Multi-GPU and input lag

[apoe]

I was wondering if going from single card to SLI would increase input lag.

Google searching tells me you get one additional frame of input lag for each extra card, so with two cards you would have one more frame of lag. However, none of these point to any sources as to where they got this information.

Are there any articles out there that talk about this? Not looking for "I don't feel any lag" kind of responses...

 

[bystander36]

Using logic, SLI at the same FPS as a single card, would put you 1 frame behind on latency. This is simply a matter of the fact that every other frame is rendered by a different card, and each frame takes twice as long to generate as the time between each displayed frame.

However, with perfect scaling, the time between frames is half the time as with a single card, giving zero latency.

As long as you get reasonable scaling on SLI, over a single card, things get more complicated. While the time between an frame starting and finishing may be a little longer than with a single card, you are also grabbing more input events. Rather than an event waiting the 16ms between them being acted on, they may be acted on every 10ms instead, which decreases latency on average.

So, if a single card gives the same FPS as SLI or CF, the single card has less latency.

If you gain a reasonable amount of FPS from SLI or CF, then latency may be reduced as events are gathered more often.

 

[Midwayman]

There is split frame mode as well. Won't scale as well, but you won't have the one card a frame ahead issue.

 

[Annisman*]

It's not so much latency that can be an issue but microstutter. I've owned about 10 different multi gpu setups and I'm back to one card now, albeit a powerful one. I do seem to have very sensitive eyes to that kind of stuff, I know lots of people can't notice it.

 

[bystander36]

There is split frame mode as well. Won't scale as well, but you won't have the one card a frame ahead issue.

I thought the split frame mode was abandoned due to pressure from AMD and Nvidia. I imagine it can still be used, but it is rare a game supports it at most. Though it has been suggested Mantle could support something similar.

 

[n0x1ous]

OP SLI is mostly set and forget assuming you are getting good scaling and have a decent cpu. I have been using SLI for a long time without issue.

 

[FalseChristian]

I found Split-frame rendering better for OpenGL, no? I've had SLI most recently with 2 eVGA GTX 460 1GB (800/1600/4150) and I've never noticed any microstuttering of any kind. And, believe me, I sometimes look for it, too.