[ubuntu-x] [Needs Testing] -intel performance workaround (greedy)

[Bryce Harrington]

I would like to propose re-enabling "greedy" migration as a solution to
performance issues with -intel.  PPA for testing is available at:

  http://people.ubuntu.com/~bryce/Testing/intel-bug359600/

Please test and let me know if you spot any regressions from Intrepid
caused by this package.


[Discussion]
One of the last[1] remaining big issues with the -intel driver for
Jaunty are performance problems that a number of people are seeing on
their hardware.

Back in Hardy when we were also seeing performance issues, I included a
patch[2] that set the "greedy" migration heuristic on by default when
using EXA.  In preliminary testing, we find this option is helping in
this case as well.  It seems to provide approximately the same
performance benefits that the "UXA" option provides, but without the
stability problems people have seen.

What the patch does is refuse to let go of certain pixmaps, preventing
various acceleration algorithms from being run.  Obviously, upstream
would prefer to see these new algorithms used rather than bypassed, so
they do not recommend using this options[3].  However, in situations
where those algorithms are not working right, this can actually improve
performance as it prevents the overhead of migrating the pixmaps around.
So, this is a horribly ugly workaround, but hey, if it works...

What I'm concerned with is crash bugs that occur with this patch
enabled, that were not present in Intrepid.  In particular, I'm
concerned about bug #246581 rearing its head again[4].  Please bring
such issues to my attention ASAP.

Rumors suggest that greedy may also cause some graphics corruption
issues.  As long as they're minor annoyances and don't prevent usage of
the machine, these may be an acceptable price to pay, but let me know.

Please test the debs here[5]:

  http://people.ubuntu.com/~bryce/Testing/intel-bug359600/

Let me know of any regressions from Intrepid that this causes.

Bryce


1: There are also some inexplicable X freezes seen esp. when running
   with compiz that we are working on.  Current theory is a regression
   in the mesa 7.3->7.4 upgrade, since downgrading to 7.3 seems to help
   (but hasn't resolved all freeze issues so we're not 100% sure).

2: http://launchpadlibrarian.net/25354124/05_intel_exa_force_greedy.patch

3: http://lists.freedesktop.org/archives/xorg/2008-May/035459.html
   (Esp. see follow up posts where it is suggested the option is not
   inappropriate to be carried at the distro level.)

4: https://bugs.edge.launchpad.net/ubuntu/+bug/246581