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Old 01-17-07, 01:58 PM   #1
Jon Cohen
Join Date: Nov 2005
Posts: 7
Default SLI AFR performance problem

I'm trying to use a simple perormance benchmark under SLI alternating frames mode (AFR). I'm running a Dell XPS 700 system with two 7900 GTX boards. I've got the SLI heads-up-display enabled, with page flipping enabled sync to vblank disabled.

My benchmark application renders a densely tesselated square with millions of triangles using VBO. I can 500-550 M tris/sec whether I run with or without SLI AFR (I wouldn't expect a speedup from SFR mode, since this has a vertex bottleneck). I am not doing any readback, glFinish, pbuffers, etc. I'm linking with FreeGLUT. I saw some mention in the GPU Programming Guide of making sure that the framebuffer doesn't need to keep the data after a swap buffers. I'm not sure how to check that in OpenGL/Linux. (If it is the same as allowing page flipping, then I guess it is covered, but I notice no performance decrease if I disallow page flipping.)

Any ideas what I'm doing wrong? BTW, the HUD shows the SLI logo as well as two white vertical lines towards the left side of the window. Do the lines show anything meaningful besides the fact that I'm in AFR mode?

I've attached the benchmark program to this post. For anyone under Linux and willing to run it, does it report different performance for you in SLI-AFR mode versus non-SLI mode?

Thanks for any insight.

Attached Files
File Type: zip GLBenchMod.zip (9.3 KB, 86 views)
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Old 01-17-07, 04:16 PM   #2
Jon Cohen
Join Date: Nov 2005
Posts: 7
Default Re: SLI AFR performance problem

SOLVED!! I had to run in full screen mode to get page flipping to really work to get AFR mode to really work. Now the SLI HUD has a green bar between the two white lines (I'm still not sure what it means exactly - it doesn't seem to be documented anywhere), and I got an increase from 550M tris/sec to 690M tris/sec. Not as dramatic as I would have expected, but something is clearly working here. I wish this was in some FAQ!!

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Old 01-18-07, 05:20 AM   #3
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Join Date: Dec 2006
Location: Greenbelt, MD
Posts: 33
Default Re: SLI AFR performance problem

Originally Posted by cohen
SOLVED!! I had to run in full screen mode...I wish this was in some FAQ!!
Great. Ditto on the FAQ!
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