SLI - Does it require specific coding?

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Scraggle

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Joined: 10th Jul 2003

Location: Yorkshire

Posted: 9th Dec 2007 19:34 Edited at: 9th Dec 2007 19:34

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I just bought a new PC with twin GeForce 8800GTX 768Mb Graphics cards in it and was eager to learn how much faster my projects would run.
So, I loaded up the bitmap font tutorial from this months newlsetter and tested it.

Old PC:
P4 3.2Ghz
GeForce 6600GT
4Gb DDR2

New PC:
Quad Core 2.6Ghz
2 x GeForce 8800 GTX 768Mb (SLI)
4Gb DDR2

Result of running the Bitmap Font Tutorial:
Old PC: 900fps
New PC with SLI disabled: 1650fps
New PC with SLI enabled: 1650fps

So, I am obviously happy that I have almost doubled my framerate with my new PC but there is no difference at all whether I use one graphics card or both. So, as per the title of the post: Does the use of SLI require specific coding or is there some other reason that there is no difference?

Thanks

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El Goorf

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Location: Uni: Manchester, Home: Dunstable

Posted: 9th Dec 2007 20:05

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i asked lee about this at the convention. he said that it's all handled by the drivers. he could release something in a patch to allow us to play with sli settings, but it'd be pointless, since the drivers will always use the most optimal settings for the game, perhaps now you could prove him wrong

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jasonhtml

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Location: OC, California, USA

Posted: 9th Dec 2007 20:19

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i dont think you should need any commands, but SLI only seems to make a big difference in games with certain settings on... (like really high resolution, anti-aliasing, ect.)

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Silvester

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Location: Netherlands

Posted: 9th Dec 2007 20:30

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I think SLI will kick in when your graphic card needs to process alot of stuff and effects at once. Not just a simple DBP program.

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dark coder

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Location: Japan

Posted: 9th Dec 2007 21:02 Edited at: 9th Dec 2007 21:02

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This is an interesting read: http://developer.nvidia.com/object/gpu_programming_guide.html

It's possible DBP uses "Compatibility mode simply uses only a single GPU to render everything (that is, the second GPU is idle at all times). This mode cannot show any performance enhancement, but also ensures an application is compatible with SLI." by default as I too haven't noticed any speed boosts with SLI(though I'm now on a single GPU system).

Also for a test like this it's probably best to use a GPU intensive test(so you know the CPU isn't the bottleneck), i.e. have a very high poly mesh with vertex/pixel shaders, large textures etc.