[Telemetry] Power metric: subtract quiescent power draw from result

tools/perf/metrics/power.py

Index: tools/perf/metrics/power.py
diff --git a/tools/perf/metrics/power.py b/tools/perf/metrics/power.py
index 869a023333537b0160180800c903a151778c4c13..bf339285e83d7f78fa36248e08b78f8eb06f7ddc 100644
--- a/tools/perf/metrics/power.py
+++ b/tools/perf/metrics/power.py
@@ -2,18 +2,29 @@
 # 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."""
 
-  def __init__(self):
+  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."""
     super(PowerMetric, self).__init__()
-    self._browser = None
+    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.
@@ -24,7 +35,7 @@ class PowerMetric(Metric):
       parent.__del__()
 
   def _StopInternal(self):
-    """ Stop monitoring power if measurement is running. This function is
+    """Stop monitoring power if measurement is running. This function is
     idempotent."""
     if not self._running:
       return
@@ -34,12 +45,24 @@ class PowerMetric(Metric):
       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."""
+    # TODO(jeremy): This only makes sense for systems where we can only measure
+    # power globally. Don't do this where power measurement is per-process.
+    if not self._browser.platform.CanMonitorPower() or not measurement_time_s:
+      return
+
+    self._browser.platform.StartMonitoringPower(self._browser)
+    time.sleep(measurement_time_s)
+    power_results = self._browser.platform.StopMonitoringPower()
+    self._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._browser = tab.browser
     self._StopInternal()
 
     # This line invokes top a few times, call before starting power measurement.
@@ -65,12 +88,23 @@ class PowerMetric(Metric):
     if not self._results:
       return
 
-    energy_consumption_mwh = self._results.get('energy_consumption_mwh')
-    if energy_consumption_mwh is not None:
+    raw_energy_consumption_mwh = self._results.get('energy_consumption_mwh')
+    if raw_energy_consumption_mwh is not None:
+      energy_consumption_mwh = max(
+          raw_energy_consumption_mwh - self._quiescent_power_draw_mwh, 0)
+
       results.AddValue(scalar.ScalarValue(
           results.current_page, 'energy_consumption_mwh', 'mWh',
           energy_consumption_mwh))
 
+      results.AddValue(scalar.ScalarValue(

[tonyg, 2014/06/26 16:19:04]

WDYT about just reporting two things:

energy_consumption_mwh: Remains unchanged and reports full system energy
consumption (we should document this in the API, as some of the providers I
think are trying to report just chrome here).
application_energy_consumption_mwh: Add this new metric which aims to estimate
just the target application (Chrome's) energy consumption. Some providers may
pass this directly in the platform API. The logic in this metric would be to
compute it if it is not given directly.

+          results.current_page, 'raw_energy_consumption_mwh', 'mWh',
+          raw_energy_consumption_mwh))
+
+      results.AddValue(scalar.ScalarValue(
+          results.current_page, 'quiescent_energy_consumption_mwh', 'mWh',
+          self._quiescent_power_draw_mwh))
+
     component_utilization = self._results.get('component_utilization', {})
     # GPU Frequency.
     gpu_power = component_utilization.get('gpu', {})