mac: Add a profiler for startup time.

chrome/browser/mac/mac_startup_profiler.h

+// Copyright 2014 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 CHROME_BROWSER_MAC_MAC_STARTUP_PROFILER_H_
+#define CHROME_BROWSER_MAC_MAC_STARTUP_PROFILER_H_
+
+#include <map>
+
+#include "base/memory/singleton.h"
+#include "base/time/time.h"
+
+// A lightweight profiler of startup performance. Records UMA metrics for the
+// time delta between Chrome's launch and major initialization phases.
+class MacStartupProfiler {

[jeremy, 2014/07/16 08:19:24]

This seems to parallel the code in components/startup_metric_utils/ - do you
think this should stay separate or should we merge the two and adding the calls
for the non-Mac-specific parts to cross-platform code so we're recording these
stats for all platforms where it makes sense?

[erikchen, 2014/07/16 22:00:33]

I like to only record histograms that are intended to be used. Of the seven
histograms, 4 of them are platform-independent. If you intend to use them (or
know someone who does), I will gladly add them to the other platforms, and port
this code into components/startup_metric_utils. Otherwise, I will not move this
class. 

+ public:
+  // Returns pointer to the singleton instance for the current process.
+  static MacStartupProfiler* GetInstance();
+
+  MacStartupProfiler();
+  ~MacStartupProfiler();
+
+  // These locations correspond to major phases of Chrome startup.
+  // Profiling of these locations should occur at the beginning of the method
+  // indicated by the enum name.
+  // The ordering of the enum matches the order in which the initialization
+  // phases are reached.
+  enum Location {
+    PRE_MAIN_MESSAGE_LOOP_START,
+    AWAKE_FROM_NIB,
+    POST_MAIN_MESSAGE_LOOP_START,
+    PRE_PROFILE_INIT,
+    POST_PROFILE_INIT,
+    WILL_FINISH_LAUNCHING,
+    DID_FINISH_LAUNCHING,
+  };
+
+  // This method should be called at the earliest convenient time after Chrome
+  // has launched.

[jeremy, 2014/07/16 08:19:24]

We currently record Startup.BrowserMessageLoopStartTimeFromMainEntry and
Startup.BrowserMessageLoopStartTime , the difference between these is when we
start the clock - at process creation time or main entry.
The first one is to try to reduce noise by removing the time it takes to load
the Chrome binary.

[erikchen, 2014/07/16 22:00:33]

Ah, that is good to know. Thanks!

+  void RecordLaunchTime();
+
+  // Saves the current time for later processing.
+  void Profile(Location location);
+
+  // Records UMA metrics from all profiled locations.
+  void RecordMetrics();
+
+ private:
+  friend struct DefaultSingletonTraits<MacStartupProfiler>;
+
+  // Returns the name of the histogram for the given location.
+  const std::string HistogramName(Location location);
+
+  // Records UMA metrics for a specific location.
+  void RecordHistogram(Location location, const base::TimeDelta& delta);
+
+  // Keeps track of the time at which each initialization phase was reached.
+  std::map<Location, base::Time> profiled_times_;
+
+  // Whether UMA metrics have been recorded. Only record UMA metrics once.
+  bool recorded_metrics_;
+
+  // The time at which Chrome was launched.
+  base::Time launch_time_;
+
+  DISALLOW_COPY_AND_ASSIGN(MacStartupProfiler);
+};
+
+#endif  // CHROME_BROWSER_MAC_MAC_STARTUP_PROFILER_H_