9800 GX2 - one device or two?

I developed some image-processing code on an 8600, then got a new 9800 GX2 to run it on. The code runs at about 80% of what I need it to; most of its time is spent in CUFFT calls, which of course I can’t rewrite for speed.

My questions: does CUDA see the GX2 as two devices, or one? Since I haven’t modified the code to specifically use two devices, does that mean I’m only using half of my 9800 GX2?

(I’m using CUDA 1.1 on WinXP, if you’re curious; XP sees the GX2 as two graphics adapters.)

It’s seen as two devices, because it really is two independent GPUs with independent RAM. They just share the same PCIe interface and physical package.

You probably also need to disable SLI.

So yes, you’re running on only half of your 9000GX2.

Well, that’s actually good news, then, since I can just send alternate frames to each processor, and my throughput almost doubles (all I really need is a 25% increase), while my (acceptable) latency stays the same.

One question about disabling SLI: my motherboard doesn’t support it, and I don’t see anything that says “SLI” in the nVidia Control Panel. Does this mean I’m not using SLI? Or do I have to make some library call in addition? (It’s not clear to me whether the two GPUs on the 9800 GX2 are linked together with SLI or not.)

Also, do I have to turn off “Multi-GPU mode” (in NVIDIA Control Panel -> 3D Settings -> Set multi-GPU configuration)?

Thanks for the info.

While I haven’t used a GX2, I think that’s the case. You should be able to use the deviceQuery SDK sample to see how many GPUs are present–if it returns two separate 9800 GX2 devices, then everything is appropriately configured.

Bingo. With it set to “Multi-GPU mode”, deviceQuery only saw 1 device. I changed it to “Multi-display mode”, and now deviceQuery sees 2 devices.

(This is in NVIDIA Control Panel -> 3D Settings -> Set multi-GPU configuration, for those who may be joining in late.)

(Is that multi-GPU mode an SLI thing, or something different?)

Thanks for the help, guys!

yeah, that would be SLI. Good luck with your coding…