Hi all,

Earlier today, I pushed the patch that enables OMTC on Linux[1][2], meaning we 
will now have OMTC enabled on all platforms. Linux is slightly different to 
other platforms, as we currently have hardware-accelerated layers disabled. 
This means it uses the BasicCompositor, which before this point, is not hugely 
well-tested. Please keep an eye out for any visual regressions, and we'll do 
our best to stay on top of them.

There is an expected, marked performance regression in some tests when enabling 
this feature. This is mainly due to measurements not taking into account the 
asynchronicity of X (and the omtc path having a couple more XSync's) - Some of 
these results are false, and so the new numbers don't actually represent as 
much of a regression as you would initially think. It's worth noting that Linux 
numbers are now more in line with our other platforms.

Some of the performance regression is real, due to unoptimised display paths in 
BasicCompositor[3][4]. These will eventually be resolved, as I believe we'll 
require this path on other platforms.

There are also performance wins; Scrolling performance is improved and OMTC 
allows us to have asynchronous video playback, which is also a huge 
improvement. Canvas performance also improves (for some unknown reason...) The 
gfx team has discussed this at length and we believe the performance trade-off 
is very much worth it, especially considering the things OMTC allows us to 
enable/do in the future.

If you have issues, me, Bas Scouten and Nicolas Silva should be able to address 
them.

Brace yourselves!

--Chris

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=994541
[2] When it gets merged: https://hg.mozilla.org/mozilla-central/rev/ea2341b06e6f
[3] https://bugzilla.mozilla.org/show_bug.cgi?id=994554
[4] https://bugzilla.mozilla.org/show_bug.cgi?id=994556
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