I ran into this while debugging why the testcase in bug 497788 continued to run at about 68-70fps no matter what I did. That testcase has the following structure: var frames; var prevFrames = 0; var checkpoint = new Date(); function doSomething() { ++frames; /* do stuff that takes about 4-5ms */ } function measure() { var curTime = new Date(); log((frames - prevFrames)*1000.0/(curTime - checkpoint)); checkpoint = curTime; prevFrames = frames; } setInterval(doSomething, 5); setInterval(measure, 3000); Now the way our interval code works is that it ensures that the new firing time for the interval is at least 10ms after the _completion_ of the interval function execution. So in my case, we were resetting it for typically 14-15ms, which of course gives 67-71fps. Wouldn't it make more sense to enforce 10ms from the time the interval fired instead?

Oh, and I can pretty easily write a test for this; I'm just not sure I can easily write one that won't randomly go random orange on tinderboxen under load....

Comment on attachment 412007 [details] [diff] [review] Proposed patch Makes sense to me. Yes, tests would be nice, but it's fairly hard (I gave up in the clamping bug).

Pushed http://hg.mozilla.org/mozilla-central/rev/3dae66874786 I think this is probably too risky for 1.9.2 at this stage... any objections?