Report timing statistics for audio controller methods via UMA.

media/base/scoped_uma_timer.h

+// Copyright 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.
+
+#ifndef MEDIA_BASE_SCOPED_UMA_TIMER_H_
+#define MEDIA_BASE_SCOPED_UMA_TIMER_H_
+
+#include "base/metrics/histogram.h"
+#include "base/time.h"
+
+// Scoped class which logs its time on this earth as a UMA statistic.  Must be
+// a #define macro since UMA macros prevent variables as names.
+#define SCOPED_UMA_HISTOGRAM_TIMER(name) \
+  class ScopedHistogramTimer { \

[jar (doing other things), 2013/05/03 21:50:26]

Since you messed with __LINE__ on line 22 so that you can have two of these in a
single scope, you presumably also need to do so here if you want it to work:

class ScopedHistogramTimer##__LINE__

[DaleCurtis, 2013/05/03 23:16:57]

Ah, right.  Actually none of this works at all.  In order to get unique names I
need another two macros essentially.

I don't think it actually makes sense to allow two of these within the same
scope.  Without the ##LINE business you can still do

SCOPED_UMA_HISTOGRAM_TIMER("global");
{
  SCOPED_UMA_HISTOGRAM_TIMER("local");
}

So I'm just going to remove it and make the variable name easier to understand
if that mistake is made.

[jar (doing other things), 2013/05/04 08:58:31]

<Doh>... I feel embarrassed.. I used to be a real expert at this stuff... and it
is lame for me to have forgotten.

It will be nice to get it working in a scope, so then you can do:

SCOPED_HIST_TIME("Whole task");
DoStart();
SCOPED_HIST_TIME("last.part")
FinishUp();



On 2013/05/03 23:16:57, DaleCurtis wrote:

[DaleCurtis, 2013/05/07 20:08:51]

Done.

+   public: \
+    ScopedHistogramTimer() : constructed_(base::Time::Now()) {} \

[jar (doing other things), 2013/05/03 21:50:26]

You should use TimeTicks, as it is guaranteed monotonic.

[DaleCurtis, 2013/05/03 23:16:57]

TimeTicks is a kind of expensive per the comment in base/time.h.  On Linux it's
the same as Now(), on Windows Now() occasionally calls TimeTicks::Now(), and Mac
uses a whole different routine for TimeTicks::Now().

Do you think these concerns are valid?

[jar (doing other things), 2013/05/04 08:58:31]

Definately use TimeTicks, not Time

Performance is not that bad at all, and correctness trumps performance (...and
we did have a LOT of trouble with this until we consistently migrated to only
using TimeTicks in a wide variety of cases).

On 2013/05/03 23:16:57, DaleCurtis wrote:

[DaleCurtis, 2013/05/07 20:08:51]

Done.

+    ~ScopedHistogramTimer() { \
+      UMA_HISTOGRAM_TIMES(name, base::Time::Now() - constructed_); \
+    } \
+   private: \
+    base::Time constructed_; \

[jar (doing other things), 2013/05/03 21:50:26]

nit: You probably should have NO_DEFAULT_COPY_OR_ASSIGN as is done in all
classes without a compelling reason to contrary.

[DaleCurtis, 2013/05/03 23:16:57]

Done.

[DaleCurtis, 2013/05/08 00:01:30]

Actually removed in the latest patch set.  VC++ won't expand the macro with the
unique ##key appended to it.  I couldn't find a workaround and keeping it
doesn't seem worth the effort.

[jar (doing other things), 2013/05/08 00:55:23]

If you're interested, the following should work (if my preprocessor jui-jitsu
skills are intact):

#define EXPAND_THEN_PASTE(a, b) PASTE(a, b)
#define PASTE(a, b) a##b

#define SCOPED_UMA_HISTOGRAM_TIMER(name) \
    class EXPAND_THEN_PASTE(ScopedHistogramTimer, __LINE__) { \
...
} EXPAND_THEN_PASTE(scoped_uma_timer, __LINE__)

[DaleCurtis, 2013/05/08 01:34:04]

Sorry for the confusion, my above comment about VC++ refer specifically to the
DISALLOW_COPY_AND_ASSIGN(xx##yy) usage, which isn't expanded correctly w/ the
macro hops from SCOPED_UMA_HISTOGRAM_TIMER() -> ...EXPANDER() -> ...UNIQUE(). 
Everything else works correctly in the latest patch set.  Even switching to your
EXPAND_THEN_PASTE macro doesn't resolve this issue with VC++ and the DISALLOW
macro (both approaches work fine in clang).

+  } scoped_uma_timer##__LINE__
+
+#endif  // MEDIA_BASE_SCOPED_UMA_TIMER_H_