[Telemetry] Add power usage 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 logging
+
+from metrics import Metric
+from telemetry.core import exceptions
+from telemetry.core import platform
+
+
+class PowerMetric(Metric):
+  """A metric for measuring power usage."""
+
+  def __init__(self):
+    super(PowerMetric, self).__init__()
+    self._backend = None
+    self._results = None
+
+  # pylint: disable=W0613
+  def Start(self, unused_page, unused_tab):
+    if not self._backend:
+      self._backend = platform.CreatePlatformBackendForCurrentOS()

[tonyg, 2014/01/23 18:23:40]

We should access this via tab.browser.platform and not store a reference to it.

[jeremy, 2014/01/23 18:58:41]

Done.

+
+    if self._results:
+      raise exceptions.ProfilingException(
+          "Reusing a power profiler is not supported")

[tonyg, 2014/01/23 18:23:40]

Why? Shouldn't we just clear self._results in AddResults()?

[jeremy, 2014/01/23 18:58:41]

Done.

+
+    if not self._backend.CanMonitorPowerAsync():
+      logging.warning("System doesn't support async power monitoring.")
+      return
+
+    self._backend.StartMonitoringPowerAsync()
+
+  # pylint: disable=W0613
+  def Stop(self, unused_page, unused_tab):
+    if not self._backend.CanMonitorPowerAsync():
+      return
+
+    self._results = self._backend.StopMonitoringPowerAsync()
+
+  def AddResults(self, _, results):
+    if not self._results:
+      return
+
+    energy_consumption_mwh = self._results['energy_consumption_mwh']
+    results.Add('energy_consumption_mwh', 'mwh', energy_consumption_mwh)

[tonyg, 2014/01/23 18:23:40]

For the unit display string, let's use the correct capitalization: mWh.

Also, we'll need to add mWh to tools/perf/unit-info.json in order for the
dashboard to know that lower is better.

[jeremy, 2014/01/23 18:58:41]

Done.