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

src/heap/memory-reducer.h

+// 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.
+
+#ifndef V8_HEAP_memory_reducer_H
+#define V8_HEAP_memory_reducer_H
+
+#include "include/v8-platform.h"
+#include "src/base/macros.h"
+
+namespace v8 {
+namespace internal {
+
+class Heap;
+
+
+// The goal of the MemoryReducer class is to detect transition of the mutator
+// from high allocation phase to low allocation phase and to collect potential
+// garbage created in the high allocation phase.
+//
+// The class implements an automaton with the following states and transitions.
+//
+// States:
+// - DONE
+// - WAIT <started_gcs> <next_gc_start_ms>
+// - RUN <started_gcs>
+// The <started_gcs> is an integer in range from 0..kMaxNumberOfGCs that stores
+// the number of GCs initiated by the MemoryReducer since it left the DONE
+// state.
+// The <next_gc_start_ms> is a double that stores the earliest time the next GC
+// can be initiated by the MemoryReducer.
+// The DONE state means that the MemoryReducer is not active.
+// The WAIT state means that the MemoryReducer is waiting for mutator allocation
+// rate to drop. The check for the allocation rate happens in the timer task
+// callback.
+// The RUN state means that the MemoryReducer started incremental marking and is
+// waiting for it to finish. Incremental marking steps are performed as usual
+// in the idle notification and in the mutator.
+//
+// Transitions:
+// DONE -> WAIT 0 (now_ms + long_delay_ms) happens:
+//     - on context disposal,
+//     - at the end of mark-compact GC initiated by the mutator.
+// This signals that there is potential garbage to be collected.
+//
+// WAIT n x -> WAIT n (now_ms + long_delay_ms) happens:
+//     - on mark-compact GC initiated by the mutator,
+//     - in the timer callback if the mutator allocation rate is high or
+//       incremental GC is in progress.
+//
+// WAIT n x -> RUN (n+1) happens:
+//     - in the timer callback if the mutator allocation rate is low
+//       and now_ms >= x and there is no incremental GC in progress.
+// The MemoryReducer starts incremental marking on this transition.
+//
+// RUN n -> DONE happens:
+//     - at end of the incremental GC initiated by the MemoryReducer if
+//       (n > 1 and there is no more garbage to be collected) or
+//       n == kMaxNumberOfGCs.
+// RUN n -> WAIT n (now_ms + short_delay_ms) happens:
+//     - at end of the incremental GC initiated by the MemoryReducer if
+//       (n == 1 or there is more garbage to be collected) and
+//       n < kMaxNumberOfGCs.
+//
+// now_ms is the current time, long_delay_ms and short_delay_ms are constants.
+class MemoryReducer {
+ public:
+  enum Action { kDone, kWait, kRun };
+
+  struct State {
+    State(Action action, int started_gcs, double next_gc_start_ms)
+        : action(action),
+          started_gcs(started_gcs),
+          next_gc_start_ms(next_gc_start_ms) {}
+    Action action;
+    int started_gcs;
+    double next_gc_start_ms;
+  };
+
+  enum EventType {
+    kTimer,
+    kMarkCompact,
+    kContextDisposed,
+  };
+
+  struct Event {
+    EventType type;
+    double time_ms;
+    bool low_allocation_rate;
+    bool next_gc_likely_to_collect_more;
+    bool can_start_incremental_gc;
+  };
+
+  explicit MemoryReducer(Heap* heap) : heap_(heap), state_(kDone, 0, 0.0) {}
+  // Callbacks.
+  void NotifyTimer(const Event& event);
+  void NotifyMarkCompact(const Event& event);
+  void NotifyScavenge(const Event& event);
+  void NotifyContextDisposed(const Event& event);
+  // The step function that computes the next state from the current state and
+  // the incoming event.
+  static State Step(const State& state, const Event& event);
+  // Posts a timer task that will call NotifyTimer after the given delay.
+  void ScheduleTimer(double delay_ms);
+
+  static const int kLongDelayMs;
+  static const int kShortDelayMs;
+  static const int kMaxNumberOfGCs;
+
+  Heap* heap() { return heap_; }
+
+ private:
+  class TimerTask : public v8::Task {
+   public:
+    explicit TimerTask(MemoryReducer* memory_reducer)
+        : memory_reducer_(memory_reducer) {}
+    virtual ~TimerTask() {}
+
+   private:
+    // v8::Task overrides.
+    void Run() override;
+    MemoryReducer* memory_reducer_;
+    DISALLOW_COPY_AND_ASSIGN(TimerTask);
+  };
+
+  Heap* heap_;
+  State state_;
+
+  DISALLOW_COPY_AND_ASSIGN(MemoryReducer);
+};
+
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_HEAP_memory_reducer_H