[Bug 70948] [X regression] aa10text performance reduced ~12% with gnome-session

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=70948

--- Comment #8 from Chris Wilson <chris at chris-wilson.co.uk> ---
(In reply to Eero Tamminen from comment #7)
> > This is an artifact of sending damage events to the compositor quicker. I suspect the issue is that the compositor is rendering more frames during the test rather than the overhead of sending more often
> 
> I don't follow how just reducing damage event latency would cause more
> frames to be rendered, unless damage events were earlier clumped more
> together, so that single compositor update handled multiple events, and now
> the events are more spread out so that they get handled separately.

Yes, that was exactly the change. I think the culprit is

commit 9bf46610a9d20962854016032de4567974e87957
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date:   Fri Jun 21 22:58:31 2013 +0100

    os: Immediately queue initial WriteToClient

which would have had the effect of sending more damage events to the
compositor. And if the compositor was not coalescing them back to the
equivalent of the earlier stream, it would process more frames. (E.g. if it was
effectively doing a do { while(read(X)) accumulate(); render(); } while(1), it
would be processing more work than if it just rendered at a maximum frequency.)

The other aspect of that change was that it affected cpufreq and process
placement across cores.

> To validate the assumption of damage events causing the issue, one could
> e.g. check with xresponse [1] whether the number or timing of damage events
> changed.
> 
> [1] https://maemo.gitorious.org/maemo-tools/xresponse

Sounds like a useful tool to build into out testing...

> But if you know what the related changes were and are sure it was latency
> reduction, notabug sounds fine too.

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