[Intel-gfx] Short stalls of xorg display updates

[Jan Niehusmann]

Hi,

this is just some random observation which may be interesting to you as
a data point, not a call for help, as I already solved / worked around
the problem, at least for my computer.

TL;DR: Switching from uxa to sna made the issue go away.

On a Thinkpad X201s with a i7-620L CPU I had a major usability issue with
xorg: Quite often (like every few seconds, during normal desktop usage)
display updates 'hung' for a short moment. The system itself did
continue to run, so it wasn't a full lockup.

As I didn't find a way to measure this, I don't have accurate numbers,
but I'd say the stalls had a length of about 1/2 second.

This didn't make the computer unusable, but was very annoying, and made
the GUI feel really slow.

The issue didn't happen with all workloads: Video playback did work
quite well, while switching between two windows quickly (using keyboard
repeat) did trigger the issue reliably.

Strangly, I think the problem didn't show up before I upgraded the BIOS
from 1.05 to 1.40. But I'm not 100% sure about this, as I only had the
notebook for a few days before upgrading.

The software on this computer is based on debian wheezy, but with a
self-compiled kernel:

Linux kernel 3.9.6
xserver-xorg-core 2:1.12.4-6
xserver-xorg-video-intel 2:2.19.0-6

(xorg versions are the full debian version numbers)

I did try several things like upgrading to the latest 3.10 RC kernel,
upgrading the xorg intel driver to 2.20.14 (from debian experimental)
etc. without success.

The change that finally helped was setting the AccelMethod to "sna"
in xorg.conf.


Please don't waste your time on this, as my workaround works perfectly
fine for me. But if someone else reports similar issues, perhaps this
may help pinpointing the cause. And, of course, feel free to ask in case
you need more information I may provide.

Regards,
Jan