Card pre-cleaning.
We now pre-clean cards before the pause in the concurrent mark sweep
collectors. This provides substantial a pause time reduction for GC
iterations which have a lot of dirty cards. The only downside is a
slight GC time increase for large heaps.
Benchmark FormulaEvaluationActions.EvaluateAndApplyChanges:
Before:
Partial average pause: 5.47ms
Sticky average pause: 2.91ms
Total GC time: 25.8s
After:
Partial average pause: 1.98ms
Sticky average pause: 1.66ms
Total GC time: 27.0s
Benchmark score difference in the noise.
Change-Id: If9f01f8c1501f122e19432438108d48e723b332e
diff --git a/runtime/gc/collector/mark_sweep.cc b/runtime/gc/collector/mark_sweep.cc
index fb797e0..9ab2d2e 100644
--- a/runtime/gc/collector/mark_sweep.cc
+++ b/runtime/gc/collector/mark_sweep.cc
@@ -65,6 +65,7 @@
constexpr bool kUseRecursiveMark = false;
constexpr bool kUseMarkStackPrefetch = true;
constexpr size_t kSweepArrayChunkFreeSize = 1024;
+constexpr bool kPreCleanCards = true;
// Parallelism options.
constexpr bool kParallelCardScan = true;
@@ -232,6 +233,25 @@
return is_concurrent_;
}
+void MarkSweep::PreCleanCards() {
+ // Don't do this for non concurrent GCs since they don't have any dirty cards.
+ if (kPreCleanCards && IsConcurrent()) {
+ Thread* self = Thread::Current();
+ CHECK(!Locks::mutator_lock_->IsExclusiveHeld(self));
+ // Process dirty cards and add dirty cards to mod union tables, also ages cards.
+ heap_->ProcessCards(timings_);
+ // Required so that we see aged cards before we start scanning the cards.
+ MarkThreadRoots(self);
+ // TODO: Only mark the dirty roots.
+ MarkNonThreadRoots();
+ MarkConcurrentRoots();
+ // Process the newly aged cards.
+ RecursiveMarkDirtyObjects(false, accounting::CardTable::kCardDirty - 1);
+ // TODO: Empty allocation stack to reduce the number of objects we need to test / mark as live
+ // in the next GC.
+ }
+}
+
void MarkSweep::MarkingPhase() {
TimingLogger::ScopedSplit split("MarkingPhase", &timings_);
Thread* self = Thread::Current();
@@ -263,6 +283,8 @@
MarkConcurrentRoots();
UpdateAndMarkModUnion();
MarkReachableObjects();
+ // Pre-clean dirtied cards to reduce pauses.
+ PreCleanCards();
}
void MarkSweep::UpdateAndMarkModUnion() {
@@ -313,20 +335,24 @@
TimingLogger::ScopedSplit split("UnMarkAllocStack", &timings_);
WriterMutexLock mu(self, *Locks::heap_bitmap_lock_);
accounting::ObjectStack* allocation_stack = GetHeap()->allocation_stack_.get();
- // The allocation stack contains things allocated since the start of the GC. These may have been
- // marked during this GC meaning they won't be eligible for reclaiming in the next sticky GC.
- // Remove these objects from the mark bitmaps so that they will be eligible for sticky
- // collection.
- // There is a race here which is safely handled. Another thread such as the hprof could
- // have flushed the alloc stack after we resumed the threads. This is safe however, since
- // reseting the allocation stack zeros it out with madvise. This means that we will either
- // read NULLs or attempt to unmark a newly allocated object which will not be marked in the
- // first place.
- mirror::Object** end = allocation_stack->End();
- for (mirror::Object** it = allocation_stack->Begin(); it != end; ++it) {
- const Object* obj = *it;
- if (obj != NULL) {
- UnMarkObjectNonNull(obj);
+ if (!kPreCleanCards) {
+ // The allocation stack contains things allocated since the start of the GC. These may have
+ // been marked during this GC meaning they won't be eligible for reclaiming in the next
+ // sticky GC. Unmark these objects so that they are eligible for reclaiming in the next
+ // sticky GC.
+ // There is a race here which is safely handled. Another thread such as the hprof could
+ // have flushed the alloc stack after we resumed the threads. This is safe however, since
+ // reseting the allocation stack zeros it out with madvise. This means that we will either
+ // read NULLs or attempt to unmark a newly allocated object which will not be marked in the
+ // first place.
+ // We can't do this if we pre-clean cards since we will unmark objects which are no longer on
+ // a dirty card since we aged cards during the pre-cleaning process.
+ mirror::Object** end = allocation_stack->End();
+ for (mirror::Object** it = allocation_stack->Begin(); it != end; ++it) {
+ const Object* obj = *it;
+ if (obj != nullptr) {
+ UnMarkObjectNonNull(obj);
+ }
}
}
}