PerformanceMonitor: Add a new UMA histograms to track average CPU utilization

base/process/process_metrics.h

 // Copyright (c) 2013 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.
 
 // This file contains routines for gathering resource statistics for processes
 // running on the system.
 
 #ifndef BASE_PROCESS_PROCESS_METRICS_H_
 #define BASE_PROCESS_PROCESS_METRICS_H_
 
@@ skipping 150 matching lines @@
   bool CalculateFreeMemory(FreeMBytes* free) const;
 
   // Returns the CPU usage in percent since the last time this method was
   // called. The first time this method is called it returns 0 and will return
   // the actual CPU info on subsequent calls.
   // On Windows, the CPU usage value is for all CPUs. So if you have 2 CPUs and
   // your process is using all the cycles of 1 CPU and not the other CPU, this
   // method returns 50.
   double GetCPUUsage();
 
+  // Same as GetCPUUsage(), but will return consistent returns on all platforms
+  // including Windows.

[Alexei Svitkine (slow), 2013/10/24 14:52:00]

Mention what the consistent values actually are.

[oystein (OOO til 10th of July), 2013/10/24 15:01:06]

Actually meant to write "values" rather than "returns" (fixed now), but
otherwise do you mean basically repeating the comment for GetCPUUsage() above?

[Alexei Svitkine (slow), 2013/10/24 15:13:33]

Not necessarily repeating, you can refer to it - just make it clear that it
makes Windows consistent with other platforms and not vice versa, but being
super explicit is even better (e.g. specify that 1 core maxed is 100% and 2
cores maxed is 200%, etc.)

Also, now that I look at the comment above, it says "Returns the CPU usage in
percent since the last time this method was called." Probably now it should say
"since the last GetCPUUsage() or GetPlatformIndepententCPUUsage() call".

[oystein (OOO til 10th of July), 2013/10/24 15:25:51]

Good point; done.

+  double GetPlatformIndependentCPUUsage();
+
   // Retrieves accounting information for all I/O operations performed by the
   // process.
   // If IO information is retrieved successfully, the function returns true
   // and fills in the IO_COUNTERS passed in. The function returns false
   // otherwise.
   bool GetIOCounters(IoCounters* io_counters) const;
 
  private:
 #if !defined(OS_MACOSX) || defined(OS_IOS)
   explicit ProcessMetrics(ProcessHandle process);
@@ skipping 175 matching lines @@
   SystemDiskInfo disk_info_;
 #endif
 #if defined(OS_CHROMEOS)
   SwapInfo swap_info_;
 #endif
 };
 
 }  // namespace base
 
 #endif  // BASE_PROCESS_PROCESS_METRICS_H_