Add comments explaining Task Manager idle wakeups computation.

base/process/process_metrics_mac.cc

 // 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.
 
 #include "base/process/process_metrics.h"
 
 #include <mach/mach.h>
 #include <mach/mach_vm.h>
 #include <mach/shared_region.h>
 #include <stddef.h>
@@ skipping 306 matching lines @@
   mach_msg_type_number_t power_info_count = TASK_POWER_INFO_COUNT;
   kern_return_t kr = task_info(task,
                                TASK_POWER_INFO,
                                reinterpret_cast<task_info_t>(&power_info_data),
                                &power_info_count);
   if (kr != KERN_SUCCESS) {
     // Most likely cause: |task| is a zombie, or this is on a pre-10.8.4 system
     // where TASK_POWER_INFO isn't supported yet.
     return 0;
   }
+
+  // The task_power_info struct contains two wakeup counters:
+  // task_interrupt_wakeups and task_platform_idle_wakeups.
+  // task_interrupt_wakeups is the total number of wakeups generated by the
+  // process, and is the number that Activity Monitor reports.
+  // task_platform_idle_wakeups is a subset of task_interrupt_wakeups that
+  // tallies the number of times the processor was taken out of its low-power
+  // idle state to handle a wakeup. task_platform_idle_wakeups therefore result
+  // in a greater power increase than the other interrupts which occur while the
+  // CPU is already working, and reducing them has a greater overall impact on
+  // power usage. See the powermetrics man page for more info.
   return CalculateIdleWakeupsPerSecond(
       power_info_data.task_platform_idle_wakeups);
 }
 
 bool ProcessMetrics::GetIOCounters(IoCounters* io_counters) const {
   return false;
 }
 
 ProcessMetrics::ProcessMetrics(ProcessHandle process,
                                PortProvider* port_provider)
@@ skipping 51 matching lines @@
     return false;
   }
 
   meminfo->free = static_cast<int>(
       (vm_info.free_count - vm_info.speculative_count) * PAGE_SIZE / 1024);
 
   return true;
 }
 
 }  // namespace base