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Old 09-01-09, 04:57 PM   #1
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Join Date: May 2003
Location: flat, steamy Champaign, IL
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Default Xinerama: catastrophic opengl performance + fix (disable Composite extension)

A colleague's system has multiple 4500X2 Quadro cards configured in Xinerama mode to drive a high-resolution display.

They had been running RHEL 4 and getting reasonable OpenGL speed, but
after switching to RHEL 5, graphics performance became extremely slow, and sometimes
completely nonfunctional (simple apps like glxgears would start up and then hang).
They'd tried various nvidia drivers, including 173.14.something, 177.70.31, and 185.18.14.
The /usr/share/doc/NVIDIA_GLX-1.0/README.txt file didn't suggest anything special.

Playing with a system here (Xinerama on a pair of 5600s, also under RHEL5),
glClear() on an 8Kx2400 Xinerama framebuffer would take about 10 seconds.

Some tinkering turned up a workaround: disable the Composite extension!
I assume it had been nonexistent in RHEL 4's X server, but is on by default
in more recent X releases, so it needed to be explicitly disabled.

With this change, OpenGL works impressively fast. (I can load and display 4K stereo textures at 30 Hz.)

In another thread, I see that AaronP suggests a Xinerama config-file-making recipe,

nvidia-xconfig --enable-all-gpus --twinview --xinerama

Indeed this works fine, generating an X config file with Composite disabled
and printing a nice message explaining why. The README.txt file lists a number of caveats for setting up Xinerama already; I hope some note about Composite can be added to the Xinerama section, for those who compose their own xorg.conf files rather than using nvidia-xconfig.

Last edited by stuartlevy; 09-01-09 at 04:58 PM. Reason: Fix dangling sentence.
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