[Bug 36596] Major 2D performance bottleneck (most noticeable with compiz)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=36596

--- Comment #10 from Jason Cassell <bluesloth600 at gmail.com> 2011-05-06 23:29:03 PDT ---
(In reply to comment #9)
> Really sounds like the GPU IRQ doesn't work (reliably). Do the numbers on the
> radeon line in /proc/interrupts still increase when the problem occurs?

Not a lot.  When everything's working, moving wobbly windows involves hundreds
of interrupts.  When it's at 2 fps, there are only a few, at most.  Possibly
none.

> Maybe try some IRQ related kernel debugging options as applicable, to see if
> any of them works around the problem.

noapic didn't make any difference.

I can't make the "irq 19: nobody cared" problem happen any more.  I don't
remember what kernel version I had last time it happened, but with 2.6.38.5 at
least, it's not happening for me anymore (so far).  When it was happening,
irqpoll and irqfixup didn't help.

Debian sid just went through a lot of upgrades.  I now have xserver 1.10.1 and
llvm 2.8.  Yesterday I did a git pull and rebuilt Mesa again.  The performance
problem's still there.

My hunch, based on my immense lack of knowledge of video drivers and GPUs, is
that when things slow down, it's because textures are being moved around in
memory.  When I rotate the cube in compiz to go to another desktop, sometimes
it stutters a bit, as if the texture for the other desktop isn't in the GPU's
memory yet, or something.  When I resize a window, the newly sized window is a
new texture, and something needs to be done with that new texture which
apparently takes 5 seconds and keeps the GPU too busy to do much else.

You probably shouldn't listen to me though, since I'm just guessing and I don't
really know how anything works.

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