SLI - Strange rumour about different Vram Gpus...

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crystal3d

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Nov 17, 2009
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Hello all,

I have been told that in `SLI`, both cards use the Vram of the GPU which is plugged onto the displays...

Is the second GPU's Vram not used at all?

The same guy further explains that it is possible to pair two 680s with 2gb and 4gb s and still achieve 4gb of actual memory in case the HDMI cables are plugged onto the GPU with more memory only...

I have read alot of times that it should not , but again their knowledge did not seem as `first hand experince`

let me know
ty


update:

clutchc: So, both cards use their own Vram, the second GPU does not use the 1st GPU's Vram, the SLI brdige cable is for Sync information but not RAW data and that guy on youtube is spreading false information and so on...

ah...wish it was true...

thanks clutch!

 
Solution
clutchc
With alternate frame rendering, each GPU uses its own VRAM to store the frames. So yes, both cards' VRAM is used. But you can't "add" them. 2 x 2GB cards does not equal 4GB VRAM.
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bystander

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Dec 9, 2009
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It is true that AFR (alternate frame rendering) requires both cards to load all the same information in order for them to render each frame. They do not assist each other on any part of the frame, except when using the the 2nd card for applying AA only, and not as a traditional SLI (this is only worth using if a game doesn't support SLI).

Another thing you need to know, SLI will not work if the cards have different amounts of VRAM unless using Coolbits to make Windows treat both as having only the amount of VRAM on the one with the least amount. So if you had a 4GB and 2GB version of the same card, you need to use Coolbits to make them behave like they both have 2GB of VRAM.
 
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clutchc

clutchc

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Apr 22, 2009
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I always had the hair-brained theory that with SFR the VRAM of both cards would effectively double due to the fact that each card is only rendering its own 'half' of the frame. As a result, if each card had 2GB... each frame would have access to 2 GB + 2 GB of VRAM. But one of the regular posters here told me I was thinking wrong.
 
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bystander

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Technically, you have double the VRAM in any of these cases, the problem is, in AFR, they have to load up all the same stuff, and utilize the same size buffers to render a frame.

With SFR, both cards still have to load all the same textures and resources into memory as the next card, because they are all still needed to draw either half of the screen. The one thing that may be different is they may have smaller buffers, which may have an impact on how much VRAM is needed.

Of course I have not done this myself, so I could have it all wrong.
 
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bystander

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I didn't say they'd need full access to the frame buffer, but they still need to have access to all the same textures, as they can happen at the top or bottom of any image, or on both. All the same resources needed to create a full frame have to be present to render half of a frame. The only thing that may be different is the buffer size.
 
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