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Fix PGC edenSurvivalRate overflow issues in CopyForward #24347

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Fix PGC edenSurvivalRate overflow issues in CopyForward #24347

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2b57b21
Fix PGC edenSurvivalRate overflow issues in CopyForward
LinHu2016 Jun 30, 2026
ddd7264
Simplify the changes
LinHu2016 Jul 13, 2026
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36 changes: 27 additions & 9 deletions runtime/gc_vlhgc/SchedulingDelegate.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -372,7 +372,15 @@ MM_SchedulingDelegate::partialGarbageCollectCompleted(MM_EnvironmentVLHGC *env,
bool globalSweepHappened = _globalSweepRequired;
_globalSweepRequired = false;
/* copy out the Eden size of the previous interval (between the last PGC and this one) before we recalculate the next one */
uintptr_t edenCountBeforeCollect = getCurrentEdenSizeInRegions(env);
/* During GC, the heap may expand to allow the current collection to continue.
* A heap resize triggers an eden size recalculation without allowing further
* heap expansion (heap reconfiguration). As a result, the newly calculated
* eden size can differ significantly from the eden size at the start of the
* collection, causing the SurvivalRate calculation to become inaccurate.
* Therefore, set copyForwardStats->_edenEvacuateRegionCount (instead of getCurrentEdenSizeInRegions(env)) to
* edenCountBeforeCollect.
*/
uintptr_t edenCountBeforeCollect = copyForwardStats->_edenEvacuateRegionCount;

Trc_MM_SchedulingDelegate_partialGarbageCollectCompleted_stats(env->getLanguageVMThread(),
copyForwardStats->_edenEvacuateRegionCount,
Expand All @@ -384,21 +392,31 @@ MM_SchedulingDelegate::partialGarbageCollectCompleted(MM_EnvironmentVLHGC *env,

if (env->_cycleState->_shouldRunCopyForward) {
uintptr_t regionSize = _regionManager->getRegionSize();

/* count the number of survivor regions allocated specifically to support Eden survivors */
uintptr_t edenSurvivorCount = copyForwardStats->_edenSurvivorRegionCount;
uintptr_t edenEvacuateBytes = edenCountBeforeCollect * regionSize;
uintptr_t edenSurvivorBytes = copyForwardStats->_copyBytesEden;
/* nonEdenSurvivorCount non age0 fresh survivor region count (excludes any survivor regions for age0 non-Eden evacuate regions) */
uintptr_t nonEdenSurvivorCount = copyForwardStats->_nonEdenSurvivorRegionCount;

/* estimate how many more regions we would have needed to avoid abort */
Assert_MM_true( (0 == copyForwardStats->_scanBytesEden) || copyForwardStats->_aborted || (0 != copyForwardStats->_nonEvacuateRegionCount));
Assert_MM_true( (0 == copyForwardStats->_scanBytesNonEden) || copyForwardStats->_aborted || (0 != copyForwardStats->_nonEvacuateRegionCount));
edenSurvivorCount += (copyForwardStats->_scanBytesEden + regionSize - 1) / regionSize;
edenSurvivorBytes += copyForwardStats->_scanBytesEden;
if (edenSurvivorBytes > edenEvacuateBytes) {
Assert_MM_true(copyForwardStats->_aborted);
/* when abort is in progress, every Eden object scanned in abort-recovery contributes to _scannedBytes.
* But an object that was partially copied before abort (the copy succeeded,
* the forwarding pointer was installed) is also counted again during the abort scan of the source region.
* So _scanBytesEden might be double-counted, then cause edenSurvivorBytes is bigger than edenEvacuateBytes.
* for the case, set edenSurvivorBytes = edenEvacuateBytes.
*/
edenSurvivorBytes = edenEvacuateBytes;
}
nonEdenSurvivorCount += (copyForwardStats->_scanBytesNonEden + regionSize - 1) / regionSize;

/* Eden count could be 0 in special case, after compaction if there is still no free region for scheduling eden(eden count = 0),
will skip update Survival Rate */
if (0 != edenCountBeforeCollect) {
double thisSurvivalRate = (double)edenSurvivorCount / (double)edenCountBeforeCollect;
if (0 != edenEvacuateBytes) {
/* SurvivalRate should never be over 1.0 */
double thisSurvivalRate = (double)edenSurvivorBytes / (double)edenEvacuateBytes;
Assert_MM_true(1.0 >= thisSurvivalRate);
updateSurvivalRatesAfterCopyForward(thisSurvivalRate, nonEdenSurvivorCount);
}

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