[Telemetry] Measure quiescent power once per run in power metric

tools/perf/metrics/power.py

 # 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.
 
 import time
 
 from metrics import Metric
 from telemetry.value import scalar
 
 
 class PowerMetric(Metric):
   """A metric for measuring power usage."""
 
+  # System power draw while idle.
+  _quiescent_power_draw_mwh = 0

[Dominik Grewe, 2014/07/08 13:05:19]

Would it make sense to set this to 'None' here to distinguish it from the case
where it is set to 0 (line 65f)? That would also prevent us from accidentally
using it before we call _MeasureQuiescentPower().

+
   def __init__(self, browser, quiescent_measurement_time_s=0):
     """PowerMetric Constructor.
 
     Args:
         browser: browser object to use.
         quiescent_measurement_time_s: time to measure quiescent power,
             in seconds. 0 means don't measure quiescent power."""
     super(PowerMetric, self).__init__()
     self._browser = browser
     self._running = False
     self._starting_cpu_stats = None
     self._results = None
-    self._quiescent_power_draw_mwh = 0
     self._MeasureQuiescentPower(quiescent_measurement_time_s)
 
   def __del__(self):
     # TODO(jeremy): Remove once crbug.com/350841 is fixed.
     # Don't leave power monitoring processes running on the system.
     self._StopInternal()
     parent = super(PowerMetric, self)
     if hasattr(parent, '__del__'):
       parent.__del__()
 
   def _StopInternal(self):
     """Stop monitoring power if measurement is running. This function is
     idempotent."""
     if not self._running:
       return
     self._running = False
     self._results = self._browser.platform.StopMonitoringPower()
     if self._results: # StopMonitoringPower() can return None.
       self._results['cpu_stats'] = (
           _SubtractCpuStats(self._browser.cpu_stats, self._starting_cpu_stats))
 
   def _MeasureQuiescentPower(self, measurement_time_s):
     """Measure quiescent power draw for the system."""
     platform = self._browser.platform
     if not platform.CanMonitorPower() or \
         platform.CanMeasurePerApplicationPower() or \
         not measurement_time_s:
       return
 
+    # Only perform quiescent measurement once per run.
+    if PowerMetric._quiescent_power_draw_mwh:

[Dominik Grewe, 2014/07/08 13:05:19]

I think I'd do this check in __init__ but that's just personal preference. If
you want to keep it here, maybe rename the method to something like
'_MeasureQuiescentPowerIfNeeded'?

+      return
+
     platform.StartMonitoringPower(self._browser)
     time.sleep(measurement_time_s)
     power_results = platform.StopMonitoringPower()
-    self._quiescent_power_draw_mwh = (
+    PowerMetric._quiescent_power_draw_mwh = (
         power_results.get('energy_consumption_mwh', 0))
 
   def Start(self, _, tab):
     if not tab.browser.platform.CanMonitorPower():
       return
 
     self._results = None
     self._StopInternal()
 
     # This line invokes top a few times, call before starting power measurement.
@@ skipping 18 matching lines @@
     """
     if not self._results:
       return
 
     application_energy_consumption_mwh = (
         self._results.get('application_energy_consumption_mwh'))
     total_energy_consumption_mwh = self._results.get('energy_consumption_mwh')
 
     if not application_energy_consumption_mwh and total_energy_consumption_mwh:
       application_energy_consumption_mwh = max(
-          total_energy_consumption_mwh - self._quiescent_power_draw_mwh, 0)
+          total_energy_consumption_mwh - PowerMetric._quiescent_power_draw_mwh,
+          0)
 
     if total_energy_consumption_mwh is not None:
       results.AddValue(scalar.ScalarValue(
           results.current_page, 'energy_consumption_mwh', 'mWh',
           total_energy_consumption_mwh))
 
     if application_energy_consumption_mwh is not None:
       results.AddValue(scalar.ScalarValue(
           results.current_page, 'application_energy_consumption_mwh', 'mWh',
           application_energy_consumption_mwh))
@@ skipping 41 matching lines @@
     if (not cpu_stats[process_type]) or (not start_cpu_stats[process_type]):
       continue
     # Skip if IdleWakeupCount is not present.
     if (('IdleWakeupCount' not in cpu_stats[process_type]) or
         ('IdleWakeupCount' not in start_cpu_stats[process_type])):
       continue
     idle_wakeup_delta = (cpu_stats[process_type]['IdleWakeupCount'] -
                         start_cpu_stats[process_type]['IdleWakeupCount'])
     cpu_delta[process_type] = idle_wakeup_delta
   return cpu_delta