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

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

Fix bug where idle power was measured on every browser invocation rather than once per run.

BUG=339180
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=281878

[marja, 2014-07-08 12:57:10]

lgtm

As discussed offline:
- this only make sense if we can assume that quiescent_power_draw_mwh is pretty
much a constant (if it's not, the previous situation is not necessarily much
better, since the total - quiescent computation would then be noisy no matter
whether we measure the quiescent many times or only once)
- since quiescent_power_draw_mwh is the power consumption when the browser is
not running, measuring it only once makes sense (and not every time before the
browser is started)

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

Just some minor suggestions:

https://codereview.chromium.org/371383002/diff/1/tools/perf/metrics/power.py
File tools/perf/metrics/power.py (right):

https://codereview.chromium.org/371383002/diff/1/tools/perf/metrics/power.py#...
tools/perf/metrics/power.py:15: _quiescent_power_draw_mwh = 0
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().

https://codereview.chromium.org/371383002/diff/1/tools/perf/metrics/power.py#...
tools/perf/metrics/power.py:59: if PowerMetric._quiescent_power_draw_mwh:
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'?

[jeremy, 2014-07-08 13:09:06]

Tony: WDYT?  Looking at some numbers coming from the bots idle power draw is
about 25.5W and stdev between runs is ~0.17W which comes to about 0.6%

Marja/Dominik: thanks!!

[tonyg, 2014-07-08 14:43:00]

lgtm

[jeremy, 2014-07-08 15:54:18]

The CQ bit was checked by jeremy@chromium.org

[commit-bot: I haz the power, 2014-07-08 15:55:29]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jeremy@chromium.org/371383002/1

[commit-bot: I haz the power, 2014-07-08 23:28:06]

Change committed as 281878