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

base/memory/memory_pressure_monitor_mac.h

Index: base/memory/memory_pressure_monitor_mac.h
diff --git a/base/memory/memory_pressure_monitor_mac.h b/base/memory/memory_pressure_monitor_mac.h
index 9118632bd664e4e5289c3e60f7b3c842861ab5d6..b85b6c9017dbaaffe22a11b548c139ab3a59c8e1 100644
--- a/base/memory/memory_pressure_monitor_mac.h
+++ b/base/memory/memory_pressure_monitor_mac.h
@@ -9,10 +9,12 @@
 #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 {
@@ -34,24 +36,51 @@ class BASE_EXPORT MemoryPressureMonitor : public base::MemoryPressureMonitor {
  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);
 };