Create base::win::MemoryPressureMonitor class.

base/win/memory_pressure_monitor.h

+// Copyright 2015 The Chromium 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 BASE_WIN_MEMORY_PRESSURE_MONITOR_H_
+#define BASE_WIN_MEMORY_PRESSURE_MONITOR_H_
+
+#include "base/base_export.h"
+#include "base/memory/memory_pressure_listener.h"
+#include "base/memory/memory_pressure_monitor.h"
+#include "base/memory/weak_ptr.h"
+#include "base/threading/thread_checker.h"
+#include "base/timer/timer.h"
+
+// To not pull in windows.h.
+typedef struct _MEMORYSTATUSEX MEMORYSTATUSEX;
+
+namespace base {
+namespace win {
+
+class TestMemoryPressureMonitor;
+
+// Windows memory pressure monitor. Because there is no OS provided signal this
+// polls at a low frequency (once per second), and applies internal hysteresis.
+class BASE_EXPORT MemoryPressureMonitor : public base::MemoryPressureMonitor {
+ public:
+  // Constants governing the polling and hysteresis behaviour of the observer.
+
+  // The polling interval, in milliseconds. While under critical pressure, this
+  // is also the timer to repeat cleanup attempts.
+  static const int kPollingIntervalMs;
+  // The time which should pass between 2 successive moderate memory pressure
+  // signals, in milliseconds.
+  static const int kModeratePressureCooldownMs;
+  // The number of cycles that should pass between 2 successive moderate memory
+  // pressure signals.
+  static const int kModeratePressureCooldownCycles;
+
+  MemoryPressureMonitor();
+  ~MemoryPressureMonitor() override;
+
+  // Schedules a memory pressure check to run soon. This should be called on the

[grt (UTC plus 2), 2015/05/06 16:31:09]

nit: should -> must

+  // same thread where the monitor was instantiated.
+  void CheckMemoryPressureSoon();
+
+  // Get the current memory pressure level. This can be called from any thread.
+  MemoryPressureLevel GetCurrentPressureLevel() const override;
+
+ private:
+  friend TestMemoryPressureMonitor;
+
+  // Starts observing the memory fill level. Calls to StartObserving should
+  // always be matched with calls to StopObserving.
+  void StartObserving();
+
+  // Stop observing the memory fill level. May be safely called if
+  // StartObserving has not been called. Must be called from the same thread on
+  // which the monitor was instantiated.
+  void StopObserving();
+
+  // Checks memory pressure, storing the current level, applying any hysteresis
+  // and emitting memory pressure level change signals as necessary. This
+  // function is called periodically while the monitor is observing memory
+  // pressure. This is split out from CheckMemoryPressureAndRecordStatistics so
+  // that it may be called by CheckMemoryPressureSoon and not invoke UMA
+  // logging. Must be called from the same thread on which the monitor was
+  // instantiated.
+  void CheckMemoryPressure();
+
+  // Wrapper to CheckMemoryPressure that also records the observed memory
+  // pressure level via an UMA enumeration. This is the function that is called
+  // periodically by the timer. Must be called from the same thread on which the
+  // monitor was instantiated.
+  void CheckMemoryPressureAndRecordStatistics();
+
+  // Calculates the current instantaneous memory pressure level. This does not
+  // use any hysteresis and simply returns the result at the current moment. Can
+  // be called on any thread.
+  MemoryPressureLevel CalculateCurrentPressureLevel();
+
+  // Gets system memory status. This is virtual as a unittesting hook. Returns
+  // true if the system call succeeds, false otherwise. Can be called on any
+  // thread.
+  virtual bool GetSystemMemoryStatus(MEMORYSTATUSEX* mem_status);
+
+  // A periodic timer to check for memory pressure changes.
+  base::RepeatingTimer<MemoryPressureMonitor> timer_;
+
+  // The current memory pressure.
+  MemoryPressureLevel current_memory_pressure_level_;
+
+  // To slow down the amount of moderate pressure event calls, this gets used to
+  // count the number of events since the last event occured. This is used by
+  // |CheckMemoryPressure| to apply hysteresis on the raw results of
+  // |CalculateCurrentPressureLevel|.
+  int moderate_pressure_repeat_count_;
+
+  // Weak pointer factory to ourself used for scheduling calls to
+  // CheckMemoryPressure/CheckMemoryPressureAndRecordStatistics via |timer_|.
+  base::WeakPtrFactory<MemoryPressureMonitor> weak_ptr_factory_;
+
+  // Ensures that this object is used from a single thread.
+  base::ThreadChecker thread_checker_;
+
+  DISALLOW_COPY_AND_ASSIGN(MemoryPressureMonitor);
+};
+
+}  // namespace win
+}  // namespace base
+
+#endif  // BASE_WIN_MEMORY_PRESSURE_MONITOR_H_