Revert of Replace reduce-memory mode in idle notification with delayed clean-up GC.

test/unittests/heap/gc-idle-time-handler-unittest.cc

Index: test/unittests/heap/gc-idle-time-handler-unittest.cc
diff --git a/test/unittests/heap/gc-idle-time-handler-unittest.cc b/test/unittests/heap/gc-idle-time-handler-unittest.cc
index c75fde492e5822fbc5d77fb3d5cfa0db9cd2a000..d4833b34d2710877a42bdca257fbb30bfa85df61 100644
--- a/test/unittests/heap/gc-idle-time-handler-unittest.cc
+++ b/test/unittests/heap/gc-idle-time-handler-unittest.cc
@@ -25,6 +25,7 @@
     result.contexts_disposal_rate = GCIdleTimeHandler::kHighContextDisposalRate;
     result.size_of_objects = kSizeOfObjects;
     result.incremental_marking_stopped = false;
+    result.can_start_incremental_marking = true;
     result.sweeping_in_progress = false;
     result.sweeping_completed = false;
     result.mark_compact_speed_in_bytes_per_ms = kMarkCompactSpeed;
@@ -37,13 +38,63 @@
     return result;
   }
 
+  void TransitionToReduceMemoryMode(
+      const GCIdleTimeHandler::HeapState& heap_state) {
+    handler()->NotifyScavenge();
+    EXPECT_EQ(GCIdleTimeHandler::kReduceLatency, handler()->mode());
+    double idle_time_ms = GCIdleTimeHandler::kMinLongIdleTime;
+    int limit = GCIdleTimeHandler::kLongIdleNotificationsBeforeMutatorIsIdle;
+    bool incremental = !heap_state.incremental_marking_stopped ||
+                       heap_state.can_start_incremental_marking;
+    for (int i = 0; i < limit; i++) {
+      GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+      if (incremental) {
+        EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
+      } else {
+        EXPECT_TRUE(DO_NOTHING == action.type || DONE == action.type);
+      }
+    }
+    handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_EQ(GCIdleTimeHandler::kReduceMemory, handler()->mode());
+  }
+
+  void TransitionToDoneMode(const GCIdleTimeHandler::HeapState& heap_state,
+                            double idle_time_ms,
+                            GCIdleTimeActionType expected) {
+    EXPECT_EQ(GCIdleTimeHandler::kReduceMemory, handler()->mode());
+    int limit = GCIdleTimeHandler::kMaxIdleMarkCompacts;
+    for (int i = 0; i < limit; i++) {
+      GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+      EXPECT_EQ(expected, action.type);
+      EXPECT_TRUE(action.reduce_memory);
+      handler()->NotifyMarkCompact(true);
+      handler()->NotifyIdleMarkCompact();
+    }
+    handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_EQ(GCIdleTimeHandler::kDone, handler()->mode());
+  }
+
+  void TransitionToReduceLatencyMode(
+      const GCIdleTimeHandler::HeapState& heap_state) {
+    EXPECT_EQ(GCIdleTimeHandler::kDone, handler()->mode());
+    int limit = GCIdleTimeHandler::kMarkCompactsBeforeMutatorIsActive;
+    double idle_time_ms = GCIdleTimeHandler::kMinLongIdleTime;
+    for (int i = 0; i < limit; i++) {
+      GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+      EXPECT_EQ(DONE, action.type);
+      handler()->NotifyMarkCompact(true);
+    }
+    handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_EQ(GCIdleTimeHandler::kReduceLatency, handler()->mode());
+  }
+
   static const size_t kSizeOfObjects = 100 * MB;
   static const size_t kMarkCompactSpeed = 200 * KB;
   static const size_t kMarkingSpeed = 200 * KB;
   static const size_t kScavengeSpeed = 100 * KB;
   static const size_t kNewSpaceCapacity = 1 * MB;
   static const size_t kNewSpaceAllocationThroughput = 10 * KB;
-  static const int kMaxNotifications = 100;
+  static const int kMaxNotifications = 1000;
 
  private:
   GCIdleTimeHandler handler_;
@@ -212,8 +263,11 @@
   heap_state.contexts_disposed = 1;
   heap_state.incremental_marking_stopped = true;
   double idle_time_ms = 0;
-  GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
-  EXPECT_EQ(DO_NOTHING, action.type);
+  for (int mode = 0; mode < 1; mode++) {
+    GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_EQ(DO_NOTHING, action.type);
+    TransitionToReduceMemoryMode(heap_state);
+  }
 }
 
 
@@ -224,8 +278,12 @@
       GCIdleTimeHandler::kHighContextDisposalRate - 1;
   heap_state.incremental_marking_stopped = true;
   double idle_time_ms = 0;
-  GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
-  EXPECT_EQ(DO_FULL_GC, action.type);
+  for (int mode = 0; mode < 1; mode++) {
+    GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_EQ(DO_FULL_GC, action.type);
+    heap_state.contexts_disposal_rate = 0.0;
+    TransitionToReduceMemoryMode(heap_state);
+  }
 }
 
 
@@ -235,12 +293,34 @@
   heap_state.contexts_disposal_rate = 1.0;
   heap_state.incremental_marking_stopped = true;
   double idle_time_ms = 0;
-  GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
-  EXPECT_EQ(DO_FULL_GC, action.type);
+  for (int mode = 0; mode < 1; mode++) {
+    GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_EQ(DO_FULL_GC, action.type);
+    heap_state.contexts_disposal_rate = 0.0;
+    TransitionToReduceMemoryMode(heap_state);
+  }
 }
 
 
 TEST_F(GCIdleTimeHandlerTest, AfterContextDisposeSmallIdleTime1) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  heap_state.contexts_disposed = 1;
+  heap_state.contexts_disposal_rate =
+      GCIdleTimeHandler::kHighContextDisposalRate;
+  heap_state.incremental_marking_stopped = true;
+  size_t speed = heap_state.mark_compact_speed_in_bytes_per_ms;
+  double idle_time_ms =
+      static_cast<double>(heap_state.size_of_objects / speed - 1);
+  for (int mode = 0; mode < 1; mode++) {
+    GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
+    heap_state.contexts_disposal_rate = 0.0;
+    TransitionToReduceMemoryMode(heap_state);
+  }
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, AfterContextDisposeSmallIdleTime2) {
   GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
   heap_state.contexts_disposed = 1;
   heap_state.contexts_disposal_rate =
@@ -248,160 +328,332 @@
   size_t speed = heap_state.mark_compact_speed_in_bytes_per_ms;
   double idle_time_ms =
       static_cast<double>(heap_state.size_of_objects / speed - 1);
-  GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
-  EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
-}
-
-
-TEST_F(GCIdleTimeHandlerTest, AfterContextDisposeSmallIdleTime2) {
-  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
-  heap_state.contexts_disposed = 1;
-  heap_state.contexts_disposal_rate =
-      GCIdleTimeHandler::kHighContextDisposalRate;
+  for (int mode = 0; mode < 1; mode++) {
+    GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
+    heap_state.contexts_disposal_rate = 0.0;
+    TransitionToReduceMemoryMode(heap_state);
+  }
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, IncrementalMarking1) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  size_t speed = heap_state.incremental_marking_speed_in_bytes_per_ms;
+  double idle_time_ms = 10;
+  for (int mode = 0; mode < 1; mode++) {
+    GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
+    EXPECT_GT(speed * static_cast<size_t>(idle_time_ms),
+              static_cast<size_t>(action.parameter));
+    EXPECT_LT(0, action.parameter);
+    TransitionToReduceMemoryMode(heap_state);
+  }
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, IncrementalMarking2) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  heap_state.incremental_marking_stopped = true;
+  size_t speed = heap_state.incremental_marking_speed_in_bytes_per_ms;
+  double idle_time_ms = 10;
+  for (int mode = 0; mode < 1; mode++) {
+    GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
+    EXPECT_GT(speed * static_cast<size_t>(idle_time_ms),
+              static_cast<size_t>(action.parameter));
+    EXPECT_LT(0, action.parameter);
+    TransitionToReduceMemoryMode(heap_state);
+  }
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, NotEnoughTime) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  heap_state.incremental_marking_stopped = true;
+  heap_state.can_start_incremental_marking = false;
   size_t speed = heap_state.mark_compact_speed_in_bytes_per_ms;
   double idle_time_ms =
       static_cast<double>(heap_state.size_of_objects / speed - 1);
   GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+  EXPECT_EQ(DO_NOTHING, action.type);
+  TransitionToReduceMemoryMode(heap_state);
+  action = handler()->Compute(idle_time_ms, heap_state);
   EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
 }
 
 
-TEST_F(GCIdleTimeHandlerTest, IncrementalMarking1) {
-  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
-  size_t speed = heap_state.incremental_marking_speed_in_bytes_per_ms;
-  double idle_time_ms = 10;
-  GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
-  EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
-  EXPECT_GT(speed * static_cast<size_t>(idle_time_ms),
-            static_cast<size_t>(action.parameter));
-  EXPECT_LT(0, action.parameter);
-}
-
-
-TEST_F(GCIdleTimeHandlerTest, IncrementalMarking2) {
-  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
-  size_t speed = heap_state.incremental_marking_speed_in_bytes_per_ms;
-  double idle_time_ms = 10;
-  GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
-  EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
-  EXPECT_GT(speed * static_cast<size_t>(idle_time_ms),
-            static_cast<size_t>(action.parameter));
-  EXPECT_LT(0, action.parameter);
-}
-
-
-TEST_F(GCIdleTimeHandlerTest, NotEnoughTime) {
-  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
-  heap_state.incremental_marking_stopped = true;
+TEST_F(GCIdleTimeHandlerTest, FinalizeSweeping) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  heap_state.incremental_marking_stopped = true;
+  heap_state.can_start_incremental_marking = false;
+  for (int mode = 0; mode < 1; mode++) {
+    heap_state.sweeping_in_progress = true;
+    heap_state.sweeping_completed = true;
+    double idle_time_ms = 10.0;
+    GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_EQ(DO_FINALIZE_SWEEPING, action.type);
+    heap_state.sweeping_in_progress = false;
+    heap_state.sweeping_completed = false;
+    TransitionToReduceMemoryMode(heap_state);
+  }
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, CannotFinalizeSweeping) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  heap_state.incremental_marking_stopped = true;
+  heap_state.can_start_incremental_marking = false;
+  for (int mode = 0; mode < 1; mode++) {
+    heap_state.sweeping_in_progress = true;
+    heap_state.sweeping_completed = false;
+    double idle_time_ms = 10.0;
+    GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_EQ(DO_NOTHING, action.type);
+    heap_state.sweeping_in_progress = false;
+    heap_state.sweeping_completed = false;
+    TransitionToReduceMemoryMode(heap_state);
+  }
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, Scavenge) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  int idle_time_ms = 10;
+  for (int mode = 0; mode < 1; mode++) {
+    heap_state.used_new_space_size =
+        heap_state.new_space_capacity -
+        (kNewSpaceAllocationThroughput * idle_time_ms);
+    GCIdleTimeAction action =
+        handler()->Compute(static_cast<double>(idle_time_ms), heap_state);
+    EXPECT_EQ(DO_SCAVENGE, action.type);
+    heap_state.used_new_space_size = 0;
+    TransitionToReduceMemoryMode(heap_state);
+  }
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, ScavengeAndDone) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  int idle_time_ms = 10;
+  heap_state.can_start_incremental_marking = false;
+  heap_state.incremental_marking_stopped = true;
+  for (int mode = 0; mode < 1; mode++) {
+    heap_state.used_new_space_size =
+        heap_state.new_space_capacity -
+        (kNewSpaceAllocationThroughput * idle_time_ms);
+    GCIdleTimeAction action =
+        handler()->Compute(static_cast<double>(idle_time_ms), heap_state);
+    EXPECT_EQ(DO_SCAVENGE, action.type);
+    heap_state.used_new_space_size = 0;
+    action = handler()->Compute(static_cast<double>(idle_time_ms), heap_state);
+    EXPECT_EQ(DO_NOTHING, action.type);
+    TransitionToReduceMemoryMode(heap_state);
+  }
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, StopEventually1) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  heap_state.incremental_marking_stopped = true;
+  heap_state.can_start_incremental_marking = false;
+  double idle_time_ms = GCIdleTimeHandler::kMinLongIdleTime;
+  bool stopped = false;
+  for (int i = 0; i < kMaxNotifications && !stopped; i++) {
+    GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+    if (action.type == DO_INCREMENTAL_MARKING || action.type == DO_FULL_GC) {
+      handler()->NotifyMarkCompact(true);
+      handler()->NotifyIdleMarkCompact();
+    }
+    if (action.type == DONE) stopped = true;
+  }
+  EXPECT_TRUE(stopped);
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, StopEventually2) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  heap_state.incremental_marking_stopped = true;
+  heap_state.can_start_incremental_marking = false;
   size_t speed = heap_state.mark_compact_speed_in_bytes_per_ms;
   double idle_time_ms =
-      static_cast<double>(heap_state.size_of_objects / speed - 1);
+      static_cast<double>(heap_state.size_of_objects / speed + 1);
+  TransitionToReduceMemoryMode(heap_state);
+  TransitionToDoneMode(heap_state, idle_time_ms, DO_FULL_GC);
   GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
   EXPECT_EQ(DONE, action.type);
 }
 
 
-TEST_F(GCIdleTimeHandlerTest, FinalizeSweeping) {
-  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
-  heap_state.incremental_marking_stopped = true;
-  heap_state.sweeping_in_progress = true;
-  heap_state.sweeping_completed = true;
-  double idle_time_ms = 10.0;
-  GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
-  EXPECT_EQ(DO_FINALIZE_SWEEPING, action.type);
-}
-
-
-TEST_F(GCIdleTimeHandlerTest, CannotFinalizeSweeping) {
-  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
-  heap_state.incremental_marking_stopped = true;
+TEST_F(GCIdleTimeHandlerTest, StopEventually3) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  heap_state.incremental_marking_stopped = true;
+  heap_state.can_start_incremental_marking = false;
+  double idle_time_ms = 10;
+  TransitionToReduceMemoryMode(heap_state);
+  TransitionToDoneMode(heap_state, idle_time_ms, DO_INCREMENTAL_MARKING);
+  GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+  EXPECT_EQ(DONE, action.type);
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, ContinueAfterStop1) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  heap_state.incremental_marking_stopped = true;
+  heap_state.can_start_incremental_marking = false;
+  size_t speed = heap_state.mark_compact_speed_in_bytes_per_ms;
+  double idle_time_ms =
+      static_cast<double>(heap_state.size_of_objects / speed + 1);
+  TransitionToReduceMemoryMode(heap_state);
+  TransitionToDoneMode(heap_state, idle_time_ms, DO_FULL_GC);
+  GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+  EXPECT_EQ(DONE, action.type);
+  TransitionToReduceLatencyMode(heap_state);
+  heap_state.can_start_incremental_marking = true;
+  action = handler()->Compute(idle_time_ms, heap_state);
+  EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
+  EXPECT_FALSE(action.reduce_memory);
+  EXPECT_EQ(GCIdleTimeHandler::kReduceLatency, handler()->mode());
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, ContinueAfterStop2) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  heap_state.incremental_marking_stopped = true;
+  heap_state.can_start_incremental_marking = false;
+  double idle_time_ms = 10;
+  TransitionToReduceMemoryMode(heap_state);
+  TransitionToDoneMode(heap_state, idle_time_ms, DO_INCREMENTAL_MARKING);
+  GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+  EXPECT_EQ(DONE, action.type);
+  TransitionToReduceLatencyMode(heap_state);
+  heap_state.can_start_incremental_marking = true;
+  action = handler()->Compute(idle_time_ms, heap_state);
+  EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
+  EXPECT_FALSE(action.reduce_memory);
+  EXPECT_EQ(GCIdleTimeHandler::kReduceLatency, handler()->mode());
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, ZeroIdleTimeNothingToDo) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  for (int i = 0; i < kMaxNotifications; i++) {
+    GCIdleTimeAction action = handler()->Compute(0, heap_state);
+    EXPECT_EQ(DO_NOTHING, action.type);
+  }
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, SmallIdleTimeNothingToDo) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  heap_state.incremental_marking_stopped = true;
+  heap_state.can_start_incremental_marking = false;
+  for (int i = 0; i < kMaxNotifications; i++) {
+    GCIdleTimeAction action = handler()->Compute(10, heap_state);
+    EXPECT_TRUE(DO_NOTHING == action.type || DONE == action.type);
+  }
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, StayInReduceLatencyModeBecauseOfScavenges) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  heap_state.incremental_marking_stopped = true;
+  heap_state.can_start_incremental_marking = false;
+  double idle_time_ms = GCIdleTimeHandler::kMinLongIdleTime;
+  int limit = GCIdleTimeHandler::kLongIdleNotificationsBeforeMutatorIsIdle;
+  for (int i = 0; i < kMaxNotifications; i++) {
+    GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_TRUE(DO_NOTHING == action.type || DONE == action.type);
+    if ((i + 1) % limit == 0) handler()->NotifyScavenge();
+    EXPECT_EQ(GCIdleTimeHandler::kReduceLatency, handler()->mode());
+  }
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, StayInReduceLatencyModeBecauseOfMarkCompacts) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  heap_state.incremental_marking_stopped = true;
+  heap_state.can_start_incremental_marking = false;
+  double idle_time_ms = GCIdleTimeHandler::kMinLongIdleTime;
+  int limit = GCIdleTimeHandler::kLongIdleNotificationsBeforeMutatorIsIdle;
+  for (int i = 0; i < kMaxNotifications; i++) {
+    GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_TRUE(DO_NOTHING == action.type || DONE == action.type);
+    if ((i + 1) % limit == 0) handler()->NotifyMarkCompact(true);
+    EXPECT_EQ(GCIdleTimeHandler::kReduceLatency, handler()->mode());
+  }
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, ReduceMemoryToReduceLatency) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  heap_state.incremental_marking_stopped = true;
+  heap_state.can_start_incremental_marking = false;
+  double idle_time_ms = GCIdleTimeHandler::kMinLongIdleTime;
+  int limit = GCIdleTimeHandler::kMaxIdleMarkCompacts;
+  for (int idle_gc = 0; idle_gc < limit; idle_gc++) {
+    TransitionToReduceMemoryMode(heap_state);
+    GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
+    EXPECT_TRUE(action.reduce_memory);
+    EXPECT_EQ(GCIdleTimeHandler::kReduceMemory, handler()->mode());
+    for (int i = 0; i < idle_gc; i++) {
+      action = handler()->Compute(idle_time_ms, heap_state);
+      EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
+      EXPECT_TRUE(action.reduce_memory);
+      // ReduceMemory mode should tolerate one mutator GC per idle GC.
+      handler()->NotifyScavenge();
+      // Notify idle GC.
+      handler()->NotifyMarkCompact(true);
+      handler()->NotifyIdleMarkCompact();
+    }
+    // Transition to ReduceLatency mode after doing |idle_gc| idle GCs.
+    handler()->NotifyScavenge();
+    action = handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_EQ(DO_NOTHING, action.type);
+    EXPECT_FALSE(action.reduce_memory);
+    EXPECT_EQ(GCIdleTimeHandler::kReduceLatency, handler()->mode());
+  }
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, ReduceMemoryToDone) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  heap_state.incremental_marking_stopped = true;
+  heap_state.can_start_incremental_marking = false;
+  double idle_time_ms = GCIdleTimeHandler::kMinLongIdleTime;
+  int limit = GCIdleTimeHandler::kMaxIdleMarkCompacts;
+  TransitionToReduceMemoryMode(heap_state);
+  GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+  EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
+  EXPECT_TRUE(action.reduce_memory);
+  for (int i = 0; i < limit; i++) {
+    action = handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
+    EXPECT_TRUE(action.reduce_memory);
+    EXPECT_EQ(GCIdleTimeHandler::kReduceMemory, handler()->mode());
+    // ReduceMemory mode should tolerate one mutator GC per idle GC.
+    handler()->NotifyScavenge();
+    // Notify idle GC.
+    handler()->NotifyMarkCompact(true);
+    handler()->NotifyIdleMarkCompact();
+  }
+  action = handler()->Compute(idle_time_ms, heap_state);
+  EXPECT_EQ(DONE, action.type);
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, DoneIfNotMakingProgressOnSweeping) {
+  // Regression test for crbug.com/489323.
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+
+  // Simulate sweeping being in-progress but not complete.
+  heap_state.incremental_marking_stopped = true;
+  heap_state.can_start_incremental_marking = false;
   heap_state.sweeping_in_progress = true;
   heap_state.sweeping_completed = false;
   double idle_time_ms = 10.0;
-  GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
-  EXPECT_EQ(DO_NOTHING, action.type);
-}
-
-
-TEST_F(GCIdleTimeHandlerTest, Scavenge) {
-  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
-  int idle_time_ms = 10;
-  heap_state.used_new_space_size =
-      heap_state.new_space_capacity -
-      (kNewSpaceAllocationThroughput * idle_time_ms);
-  GCIdleTimeAction action =
-      handler()->Compute(static_cast<double>(idle_time_ms), heap_state);
-  EXPECT_EQ(DO_SCAVENGE, action.type);
-  heap_state.used_new_space_size = 0;
-}
-
-
-TEST_F(GCIdleTimeHandlerTest, ScavengeAndDone) {
-  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
-  int idle_time_ms = 10;
-  heap_state.incremental_marking_stopped = true;
-  heap_state.used_new_space_size =
-      heap_state.new_space_capacity -
-      (kNewSpaceAllocationThroughput * idle_time_ms);
-  GCIdleTimeAction action =
-      handler()->Compute(static_cast<double>(idle_time_ms), heap_state);
-  EXPECT_EQ(DO_SCAVENGE, action.type);
-  heap_state.used_new_space_size = 0;
-  action = handler()->Compute(static_cast<double>(idle_time_ms), heap_state);
-  EXPECT_EQ(DONE, action.type);
-}
-
-
-TEST_F(GCIdleTimeHandlerTest, DoNotStartIncrementalMarking) {
-  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
-  heap_state.incremental_marking_stopped = true;
-  double idle_time_ms = 10.0;
-  GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
-  EXPECT_EQ(DONE, action.type);
-}
-
-
-TEST_F(GCIdleTimeHandlerTest, ContinueAfterStop) {
-  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
-  heap_state.incremental_marking_stopped = true;
-  double idle_time_ms = 10.0;
-  GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
-  EXPECT_EQ(DONE, action.type);
-  heap_state.incremental_marking_stopped = false;
-  action = handler()->Compute(idle_time_ms, heap_state);
-  EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
-}
-
-
-TEST_F(GCIdleTimeHandlerTest, ZeroIdleTimeNothingToDo) {
-  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
-  for (int i = 0; i < kMaxNotifications; i++) {
-    GCIdleTimeAction action = handler()->Compute(0, heap_state);
-    EXPECT_EQ(DO_NOTHING, action.type);
-  }
-}
-
-
-TEST_F(GCIdleTimeHandlerTest, SmallIdleTimeNothingToDo) {
-  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
-  heap_state.incremental_marking_stopped = true;
-  for (int i = 0; i < kMaxNotifications; i++) {
-    GCIdleTimeAction action = handler()->Compute(10, heap_state);
-    EXPECT_TRUE(DO_NOTHING == action.type || DONE == action.type);
-  }
-}
-
-
-TEST_F(GCIdleTimeHandlerTest, DoneIfNotMakingProgressOnSweeping) {
-  // Regression test for crbug.com/489323.
-  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
-
-  // Simulate sweeping being in-progress but not complete.
-  heap_state.incremental_marking_stopped = true;
-  heap_state.sweeping_in_progress = true;
-  heap_state.sweeping_completed = false;
-  double idle_time_ms = 10.0;
-  for (int i = 0; i < GCIdleTimeHandler::kMaxNoProgressIdleTimes; i++) {
+  for (int i = 0; i < GCIdleTimeHandler::kMaxNoProgressIdleTimesPerMode; i++) {
     GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
     EXPECT_EQ(DO_NOTHING, action.type);
   }
@@ -417,9 +669,48 @@
 
   // Simulate incremental marking stopped and not eligible to start.
   heap_state.incremental_marking_stopped = true;
+  heap_state.can_start_incremental_marking = false;
   double idle_time_ms = 10.0;
-  // We should return DONE if we cannot start incremental marking.
-  GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+  for (int i = 0; i < GCIdleTimeHandler::kMaxNoProgressIdleTimesPerMode; i++) {
+    GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_EQ(DO_NOTHING, action.type);
+  }
+  // We should return DONE after not making progress for some time.
+  GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+  EXPECT_EQ(DONE, action.type);
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, BackgroundReduceLatencyToReduceMemory) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  heap_state.incremental_marking_stopped = false;
+  heap_state.can_start_incremental_marking = true;
+  double idle_time_ms = GCIdleTimeHandler::kMinBackgroundIdleTime;
+  handler()->NotifyScavenge();
+  EXPECT_EQ(GCIdleTimeHandler::kReduceLatency, handler()->mode());
+  int limit =
+      GCIdleTimeHandler::kBackgroundIdleNotificationsBeforeMutatorIsIdle;
+  for (int i = 0; i < limit; i++) {
+    GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+    EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
+  }
+  handler()->Compute(idle_time_ms, heap_state);
+  EXPECT_EQ(GCIdleTimeHandler::kReduceMemory, handler()->mode());
+}
+
+
+TEST_F(GCIdleTimeHandlerTest, SkipUselessGCs) {
+  GCIdleTimeHandler::HeapState heap_state = DefaultHeapState();
+  heap_state.incremental_marking_stopped = false;
+  heap_state.can_start_incremental_marking = true;
+  TransitionToReduceMemoryMode(heap_state);
+  EXPECT_EQ(GCIdleTimeHandler::kReduceMemory, handler()->mode());
+  double idle_time_ms = GCIdleTimeHandler::kMinLongIdleTime;
+  GCIdleTimeAction action = handler()->Compute(idle_time_ms, heap_state);
+  EXPECT_EQ(DO_INCREMENTAL_MARKING, action.type);
+  handler()->NotifyMarkCompact(false);
+  handler()->NotifyIdleMarkCompact();
+  action = handler()->Compute(idle_time_ms, heap_state);
   EXPECT_EQ(DONE, action.type);
 }