[Mac] Collect real-time memory pressure stats, in an energy-efficient way

base/memory/memory_pressure_monitor_mac.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_MEMORY_MEMORY_PRESSURE_MONITOR_MAC_H_
 #define BASE_MEMORY_MEMORY_PRESSURE_MONITOR_MAC_H_
 
 #include <CoreFoundation/CFDate.h>
 #include <dispatch/dispatch.h>
 
 #include "base/base_export.h"
+#include "base/mac/scoped_cftyperef.h"
 #include "base/mac/scoped_dispatch_object.h"
 #include "base/macros.h"
 #include "base/memory/memory_pressure_listener.h"
 #include "base/memory/memory_pressure_monitor.h"
+#include "base/message_loop/message_pump_mac.h"
 
 namespace base {
 namespace mac {
 
 class TestMemoryPressureMonitor;
 
 // Declares the interface for the Mac MemoryPressureMonitor, which reports
 // memory pressure events and status.
 class BASE_EXPORT MemoryPressureMonitor : public base::MemoryPressureMonitor {
  public:
   MemoryPressureMonitor();
   ~MemoryPressureMonitor() override;
 
   // Returns the currently-observed memory pressure.
   MemoryPressureLevel GetCurrentPressureLevel() override;
 
   void SetDispatchCallback(const DispatchCallback& callback) override;
 
  private:
   friend TestMemoryPressureMonitor;
 
-  static MemoryPressureLevel
-      MemoryPressureLevelForMacMemoryPressure(int mac_memory_pressure);
+  static MemoryPressureLevel MemoryPressureLevelForMacMemoryPressureLevel(
+      int mac_memory_pressure_level);
+  static void OnRunLoopExit(CFRunLoopObserverRef observer,
+                            CFRunLoopActivity activity,
+                            void* info);
+  // Returns the raw memory pressure level from the macOS. Exposed for
+  // unit testing.
+  virtual int GetMacMemoryPressureLevel();
+
+  // Updates |last_pressure_level_| with the current memory pressure level.
+  void UpdatePressureLevel();
+
+  // Updates |last_pressure_level_| at the end of every run loop pass (modulo
+  // some number of seconds).
+  void UpdatePressureLevelOnRunLoopExit();
+
+  // Run |dispatch_callback| on memory pressure notifications from the OS.
   void OnMemoryPressureChanged(dispatch_source_s* event_source,
                                const DispatchCallback& dispatch_callback);
-  void SendStatisticsIfNecessary(bool pressure_level_changed);
 
+  // Returns the number of seconds per UMA tick (for statistics recording).
+  // Exposed for testing.
+  static int GetSecondsPerUMATick();
+
+  // The dispatch source that generates memory pressure change notifications.
   ScopedDispatchObject<dispatch_source_t> memory_level_event_source_;
 
+  // The callback to call upon receiving a memory pressure change notification.
   DispatchCallback dispatch_callback_;
 
-  CFTimeInterval last_statistic_report_;
+  // Last UMA report time.
+  CFTimeInterval last_statistic_report_time_;
 
+  // Most-recent memory pressure level.
   MemoryPressureLevel last_pressure_level_;
 
-  // The UMA statistic is recorded in 5 second increments. This
-  // accumulates the remaining time to be rolled into the next
-  // call.
-  CFTimeInterval reporting_error_;
+  // Observer that tracks exits from the main run loop.
+  ScopedCFTypeRef<CFRunLoopObserverRef> exit_observer_;
+
+  // Next time to update the memory pressure level when exiting the run loop.
+  CFTimeInterval next_run_loop_update_time_;
+
+  // Seconds left over from the last UMA tick calculation (to be added to the
+  // next calculation).
+  CFTimeInterval subtick_seconds_;
 
   DISALLOW_COPY_AND_ASSIGN(MemoryPressureMonitor);
 };
 
 }  // namespace mac
 }  // namespace base
 
 #endif  // BASE_MEMORY_MEMORY_PRESSURE_MONITOR_MAC_H_