryAwaitWork(ForkJoinWorkerThread w, long c) { |
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int v = w.eventCount; |
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w.nextWait = (int)c; // w's successor record |
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w.nextWait = (int)c; // w's successor record |
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long nc = (long)(v & E_MASK) | ((c - AC_UNIT) & (AC_MASK|TC_MASK)); |
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if (ctl != c || !UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) { |
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long d = ctl; // return true if lost to a deq, to force rescan |
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> |
long d = ctl; // return true if lost to a deq, to force scan |
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return (int)d != (int)c && ((d - c) & AC_MASK) >= 0L; |
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} |
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if (parallelism + (int)(c >> AC_SHIFT) == 1 && |
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> |
for (int sc = w.stealCount; sc != 0;) { // accumulate stealCount |
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long s = stealCount; |
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if (UNSAFE.compareAndSwapLong(this, stealCountOffset, s, s + sc)) |
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sc = w.stealCount = 0; |
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else if (w.eventCount != v) |
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return true; // update next time |
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} |
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if ((!shutdown || !tryTerminate(false)) && |
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(int)c != 0 && parallelism + (int)(nc >> AC_SHIFT) == 0 && |
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blockedCount == 0 && quiescerCount == 0) |
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idleAwaitWork(w, v); // quiescent -- maybe shrink |
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< |
|
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< |
boolean rescanned = false; |
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for (int sc;;) { |
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idleAwaitWork(w, nc, c, v); // quiescent |
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> |
for (boolean rescanned = false;;) { |
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if (w.eventCount != v) |
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return true; |
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< |
if ((sc = w.stealCount) != 0) { |
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< |
long s = stealCount; // accumulate stealCount |
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< |
if (UNSAFE.compareAndSwapLong(this, stealCountOffset, s, s+sc)) |
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< |
w.stealCount = 0; |
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< |
} |
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< |
else if (!rescanned) { |
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> |
if (!rescanned) { |
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int g = scanGuard, m = g & SMASK; |
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ForkJoinWorkerThread[] ws = workers; |
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if (ws != null && m < ws.length) { |
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} |
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|
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/** |
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< |
* If pool is quiescent, checks for termination, and waits for |
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< |
* event signal for up to SHRINK_RATE nanosecs. On timeout, if ctl |
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< |
* has not changed, terminates the worker. Upon its termination |
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< |
* (see deregisterWorker), it may wake up another worker to |
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< |
* possibly repeat this process. |
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> |
* If inactivating worker w has caused pool to become |
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> |
* quiescent, check for pool termination, and wait for event |
| 844 |
> |
* for up to SHRINK_RATE nanosecs (rescans are unnecessary in |
| 845 |
> |
* this case because quiescence reflects consensus about lack |
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> |
* of work). On timeout, if ctl has not changed, terminate the |
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> |
* worker. Upon its termination (see deregisterWorker), it may |
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* wake up another worker to possibly repeat this process. |
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* |
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* @param w the calling worker |
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< |
* @param v the eventCount w must wait until changed |
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< |
*/ |
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< |
private void idleAwaitWork(ForkJoinWorkerThread w, int v) { |
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< |
ForkJoinTask.helpExpungeStaleExceptions(); // help clean weak refs |
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< |
if (shutdown) |
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< |
tryTerminate(false); |
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< |
long c = ctl; |
| 858 |
< |
long nc = (((c & (AC_MASK|TC_MASK)) + AC_UNIT) | |
| 859 |
< |
(long)(w.nextWait & E_MASK)); // ctl value to release w |
| 860 |
< |
if (w.eventCount == v && |
| 861 |
< |
parallelism + (int)(c >> AC_SHIFT) == 0 && |
| 862 |
< |
blockedCount == 0 && quiescerCount == 0) { |
| 843 |
< |
long startTime = System.nanoTime(); |
| 844 |
< |
Thread.interrupted(); |
| 845 |
< |
if (w.eventCount == v) { |
| 851 |
> |
* @param currentCtl the ctl value after enqueuing w |
| 852 |
> |
* @param prevCtl the ctl value if w terminated |
| 853 |
> |
* @param v the eventCount w awaits change |
| 854 |
> |
*/ |
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> |
private void idleAwaitWork(ForkJoinWorkerThread w, long currentCtl, |
| 856 |
> |
long prevCtl, int v) { |
| 857 |
> |
if (w.eventCount == v) { |
| 858 |
> |
if (shutdown) |
| 859 |
> |
tryTerminate(false); |
| 860 |
> |
ForkJoinTask.helpExpungeStaleExceptions(); // help clean weak refs |
| 861 |
> |
while (ctl == currentCtl) { |
| 862 |
> |
long startTime = System.nanoTime(); |
| 863 |
|
w.parked = true; |
| 864 |
< |
if (w.eventCount == v) |
| 864 |
> |
if (w.eventCount == v) // must recheck |
| 865 |
|
LockSupport.parkNanos(this, SHRINK_RATE); |
| 866 |
|
w.parked = false; |
| 867 |
< |
if (w.eventCount == v && ctl == c && |
| 868 |
< |
System.nanoTime() - startTime >= SHRINK_RATE && |
| 869 |
< |
UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) { |
| 870 |
< |
w.terminate = true; |
| 871 |
< |
w.eventCount = ((int)c + EC_UNIT) & E_MASK; |
| 867 |
> |
if (w.eventCount != v) |
| 868 |
> |
break; |
| 869 |
> |
else if (System.nanoTime() - startTime < |
| 870 |
> |
SHRINK_RATE - (SHRINK_RATE / 10)) // timing slop |
| 871 |
> |
Thread.interrupted(); // spurious wakeup |
| 872 |
> |
else if (UNSAFE.compareAndSwapLong(this, ctlOffset, |
| 873 |
> |
currentCtl, prevCtl)) { |
| 874 |
> |
w.terminate = true; // restore previous |
| 875 |
> |
w.eventCount = ((int)currentCtl + EC_UNIT) & E_MASK; |
| 876 |
> |
break; |
| 877 |
|
} |
| 878 |
|
} |
| 879 |
|
} |
| 909 |
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|
| 910 |
|
/** |
| 911 |
|
* Creates or doubles submissionQueue array. |
| 912 |
< |
* Basically identical to ForkJoinWorkerThread version |
| 912 |
> |
* Basically identical to ForkJoinWorkerThread version. |
| 913 |
|
*/ |
| 914 |
|
private void growSubmissionQueue() { |
| 915 |
|
ForkJoinTask<?>[] oldQ = submissionQueue; |
| 996 |
|
do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset, // no mask |
| 997 |
|
c = ctl, c + AC_UNIT)); |
| 998 |
|
int b; |
| 999 |
< |
do {} while(!UNSAFE.compareAndSwapInt(this, blockedCountOffset, |
| 1000 |
< |
b = blockedCount, b - 1)); |
| 999 |
> |
do {} while (!UNSAFE.compareAndSwapInt(this, blockedCountOffset, |
| 1000 |
> |
b = blockedCount, b - 1)); |
| 1001 |
|
} |
| 1002 |
|
|
| 1003 |
|
/** |
| 1014 |
|
joinMe.tryAwaitDone(0L); |
| 1015 |
|
postBlock(); |
| 1016 |
|
} |
| 1017 |
< |
if ((ctl & STOP_BIT) != 0L) |
| 1017 |
> |
else if ((ctl & STOP_BIT) != 0L) |
| 1018 |
|
joinMe.cancelIgnoringExceptions(); |
| 1019 |
|
} |
| 1020 |
|
} |
| 1149 |
|
ws[k] = w; |
| 1150 |
|
nextWorkerIndex = k + 1; |
| 1151 |
|
int m = g & SMASK; |
| 1152 |
< |
g = k >= m? ((m << 1) + 1) & SMASK : g + (SG_UNIT<<1); |
| 1152 |
> |
g = (k > m) ? ((m << 1) + 1) & SMASK : g + (SG_UNIT<<1); |
| 1153 |
|
} |
| 1154 |
|
} finally { |
| 1155 |
|
scanGuard = g; |
| 1229 |
|
if ((int)(c >> AC_SHIFT) != -parallelism) |
| 1230 |
|
return false; |
| 1231 |
|
if (!shutdown || blockedCount != 0 || quiescerCount != 0 || |
| 1232 |
< |
queueTop - queueBase > 0) { |
| 1232 |
> |
queueBase != queueTop) { |
| 1233 |
|
if (ctl == c) // staleness check |
| 1234 |
|
return false; |
| 1235 |
|
continue; |
| 1238 |
|
if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, c | STOP_BIT)) |
| 1239 |
|
startTerminating(); |
| 1240 |
|
} |
| 1241 |
< |
if ((short)(c >>> TC_SHIFT) == -parallelism) { |
| 1242 |
< |
submissionLock.lock(); |
| 1243 |
< |
termination.signalAll(); |
| 1244 |
< |
submissionLock.unlock(); |
| 1241 |
> |
if ((short)(c >>> TC_SHIFT) == -parallelism) { // signal when 0 workers |
| 1242 |
> |
final ReentrantLock lock = this.submissionLock; |
| 1243 |
> |
lock.lock(); |
| 1244 |
> |
try { |
| 1245 |
> |
termination.signalAll(); |
| 1246 |
> |
} finally { |
| 1247 |
> |
lock.unlock(); |
| 1248 |
> |
} |
| 1249 |
|
} |
| 1250 |
|
return true; |
| 1251 |
|
} |
| 1252 |
|
|
| 1253 |
|
/** |
| 1254 |
|
* Runs up to three passes through workers: (0) Setting |
| 1255 |
< |
* termination status for each worker, followed by wakeups up |
| 1256 |
< |
* queued workers (1) helping cancel tasks (2) interrupting |
| 1255 |
> |
* termination status for each worker, followed by wakeups up to |
| 1256 |
> |
* queued workers; (1) helping cancel tasks; (2) interrupting |
| 1257 |
|
* lagging threads (likely in external tasks, but possibly also |
| 1258 |
|
* blocked in joins). Each pass repeats previous steps because of |
| 1259 |
|
* potential lagging thread creation. |
| 1299 |
|
|
| 1300 |
|
/** |
| 1301 |
|
* Tries to set the termination status of waiting workers, and |
| 1302 |
< |
* then wake them up (after which they will terminate). |
| 1302 |
> |
* then wakes them up (after which they will terminate). |
| 1303 |
|
*/ |
| 1304 |
|
private void terminateWaiters() { |
| 1305 |
|
ForkJoinWorkerThread[] ws = workers; |
| 1332 |
|
*/ |
| 1333 |
|
final void addQuiescerCount(int delta) { |
| 1334 |
|
int c; |
| 1335 |
< |
do {} while(!UNSAFE.compareAndSwapInt(this, quiescerCountOffset, |
| 1336 |
< |
c = quiescerCount, c + delta)); |
| 1335 |
> |
do {} while (!UNSAFE.compareAndSwapInt(this, quiescerCountOffset, |
| 1336 |
> |
c = quiescerCount, c + delta)); |
| 1337 |
|
} |
| 1338 |
|
|
| 1339 |
|
/** |
| 1358 |
|
final int idlePerActive() { |
| 1359 |
|
// Approximate at powers of two for small values, saturate past 4 |
| 1360 |
|
int p = parallelism; |
| 1361 |
< |
int a = p + (int)(ctl >> AC_SHIFT); |
| 1362 |
< |
return (a > (p >>>= 1) ? 0 : |
| 1363 |
< |
a > (p >>>= 1) ? 1 : |
| 1364 |
< |
a > (p >>>= 1) ? 2 : |
| 1365 |
< |
a > (p >>>= 1) ? 4 : |
| 1366 |
< |
8); |
| 1361 |
> |
int a = p + (int)(ctl >> AC_SHIFT); |
| 1362 |
> |
return (a > (p >>>= 1) ? 0 : |
| 1363 |
> |
a > (p >>>= 1) ? 1 : |
| 1364 |
> |
a > (p >>>= 1) ? 2 : |
| 1365 |
> |
a > (p >>>= 1) ? 4 : |
| 1366 |
> |
8); |
| 1367 |
|
} |
| 1368 |
|
|
| 1369 |
|
// Exported methods |
| 1686 |
|
*/ |
| 1687 |
|
public int getRunningThreadCount() { |
| 1688 |
|
int r = parallelism + (int)(ctl >> AC_SHIFT); |
| 1689 |
< |
return r <= 0? 0 : r; // suppress momentarily negative values |
| 1689 |
> |
return (r <= 0) ? 0 : r; // suppress momentarily negative values |
| 1690 |
|
} |
| 1691 |
|
|
| 1692 |
|
/** |
| 1698 |
|
*/ |
| 1699 |
|
public int getActiveThreadCount() { |
| 1700 |
|
int r = parallelism + (int)(ctl >> AC_SHIFT) + blockedCount; |
| 1701 |
< |
return r <= 0? 0 : r; // suppress momentarily negative values |
| 1701 |
> |
return (r <= 0) ? 0 : r; // suppress momentarily negative values |
| 1702 |
|
} |
| 1703 |
|
|
| 1704 |
|
/** |
| 1755 |
|
|
| 1756 |
|
/** |
| 1757 |
|
* Returns an estimate of the number of tasks submitted to this |
| 1758 |
< |
* pool that have not yet begun executing. This meThod may take |
| 1758 |
> |
* pool that have not yet begun executing. This method may take |
| 1759 |
|
* time proportional to the number of submissions. |
| 1760 |
|
* |
| 1761 |
|
* @return the number of queued submissions |
| 1853 |
|
int ac = rc + blockedCount; |
| 1854 |
|
String level; |
| 1855 |
|
if ((c & STOP_BIT) != 0) |
| 1856 |
< |
level = (tc == 0)? "Terminated" : "Terminating"; |
| 1856 |
> |
level = (tc == 0) ? "Terminated" : "Terminating"; |
| 1857 |
|
else |
| 1858 |
< |
level = shutdown? "Shutting down" : "Running"; |
| 1858 |
> |
level = shutdown ? "Shutting down" : "Running"; |
| 1859 |
|
return super.toString() + |
| 1860 |
|
"[" + level + |
| 1861 |
|
", parallelism = " + pc + |
| 1992 |
|
* {@code isReleasable} must return {@code true} if blocking is |
| 1993 |
|
* not necessary. Method {@code block} blocks the current thread |
| 1994 |
|
* if necessary (perhaps internally invoking {@code isReleasable} |
| 1995 |
< |
* before actually blocking). The unusual methods in this API |
| 1996 |
< |
* accommodate synchronizers that may, but don't usually, block |
| 1997 |
< |
* for long periods. Similarly, they allow more efficient internal |
| 1998 |
< |
* handling of cases in which additional workers may be, but |
| 1999 |
< |
* usually are not, needed to ensure sufficient parallelism. |
| 2000 |
< |
* Toward this end, implementations of method {@code isReleasable} |
| 2001 |
< |
* must be amenable to repeated invocation. |
| 1995 |
> |
* before actually blocking). These actions are performed by any |
| 1996 |
> |
* thread invoking {@link ForkJoinPool#managedBlock}. The |
| 1997 |
> |
* unusual methods in this API accommodate synchronizers that may, |
| 1998 |
> |
* but don't usually, block for long periods. Similarly, they |
| 1999 |
> |
* allow more efficient internal handling of cases in which |
| 2000 |
> |
* additional workers may be, but usually are not, needed to |
| 2001 |
> |
* ensure sufficient parallelism. Toward this end, |
| 2002 |
> |
* implementations of method {@code isReleasable} must be amenable |
| 2003 |
> |
* to repeated invocation. |
| 2004 |
|
* |
| 2005 |
|
* <p>For example, here is a ManagedBlocker based on a |
| 2006 |
|
* ReentrantLock: |
| 2121 |
|
int s; |
| 2122 |
|
try { |
| 2123 |
|
UNSAFE = getUnsafe(); |
| 2124 |
< |
Class k = ForkJoinPool.class; |
| 2124 |
> |
Class<?> k = ForkJoinPool.class; |
| 2125 |
|
ctlOffset = UNSAFE.objectFieldOffset |
| 2126 |
|
(k.getDeclaredField("ctl")); |
| 2127 |
|
stealCountOffset = UNSAFE.objectFieldOffset |
| 2134 |
|
(k.getDeclaredField("scanGuard")); |
| 2135 |
|
nextWorkerNumberOffset = UNSAFE.objectFieldOffset |
| 2136 |
|
(k.getDeclaredField("nextWorkerNumber")); |
| 2137 |
< |
Class a = ForkJoinTask[].class; |
| 2137 |
> |
Class<?> a = ForkJoinTask[].class; |
| 2138 |
|
ABASE = UNSAFE.arrayBaseOffset(a); |
| 2139 |
|
s = UNSAFE.arrayIndexScale(a); |
| 2140 |
|
} catch (Exception e) { |
