Powersaving Bughunting

I spent most of yesterday working on Bug 567339, which is about removing unnecessary CPU wakeups in order to save power (which is very important on mobile devices - the fewer time the CPU is woken up from sleep, the longer your battery will last, since each wakeup disturbs sleep mode for a short while after the actual activity).

So, long story short, it seem the bug is that timers for image animations are not canceled. In other words, if you view an image with animations, then a timer is created for each image, and it does the rendering at the proper rate, but the timer keeps running after the image is no longer needed. So you keep getting CPU wakeups at the appropriate rate for the animations, wasting power.

(This is actually an issue on normal Firefox as well, not just mobile - some CPU cycles are being spent on these timers. It is probably negligible, though, but still wasteful.)

The longer story of hunting the cause of the bug is as follows:

• A helpful person reports the bug, and mentions a suspicion of https being the cause (since enabling/disabling https affects the symptoms). At issue here are 4 wakeups/second which should not be happening.
• Trying to reproduce the bug, it turns out that it happens in nightly builds, but not when building from source in trunk. Hmm.
• Trying to figure out the difference, one noticeable change between the nightly builds and building from source is that the start page shows some suggestions of addons in the nightly, but building from source does not. Turns out this is because /trunk reports itself as 2.0, and no addons exist for that yet. Changing the reported version to 1.1 leads to addons being suggested, and the bug in fact occurs.
• Since fetching addon suggestions is done from AMO through https, and the original bug report strongly suspected https as the culprit, it would seem we are almost done here. Or are we?
• Actually inspecting all the wakeups (this was where most of the time I spend on this bug was; I put up a wiki page with some details of the methodology I used) eventually reveals that the vast majority of wakeups are in fact of imgContainer::notify()... and not https or anything network related.
• To confirm this, removing the animated image that was shown when loading addon suggestions removes the wakeups... and indeed, the image had 4 frames/second, exactly the number of wakeups we were hunting!

Still working on figuring out why this happens and how it can be resolved.