SLI CUDA GPUs - any crunching penalty?

[DrPop]

Finally getting my first mobo that has SLI support. I've Crossfired two ATI GPUs before and found virtually no performance penalty when crunching on the GPUs. Does the same hold true with nVidia's CUDA cards? Does anyone have experience SLIing two CUDA GPUs and crunching with them like that?
Thanks for the info, just trying to decide which GPUs to put in each rig now that I'm rebuilding Cheetah.:cool:

[c303a]

I'm running 2 x GTX 465 in SLI mode on the I7 and I don't see any difference. Someone may have better info than I do but it seems to be running 2 apps of GPU Grid with no problems. I just had to put the script into the cc config file.

[DrPop]

Thanks for the good report! I'll try it then and see what we get. :)

[spingadus]

Correct me if I'm wrong, but my understanding was that sli/cf is mainly irrelevant as Boinc will see each individual GPU regardless. So, you shouldn't get more performance. In fact, you may get less performance as sli/cf requires the cards to use a small % of processing time to keep themselves synced.

[DrPop]

That's why I was asking... ;) Actually, a while back I had tested the difference between two HD4870s in Crossfire and non-Crossfired on MW and Collatz, and there was virtually no difference at all in the speed. So not sure there is really much of a penalty like we all thought back in the day. There are a few applications where an SLI'd or Crossfire'd rig would be nicer to use - both GPUs cranking out to one monitor - so if the penalty is tiny or non-existent, then I'll be tempted to try it.

[John P. Myers]

I think the reason people thought there was a penalty is because with games, there is. Using 2 GPUs in SLI/Xfire is only 1.7x faster instead of double. So i can see why people may have thought SLI would hurt crunching to the same degree.