--- a/hotspot/src/share/vm/runtime/synchronizer.cpp Wed May 26 14:16:55 2010 -0700
+++ b/hotspot/src/share/vm/runtime/synchronizer.cpp Thu Jun 03 13:21:47 2010 -0400
@@ -185,6 +185,8 @@
} ;
static SharedGlobals GVars ;
+static int MonitorScavengeThreshold = 1000000 ;
+static volatile int ForceMonitorScavenge = 0 ; // Scavenge required and pending
// Tunables ...
@@ -746,8 +748,85 @@
ObjectMonitor * ObjectSynchronizer::gBlockList = NULL ;
ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL ;
static volatile intptr_t ListLock = 0 ; // protects global monitor free-list cache
+static volatile int MonitorFreeCount = 0 ; // # on gFreeList
+static volatile int MonitorPopulation = 0 ; // # Extant -- in circulation
#define CHAINMARKER ((oop)-1)
+// Constraining monitor pool growth via MonitorBound ...
+//
+// The monitor pool is grow-only. We scavenge at STW safepoint-time, but the
+// the rate of scavenging is driven primarily by GC. As such, we can find
+// an inordinate number of monitors in circulation.
+// To avoid that scenario we can artificially induce a STW safepoint
+// if the pool appears to be growing past some reasonable bound.
+// Generally we favor time in space-time tradeoffs, but as there's no
+// natural back-pressure on the # of extant monitors we need to impose some
+// type of limit. Beware that if MonitorBound is set to too low a value
+// we could just loop. In addition, if MonitorBound is set to a low value
+// we'll incur more safepoints, which are harmful to performance.
+// See also: GuaranteedSafepointInterval
+//
+// As noted elsewhere, the correct long-term solution is to deflate at
+// monitorexit-time, in which case the number of inflated objects is bounded
+// by the number of threads. That policy obviates the need for scavenging at
+// STW safepoint time. As an aside, scavenging can be time-consuming when the
+// # of extant monitors is large. Unfortunately there's a day-1 assumption baked
+// into much HotSpot code that the object::monitor relationship, once established
+// or observed, will remain stable except over potential safepoints.
+//
+// We can use either a blocking synchronous VM operation or an async VM operation.
+// -- If we use a blocking VM operation :
+// Calls to ScavengeCheck() should be inserted only into 'safe' locations in paths
+// that lead to ::inflate() or ::omAlloc().
+// Even though the safepoint will not directly induce GC, a GC might
+// piggyback on the safepoint operation, so the caller should hold no naked oops.
+// Furthermore, monitor::object relationships are NOT necessarily stable over this call
+// unless the caller has made provisions to "pin" the object to the monitor, say
+// by incrementing the monitor's _count field.
+// -- If we use a non-blocking asynchronous VM operation :
+// the constraints above don't apply. The safepoint will fire in the future
+// at a more convenient time. On the other hand the latency between posting and
+// running the safepoint introduces or admits "slop" or laxity during which the
+// monitor population can climb further above the threshold. The monitor population,
+// however, tends to converge asymptotically over time to a count that's slightly
+// above the target value specified by MonitorBound. That is, we avoid unbounded
+// growth, albeit with some imprecision.
+//
+// The current implementation uses asynchronous VM operations.
+//
+// Ideally we'd check if (MonitorPopulation > MonitorBound) in omAlloc()
+// immediately before trying to grow the global list via allocation.
+// If the predicate was true then we'd induce a synchronous safepoint, wait
+// for the safepoint to complete, and then again to allocate from the global
+// free list. This approach is much simpler and precise, admitting no "slop".
+// Unfortunately we can't safely safepoint in the midst of omAlloc(), so
+// instead we use asynchronous safepoints.
+
+static void InduceScavenge (Thread * Self, const char * Whence) {
+ // Induce STW safepoint to trim monitors
+ // Ultimately, this results in a call to deflate_idle_monitors() in the near future.
+ // More precisely, trigger an asynchronous STW safepoint as the number
+ // of active monitors passes the specified threshold.
+ // TODO: assert thread state is reasonable
+
+ if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) {
+ if (Knob_Verbose) {
+ ::printf ("Monitor scavenge - Induced STW @%s (%d)\n", Whence, ForceMonitorScavenge) ;
+ ::fflush(stdout) ;
+ }
+ // Induce a 'null' safepoint to scavenge monitors
+ // Must VM_Operation instance be heap allocated as the op will be enqueue and posted
+ // to the VMthread and have a lifespan longer than that of this activation record.
+ // The VMThread will delete the op when completed.
+ VMThread::execute (new VM_ForceAsyncSafepoint()) ;
+
+ if (Knob_Verbose) {
+ ::printf ("Monitor scavenge - STW posted @%s (%d)\n", Whence, ForceMonitorScavenge) ;
+ ::fflush(stdout) ;
+ }
+ }
+}
+
ObjectMonitor * ATTR ObjectSynchronizer::omAlloc (Thread * Self) {
// A large MAXPRIVATE value reduces both list lock contention
// and list coherency traffic, but also tends to increase the
@@ -770,6 +849,11 @@
Self->omFreeCount -- ;
// CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene
guarantee (m->object() == NULL, "invariant") ;
+ if (MonitorInUseLists) {
+ m->FreeNext = Self->omInUseList;
+ Self->omInUseList = m;
+ Self->omInUseCount ++;
+ }
return m ;
}
@@ -784,6 +868,7 @@
// on various locks.
Thread::muxAcquire (&ListLock, "omAlloc") ;
for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL; ) {
+ MonitorFreeCount --;
ObjectMonitor * take = gFreeList ;
gFreeList = take->FreeNext ;
guarantee (take->object() == NULL, "invariant") ;
@@ -796,6 +881,15 @@
if (Self->omFreeProvision > MAXPRIVATE ) Self->omFreeProvision = MAXPRIVATE ;
TEVENT (omFirst - reprovision) ;
continue ;
+
+ const int mx = MonitorBound ;
+ if (mx > 0 && (MonitorPopulation-MonitorFreeCount) > mx) {
+ // We can't safely induce a STW safepoint from omAlloc() as our thread
+ // state may not be appropriate for such activities and callers may hold
+ // naked oops, so instead we defer the action.
+ InduceScavenge (Self, "omAlloc") ;
+ }
+ continue;
}
// 3: allocate a block of new ObjectMonitors
@@ -836,6 +930,8 @@
// Acquire the ListLock to manipulate BlockList and FreeList.
// An Oyama-Taura-Yonezawa scheme might be more efficient.
Thread::muxAcquire (&ListLock, "omAlloc [2]") ;
+ MonitorPopulation += _BLOCKSIZE-1;
+ MonitorFreeCount += _BLOCKSIZE-1;
// Add the new block to the list of extant blocks (gBlockList).
// The very first objectMonitor in a block is reserved and dedicated.
@@ -894,7 +990,9 @@
if (List == NULL) return ;
ObjectMonitor * Tail = NULL ;
ObjectMonitor * s ;
+ int Tally = 0;
for (s = List ; s != NULL ; s = s->FreeNext) {
+ Tally ++ ;
Tail = s ;
guarantee (s->object() == NULL, "invariant") ;
guarantee (!s->is_busy(), "invariant") ;
@@ -906,6 +1004,7 @@
Thread::muxAcquire (&ListLock, "omFlush") ;
Tail->FreeNext = gFreeList ;
gFreeList = List ;
+ MonitorFreeCount += Tally;
Thread::muxRelease (&ListLock) ;
TEVENT (omFlush) ;
}
@@ -1747,16 +1846,15 @@
// Having a large number of monitors in-circulation negatively
// impacts the performance of some applications (e.g., PointBase).
// Broadly, we want to minimize the # of monitors in circulation.
-// Alternately, we could partition the active monitors into sub-lists
-// of those that need scanning and those that do not.
-// Specifically, we would add a new sub-list of objectmonitors
-// that are in-circulation and potentially active. deflate_idle_monitors()
-// would scan only that list. Other monitors could reside on a quiescent
-// list. Such sequestered monitors wouldn't need to be scanned by
-// deflate_idle_monitors(). omAlloc() would first check the global free list,
-// then the quiescent list, and, failing those, would allocate a new block.
-// Deflate_idle_monitors() would scavenge and move monitors to the
-// quiescent list.
+//
+// We have added a flag, MonitorInUseLists, which creates a list
+// of active monitors for each thread. deflate_idle_monitors()
+// only scans the per-thread inuse lists. omAlloc() puts all
+// assigned monitors on the per-thread list. deflate_idle_monitors()
+// returns the non-busy monitors to the global free list.
+// An alternative could have used a single global inuse list. The
+// downside would have been the additional cost of acquiring the global list lock
+// for every omAlloc().
//
// Perversely, the heap size -- and thus the STW safepoint rate --
// typically drives the scavenge rate. Large heaps can mean infrequent GC,
@@ -1769,18 +1867,100 @@
// An even better solution would be to deflate on-the-fly, aggressively,
// at monitorexit-time as is done in EVM's metalock or Relaxed Locks.
+
+// Deflate a single monitor if not in use
+// Return true if deflated, false if in use
+bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj,
+ ObjectMonitor** FreeHeadp, ObjectMonitor** FreeTailp) {
+ bool deflated;
+ // Normal case ... The monitor is associated with obj.
+ guarantee (obj->mark() == markOopDesc::encode(mid), "invariant") ;
+ guarantee (mid == obj->mark()->monitor(), "invariant");
+ guarantee (mid->header()->is_neutral(), "invariant");
+
+ if (mid->is_busy()) {
+ if (ClearResponsibleAtSTW) mid->_Responsible = NULL ;
+ deflated = false;
+ } else {
+ // Deflate the monitor if it is no longer being used
+ // It's idle - scavenge and return to the global free list
+ // plain old deflation ...
+ TEVENT (deflate_idle_monitors - scavenge1) ;
+ if (TraceMonitorInflation) {
+ if (obj->is_instance()) {
+ ResourceMark rm;
+ tty->print_cr("Deflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
+ (intptr_t) obj, (intptr_t) obj->mark(), Klass::cast(obj->klass())->external_name());
+ }
+ }
+
+ // Restore the header back to obj
+ obj->release_set_mark(mid->header());
+ mid->clear();
+
+ assert (mid->object() == NULL, "invariant") ;
+
+ // Move the object to the working free list defined by FreeHead,FreeTail.
+ if (*FreeHeadp == NULL) *FreeHeadp = mid;
+ if (*FreeTailp != NULL) {
+ ObjectMonitor * prevtail = *FreeTailp;
+ prevtail->FreeNext = mid;
+ }
+ *FreeTailp = mid;
+ deflated = true;
+ }
+ return deflated;
+}
+
void ObjectSynchronizer::deflate_idle_monitors() {
assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
int nInuse = 0 ; // currently associated with objects
int nInCirculation = 0 ; // extant
int nScavenged = 0 ; // reclaimed
+ bool deflated = false;
ObjectMonitor * FreeHead = NULL ; // Local SLL of scavenged monitors
ObjectMonitor * FreeTail = NULL ;
+ TEVENT (deflate_idle_monitors) ;
+ // Prevent omFlush from changing mids in Thread dtor's during deflation
+ // And in case the vm thread is acquiring a lock during a safepoint
+ // See e.g. 6320749
+ Thread::muxAcquire (&ListLock, "scavenge - return") ;
+
+ if (MonitorInUseLists) {
+ ObjectMonitor* mid;
+ ObjectMonitor* next;
+ ObjectMonitor* curmidinuse;
+ for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) {
+ curmidinuse = NULL;
+ for (mid = cur->omInUseList; mid != NULL; ) {
+ oop obj = (oop) mid->object();
+ deflated = false;
+ if (obj != NULL) {
+ deflated = deflate_monitor(mid, obj, &FreeHead, &FreeTail);
+ }
+ if (deflated) {
+ // extract from per-thread in-use-list
+ if (mid == cur->omInUseList) {
+ cur->omInUseList = mid->FreeNext;
+ } else if (curmidinuse != NULL) {
+ curmidinuse->FreeNext = mid->FreeNext; // maintain the current thread inuselist
+ }
+ next = mid->FreeNext;
+ mid->FreeNext = NULL; // This mid is current tail in the FreeHead list
+ mid = next;
+ cur->omInUseCount--;
+ nScavenged ++ ;
+ } else {
+ curmidinuse = mid;
+ mid = mid->FreeNext;
+ nInuse ++;
+ }
+ }
+ }
+ } else for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
// Iterate over all extant monitors - Scavenge all idle monitors.
- TEVENT (deflate_idle_monitors) ;
- for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
assert(block->object() == CHAINMARKER, "must be a block header");
nInCirculation += _BLOCKSIZE ;
for (int i = 1 ; i < _BLOCKSIZE; i++) {
@@ -1795,61 +1975,39 @@
guarantee (!mid->is_busy(), "invariant") ;
continue ;
}
-
- // Normal case ... The monitor is associated with obj.
- guarantee (obj->mark() == markOopDesc::encode(mid), "invariant") ;
- guarantee (mid == obj->mark()->monitor(), "invariant");
- guarantee (mid->header()->is_neutral(), "invariant");
-
- if (mid->is_busy()) {
- if (ClearResponsibleAtSTW) mid->_Responsible = NULL ;
- nInuse ++ ;
+ deflated = deflate_monitor(mid, obj, &FreeHead, &FreeTail);
+
+ if (deflated) {
+ mid->FreeNext = NULL ;
+ nScavenged ++ ;
} else {
- // Deflate the monitor if it is no longer being used
- // It's idle - scavenge and return to the global free list
- // plain old deflation ...
- TEVENT (deflate_idle_monitors - scavenge1) ;
- if (TraceMonitorInflation) {
- if (obj->is_instance()) {
- ResourceMark rm;
- tty->print_cr("Deflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
- (intptr_t) obj, (intptr_t) obj->mark(), Klass::cast(obj->klass())->external_name());
- }
- }
-
- // Restore the header back to obj
- obj->release_set_mark(mid->header());
- mid->clear();
-
- assert (mid->object() == NULL, "invariant") ;
-
- // Move the object to the working free list defined by FreeHead,FreeTail.
- mid->FreeNext = NULL ;
- if (FreeHead == NULL) FreeHead = mid ;
- if (FreeTail != NULL) FreeTail->FreeNext = mid ;
- FreeTail = mid ;
- nScavenged ++ ;
+ nInuse ++;
}
}
}
+ MonitorFreeCount += nScavenged;
+
+ // Consider: audit gFreeList to ensure that MonitorFreeCount and list agree.
+
+ if (Knob_Verbose) {
+ ::printf ("Deflate: InCirc=%d InUse=%d Scavenged=%d ForceMonitorScavenge=%d : pop=%d free=%d\n",
+ nInCirculation, nInuse, nScavenged, ForceMonitorScavenge,
+ MonitorPopulation, MonitorFreeCount) ;
+ ::fflush(stdout) ;
+ }
+
+ ForceMonitorScavenge = 0; // Reset
+
// Move the scavenged monitors back to the global free list.
- // In theory we don't need the freelist lock as we're at a STW safepoint.
- // omAlloc() and omFree() can only be called while a thread is _not in safepoint state.
- // But it's remotely possible that omFlush() or release_monitors_owned_by_thread()
- // might be called while not at a global STW safepoint. In the interest of
- // safety we protect the following access with ListLock.
- // An even more conservative and prudent approach would be to guard
- // the main loop in scavenge_idle_monitors() with ListLock.
if (FreeHead != NULL) {
guarantee (FreeTail != NULL && nScavenged > 0, "invariant") ;
assert (FreeTail->FreeNext == NULL, "invariant") ;
// constant-time list splice - prepend scavenged segment to gFreeList
- Thread::muxAcquire (&ListLock, "scavenge - return") ;
FreeTail->FreeNext = gFreeList ;
gFreeList = FreeHead ;
- Thread::muxRelease (&ListLock) ;
}
+ Thread::muxRelease (&ListLock) ;
if (_sync_Deflations != NULL) _sync_Deflations->inc(nScavenged) ;
if (_sync_MonExtant != NULL) _sync_MonExtant ->set_value(nInCirculation);