Question about VRAM, SLI, multiple monitors, and larger screens.

[Tohst]

People say VRAM does not stack when you SLI two graphics cards. i.e. 2GB card + 2 GB card does not = 4GB card. However, you usually need to SLI cards or install a 3 or 4GB card when using multiple monitors. Why is this? What is the difference between SLI and a larger GB card if SLI does not add VRAM?

My second question has to do with gaming on a large screen, such as a 50" TV. To accomplish this, would it be necessary to SLI cards or by a 3 or 4GB card? Why or why not?

 

[cookybiscuit]

Multiple screens means you are pushing 3x as many pixels, higher resolutions require more VRAM. SLI is usually recommended for multimonitor because it is usually cheaper and/or faster than going with a single GPU, takes alot of power to do multi monitor gaming.

If you are on a single screen (assuming it isn't 4K), you don't plan on running SLI, then 2GB VRAM is enough. At for example 1440p, running games at ultra will usually result in usage over 2GB, however, if a card has 2GB of VRAM it will be something like a 670/760/7850etc, not fast enough to run the game at ultra at that res; so you turn the settings down from ultra to say high, allowing you to dip under the VRAM cap. This changes with SLI as you have more power at your disposal and can run games at settings high enough to use more than 2GB of VRAM.

Screen size has 0 influence on performance requirements, its all about the resolution. The GPU doesn't care if you are sending a 1080p image to a 22" monitor or an IMAX screen, its still 1080p and is rendering the same amount of stuff.

 

[Tohst]

Multiple screens means you are pushing 3x as many pixels, higher resolutions require more VRAM. SLI is usually recommended for multimonitor because it is usually cheaper and/or faster than going with a single GPU, takes alot of power to do multi monitor gaming.

If you are on a single screen (assuming it isn't 4K), you don't plan on running SLI, then 2GB VRAM is enough. At for example 1440p, running games at ultra will usually result in usage over 2GB, however, if a card has 2GB of VRAM it will be something like a 670/760/7850etc, not fast enough to run the game at ultra at that res; so you turn the settings down from ultra to say high, allowing you to dip under the VRAM cap. This changes with SLI as you have more power at your disposal and can run games at settings high enough to use more than 2GB of VRAM.

Screen size has 0 influence on performance requirements, its all about the resolution. The GPU doesn't care if you are sending a 1080p image to a 22" monitor or an IMAX screen, its still 1080p and is rendering the same amount of stuff.

What you said about pixels and screen size makes a lot of sense. Kind of obvious actually when I think about it.

But what about SLI? It's recommended for multiple monitor setups but the vram doesn't stack? What does "stack" even mean?

What is the difference between adding another card ( via SLI) and replacing the card with a larger GB version? Do they both allow more rendering to happen?

 

[cookybiscuit]

'Stack' refers to VRAM increasing when you add more cards, for example, people often think if you take two 2GB cards, SLI them, you get 4GB effective memory, but you don't. This is because SLI (and Crossfire) use alternate frame rendering, both cards need all the information, this has to be stored twice, once for each card; adding another card will give you another 2GB of VRAM, but you're VRAM usage will double, cancelling it out.

Normally, you wouldn't SLI right off the bat. People usually go for an SLI setup because they bought say a 670 a year ago and want more speed now, so rather than lose cash by selling it off and buying say a 780 or 290, you add another card. So long as you have one display, you can pretty much ignore how much VRAM a card has.

However, say you are at 1440p. SLI 760 and 1x 780 are pretty comparable in performance, yet they have 2GB/4GB and 3GB respectively. Because of the larger res, and the fact that both setups have enough power to max games, you'd want more VRAM, so you'd either want a 780 or the 4GB models of the 760's. This doesn't apply when you have one mid range card, as explained above.

 

[Tohst]

This is because SLI (and Crossfire) use alternate frame rendering, both cards need all the information, this has to be stored twice, once for each card; adding another card will give you another 2GB of VRAM, but you're VRAM usage will double, cancelling it out.

So one card renders one frame, and the other renders the next, and so on, right?

And if VRAM usage is doubled, doesn't that mean that SLI does give you more VRAM at your disposal? That sounds like it gives you more VRAM.

If the above is true, than I'm confused. 2GB card + 2GB card /= 4GB card but it doubles your VRAM usage? The two sound like the same thing to me. What's the difference between two cards in SLI and a larger GB card? It seems to me that they both accomplish the same thing.