Multi GPU's and SLI configurations

[csmallfield]

Hey guys,

In my office we are putting together a "GPU Render Beast", specifically to work well for Vray RT. I'm struggling to find good information on what we are trying to do, so if anyone could answer the following questions I would greatly appreciate it.

1. Does Vray RT work with SLI? Does it help?
2. As a straight comparison, GTX 780ti 6gb versus a Titan Black?
3. What is the performance increase (approx.) for 1, 2, 3 or 4 cards?

Of course money is an issue, but for the purpose of my questions I only care about pure performance. We will likely be building a machine with either 2 GTX 780 ti, or 2 Titan Blacks. Ideally with an extra slot or two for adding another 1 or 2 cards down the road. Any advice is welcome.

Thanks,
Chris

[super gnu]

hm. not sure i can help much with the answers, as im still waiting to dive into the gpu race.

however unless im wrong, the 780TI is only available as a 3gb card?

im waiting a month or so to see if the Titan X is really a 12gb card.. then i might take the plunge.

[csmallfield]

There is a 780 ti made by another manufacturer with 6gb. We're actually also looking at dual, Titan Zs, which have 12gb, but not really, since it's 2 GPUs on a single board, with 6GB each.

Still it would be great to know about SLI and if it matters, is utilized or would actually be a problem.

Thanks,
Chris

[super gnu]

ah sorry, yes that i can help with.. vry doesnt use SLI, and iirc vlado even suggests disabling it.


could you provide a link to a 6gb 780ti? ive looked everywhere.

[savage309]

[*]VRay RT can use as many GPUs as the operating system recognize. You should turn off SLI (and the memory does not stack, 3 GPUs, 2 Gigs each makes 2 Gigs, not 6).
[*]We haven't benchmarked Titan any soon I think, but here are some results with the latest V-Ray http://forums.chaosgroup.com/showthr...282#post640282
[*]It scales almost linearly (meaning that adding another GPU as the one you have makes it 2 times faster, adding 3 - 3 times, etc.). There might be some overhead, but it is not a lot.

[csmallfield]

Thanks for the information Blagovest!

Is there any reason that the Titan might not perform as expected? Is the performance pretty directly related to number of CUDA cores?

Also, there does not seem to be an option to render with both CPU and GPU power, is this something just not implemented yet or is it something that is just problematic due to the different natures of CPUs and GPU and they don't mix?

[savage309]

Titans should render fine.

There is some correlation between the number of cores and the rendering speed (the more the better). But many other stuff also do matter - frequency, cache size, memory bus, schedulers, architecture, etc.

We have some internal experiments with CUDA running on the CPU and the GPU.
The problem is that by nature, the CPU produces a bit different result from the GPU (because of the way they handle floating point calculations). So they have different noise patterns and every time you re-render a scene, it will have different noise. This may be fine for some cases, but it is not for many others. We have to see if it will be added in the official build.