heap: Only compact heap on memory reducing unified heap GCs

These GCs are already expensive or are executed in idleness or when
entering background. Use the opportunity to perform compaction.

Bug: 958615, 961231

Change-Id: I19c571e9b1b773fe11516c02b0918019fbffb852
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1588561
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659002}

third_party/blink/renderer/platform/heap/blink_gc.cc

@@ -24,6 +24,8 @@ const char* BlinkGC::ToString(BlinkGC::GCReason reason) {
       return "IncrementalV8FollowupGC";
     case BlinkGC::GCReason::kUnifiedHeapGC:
       return "UnifiedHeapGC";
+    case BlinkGC::GCReason::kUnifiedHeapForMemoryReductionGC:
+      return "UnifiedHeapForMemoryReductionGC";
   }
   IMMEDIATE_CRASH();
 }

third_party/blink/renderer/platform/heap/blink_gc.h

@@ -100,7 +100,8 @@ class PLATFORM_EXPORT BlinkGC final {
     // kIncrementalIdleGC = 8,
     kIncrementalV8FollowupGC = 9,
     kUnifiedHeapGC = 10,
-    kMaxValue = kUnifiedHeapGC,
+    kUnifiedHeapForMemoryReductionGC = 11,
+    kMaxValue = kUnifiedHeapForMemoryReductionGC,
   };
 
   enum ArenaIndices {

third_party/blink/renderer/platform/heap/heap_compact.cc

@@ -346,11 +346,13 @@ bool HeapCompact::ShouldCompact(BlinkGC::StackState stack_state,
     return true;
   }
 
-  // TODO(mlippautz): Only enable compaction when doing garbage collections that
-  // should aggressively reduce memory footprint.
-  return gc_count_since_last_compaction_ >
-             kGCCountSinceLastCompactionThreshold &&
-         free_list_size_ > kFreeListSizeThreshold;
+  // Only enable compaction when in a memory reduction garbage collection as it
+  // may significantly increase the final garbage collection pause.
+  if (reason == BlinkGC::GCReason::kUnifiedHeapForMemoryReductionGC) {
+    return free_list_size_ > kFreeListSizeThreshold;
+  }
+
+  return false;
 }
 
 void HeapCompact::Initialize(ThreadState* state) {

third_party/blink/renderer/platform/heap/heap_compact.h

@@ -118,9 +118,6 @@ class PLATFORM_EXPORT HeapCompact final {
  private:
   class MovableObjectFixups;
 
-  // Number of GCs that must have passed since last compaction GC.
-  static const int kGCCountSinceLastCompactionThreshold = 10;
-
   // Freelist size threshold that must be exceeded before compaction
   // should be considered.
   static const size_t kFreeListSizeThreshold = 512 * 1024;

third_party/blink/renderer/platform/heap/thread_state.cc

@@ -1067,6 +1067,7 @@ void UpdateHistograms(const ThreadHeapStatsCollector::Event& event) {
     COUNT_BY_GC_REASON(ThreadTerminationGC)
     COUNT_BY_GC_REASON(IncrementalV8FollowupGC)
     COUNT_BY_GC_REASON(UnifiedHeapGC)
+    COUNT_BY_GC_REASON(UnifiedHeapForMemoryReductionGC)
 
 #undef COUNT_BY_GC_REASON
   }
@@ -1405,6 +1406,7 @@ void ThreadState::CollectGarbage(BlinkGC::StackState stack_state,
     COUNT_BY_GC_REASON(ThreadTerminationGC)
     COUNT_BY_GC_REASON(IncrementalV8FollowupGC)
     COUNT_BY_GC_REASON(UnifiedHeapGC)
+    COUNT_BY_GC_REASON(UnifiedHeapForMemoryReductionGC)
   }
 #undef COUNT_BY_GC_REASON
 

third_party/blink/renderer/platform/heap/thread_state.h

@@ -234,7 +234,9 @@ class PLATFORM_EXPORT ThreadState final : private RAILModeObserver {
   bool IsSweepingInProgress() const { return gc_phase_ == GCPhase::kSweeping; }
   bool IsUnifiedGCMarkingInProgress() const {
     return IsMarkingInProgress() &&
-           (current_gc_data_.reason == BlinkGC::GCReason::kUnifiedHeapGC);
+           (current_gc_data_.reason == BlinkGC::GCReason::kUnifiedHeapGC ||
+            current_gc_data_.reason ==
+                BlinkGC::GCReason::kUnifiedHeapForMemoryReductionGC);
   }
 
   void EnableCompactionForNextGCForTesting();

third_party/blink/renderer/platform/heap/unified_heap_controller.cc

@@ -28,7 +28,8 @@ constexpr BlinkGC::StackState ToBlinkGCStackState(
 UnifiedHeapController::UnifiedHeapController(ThreadState* thread_state)
     : thread_state_(thread_state) {}
 
-void UnifiedHeapController::TracePrologue() {
+void UnifiedHeapController::TracePrologue(
+    v8::EmbedderHeapTracer::TraceFlags v8_flags) {
   VLOG(2) << "UnifiedHeapController::TracePrologue";
   ThreadHeapStatsCollector::BlinkGCInV8Scope nested_scope(
       thread_state_->Heap().stats_collector());
@@ -40,8 +41,11 @@ void UnifiedHeapController::TracePrologue() {
 
   // Reset any previously scheduled garbage collections.
   thread_state_->SetGCState(ThreadState::kNoGCScheduled);
-  // Start incremental marking with unified tracing.
-  thread_state_->IncrementalMarkingStart(BlinkGC::GCReason::kUnifiedHeapGC);
+  BlinkGC::GCReason gc_reason =
+      (v8_flags & v8::EmbedderHeapTracer::TraceFlags::kReduceMemory)
+          ? BlinkGC::GCReason::kUnifiedHeapForMemoryReductionGC
+          : BlinkGC::GCReason::kUnifiedHeapGC;
+  thread_state_->IncrementalMarkingStart(gc_reason);
 
   is_tracing_done_ = false;
 }
@@ -59,9 +63,9 @@ void UnifiedHeapController::EnterFinalPause(EmbedderStackState stack_state) {
     ThreadHeapStatsCollector::Scope mark_prologue_scope(
         thread_state_->Heap().stats_collector(),
         ThreadHeapStatsCollector::kUnifiedMarkingAtomicPrologue);
-    thread_state_->AtomicPauseMarkPrologue(ToBlinkGCStackState(stack_state),
-                                           BlinkGC::kIncrementalMarking,
-                                           BlinkGC::GCReason::kUnifiedHeapGC);
+    thread_state_->AtomicPauseMarkPrologue(
+        ToBlinkGCStackState(stack_state), BlinkGC::kIncrementalMarking,
+        thread_state_->current_gc_data_.reason);
   }
 }
 

third_party/blink/renderer/platform/heap/unified_heap_controller.h

@@ -36,7 +36,7 @@ class PLATFORM_EXPORT UnifiedHeapController final
   explicit UnifiedHeapController(ThreadState*);
 
   // v8::EmbedderHeapTracer implementation.
-  void TracePrologue() final;
+  void TracePrologue(v8::EmbedderHeapTracer::TraceFlags) final;
   void TraceEpilogue() final;
   void EnterFinalPause(EmbedderStackState) final;
   void RegisterV8References(const std::vector<std::pair<void*, void*>>&) final;