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8215363: needless signals in ForkJoinPool

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Reviewed-by: martin, chegar, dholmes
This commit is contained in:
Doug Lea 2019-02-15 11:18:01 -08:00
parent a7ab4d7bd3
commit bb6d423ab8
1 changed files with 60 additions and 51 deletions

111
src/java.base/share/classes/java/util/concurrent/ForkJoinPool.java

@ -445,8 +445,7 @@ public class ForkJoinPool extends AbstractExecutorService {
* if to its current value). This would be extremely costly. So
* we relax it in several ways: (1) Producers only signal when
* their queue is possibly empty at some point during a push
* operation (which requires conservatively checking size zero or
* one to cover races). (2) Other workers propagate this signal
* operation. (2) Other workers propagate this signal
* when they find tasks in a queue with size greater than one. (3)
* Workers only enqueue after scanning (see below) and not finding
* any tasks. (4) Rather than CASing ctl to its current value in
@ -762,10 +761,8 @@ public class ForkJoinPool extends AbstractExecutorService {
/**
* The maximum number of top-level polls per worker before
* checking other queues, expressed as a bit shift to, in effect,
* multiply by pool size, and then use as random value mask, so
* average bound is about poolSize*(1<<TOP_BOUND_SHIFT). See
* above for rationale.
* checking other queues, expressed as a bit shift. See above for
* rationale.
*/
static final int TOP_BOUND_SHIFT = 10;
@ -841,18 +838,17 @@ public class ForkJoinPool extends AbstractExecutorService {
*/
final void push(ForkJoinTask<?> task) {
ForkJoinTask<?>[] a;
int s = top, d, cap, m;
int s = top, d = s - base, cap, m;
ForkJoinPool p = pool;
if ((a = array) != null && (cap = a.length) > 0) {
QA.setRelease(a, (m = cap - 1) & s, task);
top = s + 1;
if (((d = s - (int)BASE.getAcquire(this)) & ~1) == 0 &&
p != null) { // size 0 or 1
VarHandle.fullFence();
p.signalWork();
}
else if (d == m)
if (d == m)
growArray(false);
else if (QA.getAcquire(a, m & (s - 1)) == null && p != null) {
VarHandle.fullFence(); // was empty
p.signalWork(null);
}
}
}
@ -863,16 +859,16 @@ public class ForkJoinPool extends AbstractExecutorService {
final boolean lockedPush(ForkJoinTask<?> task) {
ForkJoinTask<?>[] a;
boolean signal = false;
int s = top, b = base, cap, d;
int s = top, d = s - base, cap, m;
if ((a = array) != null && (cap = a.length) > 0) {
a[(cap - 1) & s] = task;
a[(m = (cap - 1)) & s] = task;
top = s + 1;
if (b - s + cap - 1 == 0)
if (d == m)
growArray(true);
else {
phase = 0; // full volatile unlock
if (((s - base) & ~1) == 0) // size 0 or 1
signal = true;
if (a[m & (s - 1)] == null)
signal = true; // was empty
}
}
return signal;
@ -1014,25 +1010,30 @@ public class ForkJoinPool extends AbstractExecutorService {
* queue, up to bound n (to avoid infinite unfairness).
*/
final void topLevelExec(ForkJoinTask<?> t, WorkQueue q, int n) {
if (t != null && q != null) { // hoist checks
int nstolen = 1;
for (;;) {
int nstolen = 1;
for (int j = 0;;) {
if (t != null)
t.doExec();
if (n-- < 0)
break;
else if ((t = nextLocalTask()) == null) {
if ((t = q.poll()) == null)
break;
else
++nstolen;
}
if (j++ <= n)
t = nextLocalTask();
else {
j = 0;
t = null;
}
if (t == null) {
if (q != null && (t = q.poll()) != null) {
++nstolen;
j = 0;
}
else if (j != 0)
break;
}
ForkJoinWorkerThread thread = owner;
nsteals += nstolen;
source = 0;
if (thread != null)
thread.afterTopLevelExec();
}
ForkJoinWorkerThread thread = owner;
nsteals += nstolen;
source = 0;
if (thread != null)
thread.afterTopLevelExec();
}
/**
@ -1455,7 +1456,7 @@ public class ForkJoinPool extends AbstractExecutorService {
if (!tryTerminate(false, false) && // possibly replace worker
w != null && w.array != null) // avoid repeated failures
signalWork();
signalWork(null);
if (ex == null) // help clean on way out
ForkJoinTask.helpExpungeStaleExceptions();
@ -1465,8 +1466,9 @@ public class ForkJoinPool extends AbstractExecutorService {
/**
* Tries to create or release a worker if too few are running.
* @param q if non-null recheck if empty on CAS failure
*/
final void signalWork() {
final void signalWork(WorkQueue q) {
for (;;) {
long c; int sp; WorkQueue[] ws; int i; WorkQueue v;
if ((c = ctl) >= 0L) // enough workers
@ -1493,6 +1495,8 @@ public class ForkJoinPool extends AbstractExecutorService {
LockSupport.unpark(vt);
break;
}
else if (q != null && q.isEmpty()) // no need to retry
break;
}
}
}
@ -1613,19 +1617,24 @@ public class ForkJoinPool extends AbstractExecutorService {
else if (rc <= 0 && (md & SHUTDOWN) != 0 &&
tryTerminate(false, false))
break; // quiescent shutdown
else if (rc <= 0 && pred != 0 && phase == (int)c) {
long nc = (UC_MASK & (c - TC_UNIT)) | (SP_MASK & pred);
long d = keepAlive + System.currentTimeMillis();
LockSupport.parkUntil(this, d);
if (ctl == c && // drop on timeout if all idle
d - System.currentTimeMillis() <= TIMEOUT_SLOP &&
CTL.compareAndSet(this, c, nc)) {
w.phase = QUIET;
break;
else if (w.phase < 0) {
if (rc <= 0 && pred != 0 && phase == (int)c) {
long nc = (UC_MASK & (c - TC_UNIT)) | (SP_MASK & pred);
long d = keepAlive + System.currentTimeMillis();
LockSupport.parkUntil(this, d);
if (ctl == c && // drop on timeout if all idle
d - System.currentTimeMillis() <= TIMEOUT_SLOP &&
CTL.compareAndSet(this, c, nc)) {
w.phase = QUIET;
break;
}
}
else {
LockSupport.park(this);
if (w.phase < 0) // one spurious wakeup check
LockSupport.park(this);
}
}
else if (w.phase < 0)
LockSupport.park(this); // OK if spuriously woken
w.source = 0; // disable signal
}
}
@ -1651,10 +1660,10 @@ public class ForkJoinPool extends AbstractExecutorService {
QA.compareAndSet(a, k, t, null)) {
q.base = b;
w.source = qid;
if (q.top - b > 0)
signalWork();
if (a[(cap - 1) & b] != null)
signalWork(q); // help signal if more tasks
w.topLevelExec(t, q, // random fairness bound
r & ((n << TOP_BOUND_SHIFT) - 1));
(r | (1 << TOP_BOUND_SHIFT)) & SMASK);
}
}
return true;
@ -1900,7 +1909,7 @@ public class ForkJoinPool extends AbstractExecutorService {
r = ThreadLocalRandom.advanceProbe(r);
else {
if (q.lockedPush(task))
signalWork();
signalWork(null);
return;
}
}