How does SLI work?

[Spev]

I remember having a convo with someone on the forums here a while back, I've never done SLI, but I assumed that it was double the power...for example if you have two GTX 980s, I figured it would be 8GB and double all the other stats too. Someone told me that is not the case, how does it work?

[ThyFeared]

Enjoy

[Fate]

VRam does NOT add up, and they very rarely give you double the power in games. Most decently optimized games they give an ~80% performance gain over non-sli. 

example benchmarks: http://www.anandtech.com/bench/product/1072?vs=1073

[Spev]

Enjoy

Hmm ok. So what is microstuttering or frame time variance?

[Jumper118]

Hmm ok. So what is microstuttering or frame time variance?

https://www.youtube.com/user/Techquickie

[jkraghify]

Okay, in a perfect world, 60 fps means 1 frame every .017 seconds consistently.

In a micro-stutter situation, you still get 60 fps, but the frames are NOT consistently timed. Meaning, it could feel like the scene is jumpy and running at a lower fps...

Basically the frames could be clustered in groups and one frame takes like .04 seconds to draw on the monitor.

Micro stuttering means high fps, but not smooth.

[D2ultima]

I remember having a convo with someone on the forums here a while back, I've never done SLI, but I assumed that it was double the power...for example if you have two GTX 980s, I figured it would be 8GB and double all the other stats too. Someone told me that is not the case, how does it work?

The SLI information guide in my signature (which for some reason never gets stickied) will tell you everything you need to know about SLI.