SLI/Xfire vs single card

[Johnmini]

i posted this as a reply somewhere but thought i would post as a thread. This is how i try and explain why the memory of your GPU dosen't double if you have 2 cards in SLI/Xfire let me know if it makes sense to you.

If you are really new let me just say this if you have x2 gtx 660s that each have 2gb of memory dose NOT MEAN YOU GET 4GB of memory so adding another card like the one you have will not help if you are running into a problem running to many monitors or 3D gaming if memory of your current GPU is the problem to begin with.

Let me try and explain why.

think of a sequence where each frame is named: 1, 2, 3, 4, 5, 6, 7, and 8.

Card 1 will render frames 1, 3, 5, and 7.

Card 2 will render frames 2, 4, 6, and 8.

But both cards get frames 1 through 8 loaded to there vram.

Now imagine 2 workers that don't know each other exist (the workers being your gpu's) and they each have lots of energy, (energy being the memory of the card) they are told to do a job that 1 man can do, they will finish the job faster, because there is 2 of them but they don't know that there is someone helping them so they look at the jog as a whole. So they plan how long it will take and how much energy it will use. they go and tell there boss it will take me this long ... and will take 100% of my energy so don't give me any other jobs. they do finish faster because unknown to them someone else was helping but that dose not help them plane in the beginning when they are looking at the job as a whole and think they are doing it alone.

So to bring this back to cards. Two cards can finish rendering an image faster then one can but it still uses the same amount of memory that only one card would, this is because both cards have the same image loaded to there vram using the same amount of memory they just render different parts of that image. an SLI/Xfire set up works the same as a card with 2 gpus like a gtx 690 if it says it has 4gb of memory each gpu only gets 2gb of vram to load an image on not 4.

[WiRoZ]

an SLI/Xfire set up works the same as a card with 2 gpus like a gtx 690 if it says it has 4gb of memory each gpu only gets 2gb of vram to load an image on not 4.

That is partly true. While two 670s have 2x 2GB of VRAM, a 690 has 1x 4GB of VRAM. Although it is shared by the GPUs, you still have 4 effective GBs of VRAM. (The rest of your story is true, but I just wanted to make this clear.)