(click for fullsize version)
The settings are:
- Baseline: Fennec is not run at all. This is for comparison, to see how much power is drawn by the N900 when idle
- Fennec 200 clumped wakeups: 200 wakeups/second are scheduled, and all of them are scheduled at the same time. In other words, 200 tiny operations are done, then the CPU can sleep for almost 1 second.
- Fennec 200 separate wakeups: As before 200 wakeups/second are scheduled, but at equal intervals over the 1-second period. So the longest the CPU has to sleep is about 1/200th of a second.
- All settings were tested both with the screen off (left chart) or screen on (right chart).
For now, though, the results show that clumping wakeups can be noticeable: 38% savings when the screen is off, and 8% when on. As expected much less is saved when the screen is on, because the screen itself drains a lot of power (so our savings become less significant). Which of the 38% or 8% figures is more relevant is a good question - it depends on how often the user takes a 'break' from browsing (so the screen goes off) but leaves the browser on. But anyhow, again, these figures can't be taken literally.
FWIW, to put this in battery life terms, if you save 38% of your power drain then your battery life would be 60% longer, and if you save 8% of your power drain then your battery life would be 8% longer. Again, this is just a general indication, but I think it does show that implementing timeout clumping (properly) in Fennec can be useful.
Other Recent Work
I've also submitted patches for some other bugs related to saving power:
- 567339 / 359608 - A longstanding issue where image animation timers continue to fire even after browsing to another page. The patch uses the refresh driver to figure out when to suspend timers (but it doesn't use the refresh driver for the actual animations, which might cause more wakeups as it uses a fixed 50Hz timer).
EDIT: Improved charts