Questions about sli and crossfire

treetops

Posts: 3,064   +784
From my understanding the second card will not use it's vram when gaming. I know sli and crossfire support can be none existent\variable. That said is it possible to buy a video card without vram to reduce the cost?

I know everyone is excited about navi and super etc. But it seems like it would make sense for game developers to all start supporting crossfire\sli. Then we could see like 2x vegs 56 crossfired, one with little to no vram. current pricing 270$ for the vega 56 and 200$? For a vega 56 without vram. 470$ for 2080 performance. Or 2x 2060 for say 600$, 100$ off without that 2nd card having vram smashing the 2080 ti in performance, not to mention price.

People want to play at 2k-4k with decent fps... Why isn't this happening?
 

cliffordcooley

Posts: 13,141   +6,438
I was once told the cards split work load when connected. One card uses even vram bits. And the other uses odd vram bits. Therefore only using half the vram on both cards. Which is also why the card with the least vram is the one that dictates the total.
 

neeyik

Posts: 1,886   +2,215
Staff member
In SLI and CrossFire, both cards use their own VRAM to the fullest extent, as demanded by the game. This is because each card takes turns in rendering alternate frames - e.g. GPU 1 does frame 1, then GPU 2 does frame 2, GPU 1 does frame 3, and so on. It's called AFR (alternate frame rendering) for short.

It is possible to have both GPUs rendering separate parts of the same frame, but in modern games, this just isn't as efficient due to the techniques involves in making a frame. For example, one such common process is called render-to-texture; in this, the GPU renders the scene but doesn't display it. Instead the scene is used again, but as a texture, to be used in effects such as reflections.

If both GPUs were rendering the same frame, then the render texture would either need to be rendered fully by both GPUs or accessed off each other's VRAM. Neither is particularly welcome, as it negates the whole point of having extra GPUs. This is why AFR is the most common method of utilising two or more graphics cards.

Edit: I should point out that even when SLI/CrossFire setups are rendering the same frame, all graphics cards are loaded with the same data into the VRAM. Despite the advances in the interfaces to the GPUs (be it the PCI Express slot or card-to-card bridge), none of them are anywhere near as good as GPU-VRAM connection.
 
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