Add comments explaining Task Manager idle wakeups computation.

base/process/process_metrics.cc

 // 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.
 
 #include "base/process/process_metrics.h"
 
 #include <utility>
 
 #include "base/logging.h"
 #include "base/values.h"
@@ skipping 68 matching lines @@
   if (time_delta == 0) {
     NOTREACHED();
     return 0;
   }
 
   last_idle_wakeups_time_ = time;
   last_absolute_idle_wakeups_ = absolute_idle_wakeups;
 
   // Round to average wakeups per second.
   int64_t wakeups_delta_for_ms = wakeups_delta * Time::kMicrosecondsPerSecond;
-  return (wakeups_delta_for_ms + time_delta / 2) / time_delta;

[brucedawson, 2017/02/02 00:55:29]

This term is presumably (see comment on line 87) to round the value to nearest.
Otherwise the result is truncated, leading to systemic bias.

Adding the divisor divided by two is equivalent to going:

    (int)(((double)wakeups_delta_for_ms / time_delta) + 0.5)

but adding time_delta / 2 avoids converting to floating-point and back.

It's easiest to think of it as being equivalent (modulo computer arithmetic
quirks) to:

wakeups_delta_for_ms / time_delta + (time_delta / 2) / time_delta

[shrike, 2017/02/02 16:12:50]

Thank you brucedawson@. I saw the comment about rounding but didn't understand
it. This is a clever approach to adding 1/2, but I don't think it's clear, and I
dislike that unit-wise it doesn't make sense. I'm going to expand the comment.

+  return wakeups_delta_for_ms / time_delta;
 }
 #else
 int ProcessMetrics::GetIdleWakeupsPerSecond() {
   NOTIMPLEMENTED();  // http://crbug.com/120488
   return 0;
 }
 #endif  // defined(OS_MACOSX) || defined(OS_LINUX)
 
 }  // namespace base