--- a/hotspot/src/share/vm/runtime/objectMonitor.cpp Sat Jul 18 04:53:32 2015 +0200
+++ b/hotspot/src/share/vm/runtime/objectMonitor.cpp Tue Jul 21 07:28:37 2015 -0700
@@ -430,6 +430,8 @@
return -1;
}
+#define MAX_RECHECK_INTERVAL 1000
+
void NOINLINE ObjectMonitor::EnterI(TRAPS) {
Thread * const Self = THREAD;
assert(Self->is_Java_thread(), "invariant");
@@ -539,7 +541,7 @@
TEVENT(Inflated enter - Contention);
int nWakeups = 0;
- int RecheckInterval = 1;
+ int recheckInterval = 1;
for (;;) {
@@ -553,10 +555,12 @@
// park self
if (_Responsible == Self || (SyncFlags & 1)) {
TEVENT(Inflated enter - park TIMED);
- Self->_ParkEvent->park((jlong) RecheckInterval);
- // Increase the RecheckInterval, but clamp the value.
- RecheckInterval *= 8;
- if (RecheckInterval > 1000) RecheckInterval = 1000;
+ Self->_ParkEvent->park((jlong) recheckInterval);
+ // Increase the recheckInterval, but clamp the value.
+ recheckInterval *= 8;
+ if (recheckInterval > MAX_RECHECK_INTERVAL) {
+ recheckInterval = MAX_RECHECK_INTERVAL;
+ }
} else {
TEVENT(Inflated enter - park UNTIMED);
Self->_ParkEvent->park();
@@ -709,7 +713,7 @@
// or java_suspend_self()
jt->set_suspend_equivalent();
if (SyncFlags & 1) {
- Self->_ParkEvent->park((jlong)1000);
+ Self->_ParkEvent->park((jlong)MAX_RECHECK_INTERVAL);
} else {
Self->_ParkEvent->park();
}
@@ -1636,15 +1640,8 @@
// then instead of transferring a thread from the WaitSet to the EntryList
// we might just dequeue a thread from the WaitSet and directly unpark() it.
-void ObjectMonitor::notify(TRAPS) {
- CHECK_OWNER();
- if (_WaitSet == NULL) {
- TEVENT(Empty-Notify);
- return;
- }
- DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
-
- int Policy = Knob_MoveNotifyee;
+void ObjectMonitor::INotify(Thread * Self) {
+ const int policy = Knob_MoveNotifyee;
Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
ObjectWaiter * iterator = DequeueWaiter();
@@ -1652,74 +1649,76 @@
TEVENT(Notify1 - Transfer);
guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
guarantee(iterator->_notified == 0, "invariant");
- if (Policy != 4) {
+ // Disposition - what might we do with iterator ?
+ // a. add it directly to the EntryList - either tail (policy == 1)
+ // or head (policy == 0).
+ // b. push it onto the front of the _cxq (policy == 2).
+ // For now we use (b).
+ if (policy != 4) {
iterator->TState = ObjectWaiter::TS_ENTER;
}
iterator->_notified = 1;
- Thread * Self = THREAD;
iterator->_notifier_tid = Self->osthread()->thread_id();
- ObjectWaiter * List = _EntryList;
- if (List != NULL) {
- assert(List->_prev == NULL, "invariant");
- assert(List->TState == ObjectWaiter::TS_ENTER, "invariant");
- assert(List != iterator, "invariant");
+ ObjectWaiter * list = _EntryList;
+ if (list != NULL) {
+ assert(list->_prev == NULL, "invariant");
+ assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
+ assert(list != iterator, "invariant");
}
- if (Policy == 0) { // prepend to EntryList
- if (List == NULL) {
+ if (policy == 0) { // prepend to EntryList
+ if (list == NULL) {
iterator->_next = iterator->_prev = NULL;
_EntryList = iterator;
} else {
- List->_prev = iterator;
- iterator->_next = List;
+ list->_prev = iterator;
+ iterator->_next = list;
iterator->_prev = NULL;
_EntryList = iterator;
}
- } else if (Policy == 1) { // append to EntryList
- if (List == NULL) {
+ } else if (policy == 1) { // append to EntryList
+ if (list == NULL) {
iterator->_next = iterator->_prev = NULL;
_EntryList = iterator;
} else {
// CONSIDER: finding the tail currently requires a linear-time walk of
// the EntryList. We can make tail access constant-time by converting to
// a CDLL instead of using our current DLL.
- ObjectWaiter * Tail;
- for (Tail = List; Tail->_next != NULL; Tail = Tail->_next) /* empty */;
- assert(Tail != NULL && Tail->_next == NULL, "invariant");
- Tail->_next = iterator;
- iterator->_prev = Tail;
+ ObjectWaiter * tail;
+ for (tail = list; tail->_next != NULL; tail = tail->_next) /* empty */;
+ assert(tail != NULL && tail->_next == NULL, "invariant");
+ tail->_next = iterator;
+ iterator->_prev = tail;
iterator->_next = NULL;
}
- } else if (Policy == 2) { // prepend to cxq
- // prepend to cxq
- if (List == NULL) {
+ } else if (policy == 2) { // prepend to cxq
+ if (list == NULL) {
iterator->_next = iterator->_prev = NULL;
_EntryList = iterator;
} else {
iterator->TState = ObjectWaiter::TS_CXQ;
for (;;) {
- ObjectWaiter * Front = _cxq;
- iterator->_next = Front;
- if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
+ ObjectWaiter * front = _cxq;
+ iterator->_next = front;
+ if (Atomic::cmpxchg_ptr(iterator, &_cxq, front) == front) {
break;
}
}
}
- } else if (Policy == 3) { // append to cxq
+ } else if (policy == 3) { // append to cxq
iterator->TState = ObjectWaiter::TS_CXQ;
for (;;) {
- ObjectWaiter * Tail;
- Tail = _cxq;
- if (Tail == NULL) {
+ ObjectWaiter * tail = _cxq;
+ if (tail == NULL) {
iterator->_next = NULL;
- if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
+ if (Atomic::cmpxchg_ptr(iterator, &_cxq, NULL) == NULL) {
break;
}
} else {
- while (Tail->_next != NULL) Tail = Tail->_next;
- Tail->_next = iterator;
- iterator->_prev = Tail;
+ while (tail->_next != NULL) tail = tail->_next;
+ tail->_next = iterator;
+ iterator->_prev = tail;
iterator->_next = NULL;
break;
}
@@ -1731,10 +1730,6 @@
ev->unpark();
}
- if (Policy < 4) {
- iterator->wait_reenter_begin(this);
- }
-
// _WaitSetLock protects the wait queue, not the EntryList. We could
// move the add-to-EntryList operation, above, outside the critical section
// protected by _WaitSetLock. In practice that's not useful. With the
@@ -1742,133 +1737,63 @@
// is the only thread that grabs _WaitSetLock. There's almost no contention
// on _WaitSetLock so it's not profitable to reduce the length of the
// critical section.
+
+ if (policy < 4) {
+ iterator->wait_reenter_begin(this);
+ }
}
-
Thread::SpinRelease(&_WaitSetLock);
+}
- if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
- ObjectMonitor::_sync_Notifications->inc();
+// Consider: a not-uncommon synchronization bug is to use notify() when
+// notifyAll() is more appropriate, potentially resulting in stranded
+// threads; this is one example of a lost wakeup. A useful diagnostic
+// option is to force all notify() operations to behave as notifyAll().
+//
+// Note: We can also detect many such problems with a "minimum wait".
+// When the "minimum wait" is set to a small non-zero timeout value
+// and the program does not hang whereas it did absent "minimum wait",
+// that suggests a lost wakeup bug. The '-XX:SyncFlags=1' option uses
+// a "minimum wait" for all park() operations; see the recheckInterval
+// variable and MAX_RECHECK_INTERVAL.
+
+void ObjectMonitor::notify(TRAPS) {
+ CHECK_OWNER();
+ if (_WaitSet == NULL) {
+ TEVENT(Empty-Notify);
+ return;
+ }
+ DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
+ INotify(THREAD);
+ if (ObjectMonitor::_sync_Notifications != NULL) {
+ ObjectMonitor::_sync_Notifications->inc(1);
}
}
+// The current implementation of notifyAll() transfers the waiters one-at-a-time
+// from the waitset to the EntryList. This could be done more efficiently with a
+// single bulk transfer but in practice it's not time-critical. Beware too,
+// that in prepend-mode we invert the order of the waiters. Let's say that the
+// waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
+// mode the waitset will be empty and the EntryList will be "DCBAXYZ".
+
void ObjectMonitor::notifyAll(TRAPS) {
CHECK_OWNER();
- ObjectWaiter* iterator;
if (_WaitSet == NULL) {
TEVENT(Empty-NotifyAll);
return;
}
+
DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
-
- int Policy = Knob_MoveNotifyee;
- int Tally = 0;
- Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notifyall");
-
- for (;;) {
- iterator = DequeueWaiter();
- if (iterator == NULL) break;
- TEVENT(NotifyAll - Transfer1);
- ++Tally;
-
- // Disposition - what might we do with iterator ?
- // a. add it directly to the EntryList - either tail or head.
- // b. push it onto the front of the _cxq.
- // For now we use (a).
-
- guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
- guarantee(iterator->_notified == 0, "invariant");
- iterator->_notified = 1;
- Thread * Self = THREAD;
- iterator->_notifier_tid = Self->osthread()->thread_id();
- if (Policy != 4) {
- iterator->TState = ObjectWaiter::TS_ENTER;
- }
-
- ObjectWaiter * List = _EntryList;
- if (List != NULL) {
- assert(List->_prev == NULL, "invariant");
- assert(List->TState == ObjectWaiter::TS_ENTER, "invariant");
- assert(List != iterator, "invariant");
- }
-
- if (Policy == 0) { // prepend to EntryList
- if (List == NULL) {
- iterator->_next = iterator->_prev = NULL;
- _EntryList = iterator;
- } else {
- List->_prev = iterator;
- iterator->_next = List;
- iterator->_prev = NULL;
- _EntryList = iterator;
- }
- } else if (Policy == 1) { // append to EntryList
- if (List == NULL) {
- iterator->_next = iterator->_prev = NULL;
- _EntryList = iterator;
- } else {
- // CONSIDER: finding the tail currently requires a linear-time walk of
- // the EntryList. We can make tail access constant-time by converting to
- // a CDLL instead of using our current DLL.
- ObjectWaiter * Tail;
- for (Tail = List; Tail->_next != NULL; Tail = Tail->_next) /* empty */;
- assert(Tail != NULL && Tail->_next == NULL, "invariant");
- Tail->_next = iterator;
- iterator->_prev = Tail;
- iterator->_next = NULL;
- }
- } else if (Policy == 2) { // prepend to cxq
- // prepend to cxq
- iterator->TState = ObjectWaiter::TS_CXQ;
- for (;;) {
- ObjectWaiter * Front = _cxq;
- iterator->_next = Front;
- if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
- break;
- }
- }
- } else if (Policy == 3) { // append to cxq
- iterator->TState = ObjectWaiter::TS_CXQ;
- for (;;) {
- ObjectWaiter * Tail;
- Tail = _cxq;
- if (Tail == NULL) {
- iterator->_next = NULL;
- if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
- break;
- }
- } else {
- while (Tail->_next != NULL) Tail = Tail->_next;
- Tail->_next = iterator;
- iterator->_prev = Tail;
- iterator->_next = NULL;
- break;
- }
- }
- } else {
- ParkEvent * ev = iterator->_event;
- iterator->TState = ObjectWaiter::TS_RUN;
- OrderAccess::fence();
- ev->unpark();
- }
-
- if (Policy < 4) {
- iterator->wait_reenter_begin(this);
- }
-
- // _WaitSetLock protects the wait queue, not the EntryList. We could
- // move the add-to-EntryList operation, above, outside the critical section
- // protected by _WaitSetLock. In practice that's not useful. With the
- // exception of wait() timeouts and interrupts the monitor owner
- // is the only thread that grabs _WaitSetLock. There's almost no contention
- // on _WaitSetLock so it's not profitable to reduce the length of the
- // critical section.
+ int tally = 0;
+ while (_WaitSet != NULL) {
+ tally++;
+ INotify(THREAD);
}
- Thread::SpinRelease(&_WaitSetLock);
-
- if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
- ObjectMonitor::_sync_Notifications->inc(Tally);
+ if (tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
+ ObjectMonitor::_sync_Notifications->inc(tally);
}
}