Australis weekly update

Justin Dolske dolske at
Tue Aug 6 21:47:06 UTC 2013

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:

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 ( is now only about 4ms 
slower (3%). Ts_paint ( 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.

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