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

base/memory/memory_pressure_monitor_mac.cc

Index: base/memory/memory_pressure_monitor_mac.cc
diff --git a/base/memory/memory_pressure_monitor_mac.cc b/base/memory/memory_pressure_monitor_mac.cc
index 391589021bd3cd6988fa10d53cf36535301e3b9e..6261de226c20296c504964a6037c012cb2f2fb32 100644
--- a/base/memory/memory_pressure_monitor_mac.cc
+++ b/base/memory/memory_pressure_monitor_mac.cc
@@ -4,6 +4,8 @@
 
 #include "base/memory/memory_pressure_monitor_mac.h"
 
+#include <CoreFoundation/CoreFoundation.h>
+
 #include <dlfcn.h>
 #include <stddef.h>
 #include <sys/sysctl.h>
@@ -26,9 +28,9 @@ namespace base {
 namespace mac {
 
 MemoryPressureListener::MemoryPressureLevel
-MemoryPressureMonitor::MemoryPressureLevelForMacMemoryPressure(
-    int mac_memory_pressure) {
-  switch (mac_memory_pressure) {
+MemoryPressureMonitor::MemoryPressureLevelForMacMemoryPressureLevel(
+    int mac_memory_pressure_level) {
+  switch (mac_memory_pressure_level) {
     case DISPATCH_MEMORYPRESSURE_NORMAL:
       return MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE;
     case DISPATCH_MEMORYPRESSURE_WARN:
@@ -39,6 +41,13 @@ MemoryPressureMonitor::MemoryPressureLevelForMacMemoryPressure(
   return MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE;
 }
 
+void MemoryPressureMonitor::OnRunLoopExit(CFRunLoopObserverRef observer,
+                                          CFRunLoopActivity activity,
+                                          void* info) {
+  MemoryPressureMonitor* self = static_cast<MemoryPressureMonitor*>(info);
+  self->UpdatePressureLevelOnRunLoopExit();
+}
+
 MemoryPressureMonitor::MemoryPressureMonitor()
     : memory_level_event_source_(dispatch_source_create(
           DISPATCH_SOURCE_TYPE_MEMORYPRESSURE,
@@ -48,75 +57,138 @@ MemoryPressureMonitor::MemoryPressureMonitor()
           dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0))),
       dispatch_callback_(
           base::Bind(&MemoryPressureListener::NotifyMemoryPressure)),
-      last_statistic_report_(CFAbsoluteTimeGetCurrent()),
+      last_statistic_report_time_(CFAbsoluteTimeGetCurrent()),
       last_pressure_level_(MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE),
-      reporting_error_(0) {
-  if (memory_level_event_source_.get() != nullptr) {
+      subtick_seconds_(0) {
+  // Attach an event handler to the memory pressure event source.
+  if (memory_level_event_source_.get()) {
     dispatch_source_set_event_handler(memory_level_event_source_, ^{
       OnMemoryPressureChanged(memory_level_event_source_.get(),
                               dispatch_callback_);
     });
+
+    // Start monitoring the event source.
     dispatch_resume(memory_level_event_source_);
   }
+
+  // Create a CFRunLoopObserver to check the memory pressure at the end of
+  // every pass through the event loop (modulo kUMATickSize).
+  CFRunLoopObserverContext observer_context = {0, this, NULL, NULL, NULL};
+
+  exit_observer_.reset(
+      CFRunLoopObserverCreate(kCFAllocatorDefault, kCFRunLoopExit, true, 0,
+                              OnRunLoopExit, &observer_context));
+
+  CFRunLoopRef run_loop = CFRunLoopGetCurrent();
+  CFRunLoopAddObserver(run_loop, exit_observer_, kCFRunLoopCommonModes);
+  CFRunLoopAddObserver(run_loop, exit_observer_,
+                       kMessageLoopExclusiveRunLoopMode);
 }
 
 MemoryPressureMonitor::~MemoryPressureMonitor() {
-  if (memory_level_event_source_.get() != nullptr)
+  // Detach from the run loop.
+  CFRunLoopRef run_loop = CFRunLoopGetCurrent();
+  CFRunLoopRemoveObserver(run_loop, exit_observer_, kCFRunLoopCommonModes);
+  CFRunLoopRemoveObserver(run_loop, exit_observer_,
+                          kMessageLoopExclusiveRunLoopMode);
+
+  // Remove the memory pressure event source.
+  if (memory_level_event_source_.get()) {
     dispatch_source_cancel(memory_level_event_source_);
+  }
 }
 
-MemoryPressureListener::MemoryPressureLevel
-MemoryPressureMonitor::GetCurrentPressureLevel() {
-  int mac_memory_pressure;
+int MemoryPressureMonitor::GetMacMemoryPressureLevel() {
+  // Get the raw memory pressure level from macOS.
+  int mac_memory_pressure_level;
   size_t length = sizeof(int);
-  sysctlbyname("kern.memorystatus_vm_pressure_level", &mac_memory_pressure,
-               &length, nullptr, 0);
-  MemoryPressureListener::MemoryPressureLevel memory_pressure_level =
-      MemoryPressureLevelForMacMemoryPressure(mac_memory_pressure);
-  bool pressure_level_changed = false;
-  if (last_pressure_level_ != memory_pressure_level) {
-    pressure_level_changed = true;
-  }
-  SendStatisticsIfNecessary(pressure_level_changed);
-  last_pressure_level_ = memory_pressure_level;
-  return memory_pressure_level;
-}
+  sysctlbyname("kern.memorystatus_vm_pressure_level",
+               &mac_memory_pressure_level, &length, nullptr, 0);
 
-void MemoryPressureMonitor::OnMemoryPressureChanged(
-    dispatch_source_s* event_source,
-    const MemoryPressureMonitor::DispatchCallback& dispatch_callback) {
-  int mac_memory_pressure = dispatch_source_get_data(event_source);
-  MemoryPressureListener::MemoryPressureLevel memory_pressure_level =
-      MemoryPressureLevelForMacMemoryPressure(mac_memory_pressure);
-  bool pressure_level_changed = false;
-  if (last_pressure_level_ != memory_pressure_level) {
-    pressure_level_changed = true;
-  }
-  SendStatisticsIfNecessary(pressure_level_changed);
-  last_pressure_level_ = memory_pressure_level;
-  if (memory_pressure_level !=
-      MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE)
-    dispatch_callback.Run(memory_pressure_level);
+  return mac_memory_pressure_level;
 }
 
-void MemoryPressureMonitor::SendStatisticsIfNecessary(
-    bool pressure_level_changed) {
+void MemoryPressureMonitor::UpdatePressureLevel() {
+  // Get the current macOS pressure level and convert to the corresponding
+  // Chrome pressure level.
+  int mac_memory_pressure_level = GetMacMemoryPressureLevel();
+  MemoryPressureListener::MemoryPressureLevel new_pressure_level =
+      MemoryPressureLevelForMacMemoryPressureLevel(mac_memory_pressure_level);
+
+  // Compute the number of "ticks" spent at |last_pressure_level_| (since the
+  // last report sent to UMA).
   CFTimeInterval now = CFAbsoluteTimeGetCurrent();
-  CFTimeInterval since_last_report = now - last_statistic_report_;
-  last_statistic_report_ = now;
+  CFTimeInterval time_since_last_report = now - last_statistic_report_time_;
+  last_statistic_report_time_ = now;
 
-  double accumulated_time = since_last_report + reporting_error_;
+  double accumulated_time = time_since_last_report + subtick_seconds_;
   int ticks_to_report = static_cast<int>(accumulated_time / kUMATickSize);
-  reporting_error_ = std::fmod(accumulated_time, kUMATickSize);
+  // Save for later the seconds that didn't make it into a full tick.
+  subtick_seconds_ = std::fmod(accumulated_time, kUMATickSize);
 
-  // Round up on change to ensure we capture it
+  // Round the tick count up on a pressure level change to ensure we capture it.
+  bool pressure_level_changed = (new_pressure_level != last_pressure_level_);
   if (pressure_level_changed && ticks_to_report < 1) {
     ticks_to_report = 1;
-    reporting_error_ = 0;
+    subtick_seconds_ = 0;
   }
 
-  if (ticks_to_report >= 1)
+  // Send elapsed ticks to UMA.
+  if (ticks_to_report >= 1) {
     RecordMemoryPressure(last_pressure_level_, ticks_to_report);
+  }
+
+  // Save the now-current memory pressure level.
+  last_pressure_level_ = new_pressure_level;
+}
+
+void MemoryPressureMonitor::UpdatePressureLevelOnRunLoopExit() {
+  // Wait until it's time to check the pressure level.
+  CFTimeInterval now = CFAbsoluteTimeGetCurrent();
+  if (now >= next_run_loop_update_time_) {
+    UpdatePressureLevel();
+
+    // Update again in kUMATickSize seconds. We can update at any frequency,
+    // but because we're only checking memory pressure levels for UMA there's
+    // no need to update more frequently than we're keeping statistics on.
+    next_run_loop_update_time_ = now + kUMATickSize - subtick_seconds_;
+  }
+}
+
+// Static.
+int MemoryPressureMonitor::GetSecondsPerUMATick() {
+  return kUMATickSize;
+}
+
+MemoryPressureListener::MemoryPressureLevel
+MemoryPressureMonitor::GetCurrentPressureLevel() {
+  UpdatePressureLevel();
+  return last_pressure_level_;
+}
+
+void MemoryPressureMonitor::OnMemoryPressureChanged(
+    dispatch_source_s* event_source,
+    const MemoryPressureMonitor::DispatchCallback& dispatch_callback) {
+  // Get the Chrome-equvialent memory pressure level.
+  int mac_memory_pressure_level = dispatch_source_get_data(event_source);
+  MemoryPressureListener::MemoryPressureLevel memory_pressure_level =
+      MemoryPressureLevelForMacMemoryPressureLevel(mac_memory_pressure_level);
+
+  // Run the callback that's waiting on memory pressure change notifications.
+  // Note that we don't bother with updating |last_pressure_level_| or
+  // causing memory pressure stats to be sent to UMA. Memory pressure change
+  // notifications are delayed on the Mac, so the current actual memory pressure
+  // level may be different than the incoming pressure level from the event.
+  //
+  // In general we don't want to take action (such as freeing memory) on
+  // memory pressure change events, but that's how the current system is
+  // designed. Given that it's incorrect to act on either stale or current
+  // pressure level info, it's not clear which level is better to send. For
+  // now stick with how it's been implemented to date, which is to send the
+  // stale value.
+  if (memory_pressure_level !=
+      MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE)
+    dispatch_callback.Run(memory_pressure_level);
 }
 
 void MemoryPressureMonitor::SetDispatchCallback(