Make sure that memory reducer makes progress in incremental marking

src/heap/memory-reducer.cc

 // Copyright 2015 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/heap/memory-reducer.h"
 
 #include "src/flags.h"
 #include "src/heap/heap.h"
 #include "src/utils.h"
 #include "src/v8.h"
@@ skipping 24 matching lines @@
 }
 
 
 void MemoryReducer::NotifyTimer(const Event& event) {
   DCHECK_EQ(kTimer, event.type);
   DCHECK_EQ(kWait, state_.action);
   state_ = Step(state_, event);
   if (state_.action == kRun) {
     DCHECK(heap()->incremental_marking()->IsStopped());
     DCHECK(FLAG_incremental_marking);
-    heap()->StartIdleIncrementalMarking();
     if (FLAG_trace_gc_verbose) {
       PrintIsolate(heap()->isolate(), "Memory reducer: started GC #%d\n",
                    state_.started_gcs);
     }
+    if (heap()->ShouldOptimizeForMemoryUsage()) {
+      // Do full GC if memory usage has higher priority than latency. This is
+      // important for background tabs that do not send idle notifications.
+      heap()->CollectAllGarbage(Heap::kReduceMemoryFootprintMask,
+                                "memory reducer");
+    } else {
+      heap()->StartIdleIncrementalMarking();
+    }
   } else if (state_.action == kWait) {
+    if (!heap()->incremental_marking()->IsStopped() &&
+        heap()->ShouldOptimizeForMemoryUsage()) {
+      // Make progress with pending incremental marking if memory usage has
+      // higher priority than latency. This is important for background tabs
+      // that do not send idle notifications.
+      const int kIncrementalMarkingDelayMs = 500;
+      double deadline = heap()->MonotonicallyIncreasingTimeInMs() +
+                        kIncrementalMarkingDelayMs;
+      heap()->AdvanceIncrementalMarking(
+          0, deadline, i::IncrementalMarking::StepActions(
+                           i::IncrementalMarking::NO_GC_VIA_STACK_GUARD,
+                           i::IncrementalMarking::FORCE_MARKING,
+                           i::IncrementalMarking::FORCE_COMPLETION));
+      heap()->FinalizeIncrementalMarkingIfComplete(
+          "Memory reducer: finalize incremental marking");
+    }
     // Re-schedule the timer.
     ScheduleTimer(state_.next_gc_start_ms - event.time_ms);
     if (FLAG_trace_gc_verbose) {
       PrintIsolate(heap()->isolate(), "Memory reducer: waiting for %.f ms\n",
                    state_.next_gc_start_ms - event.time_ms);
     }
   }
 }
 
 
@@ skipping 132 matching lines @@
   auto timer_task = new MemoryReducer::TimerTask(this);
   V8::GetCurrentPlatform()->CallDelayedOnForegroundThread(
       isolate, timer_task, (delay_ms + kSlackMs) / 1000.0);
 }
 
 
 void MemoryReducer::TearDown() { state_ = State(kDone, 0, 0, 0.0); }
 
 }  // internal
 }  // v8