Over the last few months a number of us at Mozilla having been working on a profiler built into Firefox. One of the goals of this profiler is to make it as easy as possible to profile anywhere. Yesterday we had a satisfying realization of this goal.
It all started with Taras' Snappy #36 update. In a comment, a user going by kamulos reported a recent problem about laggy tab switches. kamulos posted the profile and Benoit Girard filed bug 784756. The profile showed us spending a bunch of time in TimeStamp::Now() during image decode. I wasn't particularly surprised by this because our TimeStamp::Now() implementation on Windows is not particularly fast. Ehsan and I went away and put some effort into improving the performance and have some good candidates in bug 784859. In the mean time, Robert Lickenbrock discovered that problem was recently introduced by bug 685516, which unintentionally caused a fixed time delay where we called TimeStamp::Now() in a loop. He has since posted a patch that fixes the problem.
Here we have two community members helping uncover a problem within a week of it landing, a problem that could have otherwise gone undetected for a long time. This is a great example of an open source community working beautifully.