Add UMA stats to get the quality of service of memory pressure handling scheme in ChromeOS.

base/chromeos/memory_pressure_observer_chromeos.cc

Index: base/chromeos/memory_pressure_observer_chromeos.cc
diff --git a/base/chromeos/memory_pressure_observer_chromeos.cc b/base/chromeos/memory_pressure_observer_chromeos.cc
index 8112c761e5e713e747cb015d8226e62a754c87a8..bbb8cec38bea210aa3b3b13258d8a1769eddc9b0 100644
--- a/base/chromeos/memory_pressure_observer_chromeos.cc
+++ b/base/chromeos/memory_pressure_observer_chromeos.cc
@@ -5,6 +5,7 @@
 #include "base/chromeos/memory_pressure_observer_chromeos.h"
 
 #include "base/message_loop/message_loop.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/process/process_metrics.h"
 #include "base/time/time.h"
 
@@ -29,6 +30,10 @@ const int kNormalMemoryPressureCriticalThresholdPercent = 90;
 const int kAggressiveMemoryPressureModerateThresholdPercent = 35;
 const int kAggressiveMemoryPressureCriticalThresholdPercent = 70;
 
+// Number of memory pressure levels. Should update this value if more memory
+// pressure level is introduced in MemoryPressureListener::MemoryPressureLevel.

[Mr4D (OOO till 08-26), 2015/02/04 00:11:19]

... if more memory pressure levels are introduced ...

Looking at the constants, there is a -1 .. 2. I think it would be a good idea to
map the pressure values to a new enum and store that instead (e.g. like
UMAMemoryPressureLevel). The new enum could then also have something like a
UMA_MEMORY_PRESSURE_MAX value which denotes the upper limit which is cleaner
than this.

+const int kNumMemoryPressureLevels = 3;
+
 // Converts a |MemoryPressureThreshold| value into a used memory percentage for
 // the moderate pressure event.
 int GetModerateMemoryThresholdInPercent(
@@ -51,6 +56,13 @@ int GetCriticalMemoryThresholdInPercent(
              : kNormalMemoryPressureCriticalThresholdPercent;
 }
 
+// Record UMA histogram statistics for the current memory pressure level.
+void RecordMemoryStatistics(
+    MemoryPressureListener::MemoryPressureLevel memory_pressure_level) {
+  UMA_HISTOGRAM_ENUMERATION("ChromeOS.MemoryPressureLevel",
+                            memory_pressure_level, kNumMemoryPressureLevels);
+}
+
 // Converts free percent of memory into a memory pressure value.
 MemoryPressureListener::MemoryPressureLevel GetMemoryPressureLevelFromFillLevel(
     int actual_fill_level,
@@ -92,7 +104,8 @@ void MemoryPressureObserverChromeOS::ScheduleEarlyCheck() {
 void MemoryPressureObserverChromeOS::StartObserving() {
   timer_.Start(FROM_HERE,
                TimeDelta::FromMilliseconds(kMemoryPressureIntervalMs),
-               Bind(&MemoryPressureObserverChromeOS::CheckMemoryPressure,
+               Bind(&MemoryPressureObserverChromeOS::
+                        CheckMemoryPressureAndRecordStatistics,
                     weak_ptr_factory_.GetWeakPtr()));
 }
 
@@ -101,6 +114,11 @@ void MemoryPressureObserverChromeOS::StopObserving() {
   timer_.Stop();
 }
 
+void MemoryPressureObserverChromeOS::CheckMemoryPressureAndRecordStatistics() {
+  CheckMemoryPressure();
+  RecordMemoryStatistics(current_memory_pressure_level_);

[Mr4D (OOO till 08-26), 2015/02/04 00:11:19]

In this case you can pull the content of the function into this function and add
a switch statement to convert the MemoryPressureLevel into a
UMAMemoryPressureLevel here. I would say that there is no need to create a
function with a single call which gets only called from one place.

+}
+
 void MemoryPressureObserverChromeOS::CheckMemoryPressure() {
   MemoryPressureListener::MemoryPressureLevel old_pressure =
       current_memory_pressure_level_;