Australis weekly update

This got long, so here's a tl;dr: * We still want to land as soon as perf regressions are fixed. * Current perf regressions looking good: down to 3%, only on XP. * A new test reports a 40% regression but actual impact unclear, could take months to fix. 1) What's the current land/ship plan? Six weeks ago I talked about our plans to land as soon as possible in Firefox 25, and but backout upon uplift and actually try to ship in Firefox 26. Obviously we're not landing in 25 now! But the same basic plan remains -- land as soon as we can (i.e., when performance regressions are fixed), and stay on Nightly until we're ready to ship (i.e., when quality / UX / bugs count / etc are acceptable to ride a train to release). 2) The good performance news. Work continues on fixing our ts_paint and tpaint performance regressions. We just finished reassessing the current graphs for all platforms, since the performance numbers are getting into the noise... You can't just compare any two individual data points, but have to look at trends. (As one example, tpaint on Ubuntu32 consistently fluctuates by ~20ms: http://cl.ly/image/003S0p0K0o3d) The good news is that we're looking much better. On almost every platform -- OSX (10.6, 10.7, 10.8), Windows (7, 8), and Linux (Ubuntu 32bit, 64bit) -- we are in good shape (no tpaint/ts_paint regression compared to mozilla-central). Windows 8 is even indicating a slight (~3%) performance improvement on tpaint. The only regressing platform is Windows XP. We've slowly been grinding it down, and a number of improvements landed last week (899608, 898126, 891104, 899608) to claw back gains of a few milliseconds each. But an unrelated mozilla-central regression (ICU, bug 853301) complicated matters, by making the baseline higher and noisier. Now that's backed out, and the results are a little easier to see. On XP, tpaint (http://cl.ly/image/1P1Y1p0K090H) is now only about 4ms slower (3%). Ts_paint (http://cl.ly/image/1e242Q311H2r) is now only 14ms slower (3%). You may notice a weird bimodal distribution since 8/1 in these graphs for both m-c and UX -- this appears to be a Talos bug (901713, 901715). Also note that we're looking at non-PGO numbers because of those bugs, but they've historically been comparable so I don't expect any PGO-only surprises. 3) The bad performance news. Back in the beginning of the year, we spent a couple of months working with the performance, gfx, and layout teams on reimplementing Australis tabs to fix performance issues (see bugs 738491, 813786). We addressed the concerns, got tabs animating at 60fps even on low-end hardware, and everyone was happy. Unfortunately we've now been told that a new test being written invalidates the perf team's own previous test, and that Australis tabs are actually a 40% animation regression. (Even though we're still running 60fps on the same low-end testing hardware.) We've started discussing what to do about this. It's unclear what the actual end-user impact is, or how to balance it against a third reimplementation of Australis tabs likely blowing out the schedule by another few months. :( See bug 902024.