Cycles, GPU, and SLI

Sorry in advance if this is a topic that pops up more frequently than people would like on here (it seems like it would be), but I have some questions about using SLI on my graphics card(s) and it’s affects within Cycles rendering.

Soooo… I currently have a single graphics card installed and have been contemplating finding another compatible graphics card on sale and then throwing on my SLI ribbon to use both cards in Cycles and all other graphical computing. I understand the benefits of doing it, and the limitations (for the most part), and that is why I would only do it if I found a really good deal.

I guess my question and concern would be if AFR issues have been solved with running multiple cards with SLI. I know how I am and I would get extremely frustrated with stuttering frame rates.

Anyone have any knowledge on this stuff? Want to shed some light?

SLI soesn’t benefit Cycles. Just use one or more Cycles compatible cards !

Don’t use SLI; scaling isn’t great, but can be good. Cards don’t share memory - meaning only the memory from one card is used.

Double posted

I am aware that there are no RAM benefits from SLI (ie. it will NOT double the RAM of the card), however why wouldn’t Cycles improve with SLI? When you ribbon the cards together, it doubles to CUDA cores (what cycles utilizes to compute through the GPU). Therefore, in theory, I should have double the potential compute power, but obviously it is not that efficient and the second card will only show something like a 50% improvement. Should that not improve my rendering speeds in Cycles? … I don’t see why not?

Also, what do you mean by using one or more Cycles compatible cards? Wouldn’t I need to SLI them to make use of multiple cards? Or do you mean have two cards running independently and have two instances of Blender running and have each application render alternated frames of the same file?

Yea, I am aware of this, but I should still see some rendering time improvements, even though the ram will not hold as much information (the amount of RAM shouldn’t really affect render times… should it?).

I am not trying to rebuttal you guys, I am just trying to get my facts straight.

Cycles uses as many compatible GPUs as you throw at it without SLI and without multiple instances of Blender so there is no reason for you to be worried about not having the maximum number of cores exploited. The only problem is that multiple GPUs don’t scale so well.

For example you have one GTX 580 then you add another. You don’t get 2x the speed you get only 1.9 something and it continues to decrease as you keep adding them. There are many ways to get more informed if you’d like but I’d recommend just using two cards at most without SLI as it doesn’t bring any benefits sometimes it can even be detrimental. Just read the BA forums you will see that people who have played with multiple cards in SLI have reached this conclusion.

Hope that helps ! Happy 2013 !

Very interesting.

Well that is good to know, I didn’t know that Blender automatically output to multiple GPUs! I am a noob in all ways, so I welcome the advice.

Thanks a bunch! Happy new year as well!

Sorry for the bump, but one last question.

I am assuming that since no SLI would be done, that it wouldn’t matter if the graphics cards were compatible, right? Since they would be used independently.

IE. I could have a NVidia gtx670 and a gtx560? or something like that.

Indeed you could. Just keep in mind that Cycles will only use the lowest amount of Vram from two cards. For example if you have two cards one with 3 GB Vram and one with 1.5 GB vram you will only get 1.5 GB maximum memory. Hope that helps !

I would use an ATI card from the HD 5000 series like the 5870 for the viewport navigation and one GTX 580 3GB for rendering in cycles. I think that’s the best combination for now or wait for the GTX 780 or so to show up.