ATI Radeon HD 4000 Series discussion

[trinibwoy]

You see, when you run multiple cards, both cards must have all the information in their individual memory banks, as both cards are likely to be showing the same textures as one another, as well as the same shaders. As such, you're effectively doubling the same exact info in both physical memory, as well as using the memory bandwidth of both cards to perform identical tasks. Seeing as how they can't share the same information from the same set of ram, they're effectively doing the same job twice.

No, ORBR is more accurate in this case. The duplicate transfer of textures from the host to the GPU happens over the PCIe bus and it doesn't happen every frame. So even if the same transfer occurs to both cards it doesn't have anything to do with the on-board bandwidth usage.

What he's referring to is the use of the on-board bandwidth to render the frame. The two cards are not doing "identical tasks". They are processing different geometry and rendering different framebuffers for their individual frame. However, there may be instances where the two cards generate the same render target or other buffers that normally would be reused in a single-card scenario which is a bit of a waste.

In any case both 256-bit buses are being used somewhat in parallel to process two different frames at once. So it is closer to the bandwidth of a 512-bit bus than it is to a single 256-bit one.

It's very simple really. Compare 9600GT-SLI to an 8800GTS-512. The former is much faster even though the number of processing units is the same. Why? Double bandwidth.