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RTX 6000 SLI slow down the performance

Hi, guys. I just brought two RTX 6000 cards with NVlink. But I found out the FPS of SLI(AFR mode) slightly slow down comparing to Single RTX 6000.
I used Unreal version 4.21.2.

Any suggestion? Thx!

The editor doesn’t support SLI.

At GDC 2019 Epic mentioned they will work on support for Multi-GPU, but didn’t give a release where that will happen. Currently it is not supported in 4.22.

how about the packaged game?

thx, hope that would happen recently.

I believe generally you have to work with Nvidia to build a SLI profile for your game. But there’s also this:

Too bad that doesn’t work on editor, but at least is something.

@nichaohao they stated they will work on it, but since there is a huge list of new features coming, adding multi-GPU will probably be one of the last things, because you need all features stable to add something that might cause issues or not behave well with each added new feature. Lets hope I am wrong, but since I have been looking at the changes in the rendering module for quite some time and my assets in development highly depend on it, I might not be that wrong.

:o Already follow the doc, but the FPS decreased as I said. Thx anyway.

:o I think you are right.

In the Fortnite Trailer white paper, the PC configuration that was used was described as follows:

PC Configuration:
• PC - i7 (7th Gen processor or higher)
• 64 GB RAM
• SSD Hard Drive
• 1080 GTX / 1080 Ti / P6000

They were using 3 GPUs. I am curious how they were used? It wasn’t really addressed in the paper (that I could see).

Epic probably modified the engine and separated tasks for each GPU to perform in parallel based on the amount of computing power each one have. I think the principle was the same used in the Star Wars demo, but in that case they used 4 x Titan V.

The key when paralleling different GPU tasks it to reduce as much as you can the data that needs to move from one GPU to the other (dependency on data produced on one to the other), and the speed of SLI for pascal cards were really an issue, since the new NVLink now is by far faster.

Interesting. I am going to take a look at the Star Wars case. I am interested in how such modifications are done.

They said what was done, but not how, it involves quite an amount of changes in the rendering module, while keeping compatibility with all dependent modules (ie Niagara) and if I would ever do it, I would have to pick a stable engine release. Since 4.22 and 4.23 will introduce plenty of new things, that would be hard to do it right now, you would want to have all the good stuff from Realtime Raytracing and Niagara to be part of such great endeavor, probably better wait for 4.25.