AMD Radeon HD 6950 2GB & 1GB - Crossfire

[Zemial]

Hi. I'm using two Radeon 6950 cards(2GB card master & 1GB Linked) in crossfire. MSI Afterburner GPU MEM says that I have a 3072MB of memory to use. What's going on, I always thought that if you use two cards with different memory, crossfire is then limited to 1Gb of memory? When I play BF3 multiplayer MSI GPU MEM OSD says that I using sometimes over 2200MB of GPU memory. Is this possible?

[automaticman]

Well...that is pretty interesting. Here's what I know: People usually say that, when configured in Crossfire, GPUs will work at teh lowest common denominator in terms of shaders, clock speeds, and memory usage. While it's a good rule of thumb, I don't think it's entirely accurate as it pertains to memory.

With memory, we say that 2x 1GB cards does not equal 2GB of VRAM, because each card needs to store the entire texture file so it it is quickly obtainable by that card's GPU. Each card needs it's own copy, so while you might have double the VRAM, you effectively have double the texture file size as well. The VRAM on a card is also used as the frame buffer, which is why higher resolutions require more vram - there is a big difference in filesize for a 1680x1050 froma vs a single from for an Eyefinity setup.

What I'm not entirely clear on is what happens when you have a texture file greater than the amound of VRAM on the smallest card. For instance in BF3, with textures on ultra, maybe some levels have 1.2GB worth of texture storage.

On the smaller card, whatever can't be fit into the VRAM is stored in the main system memory (game loading time is usually taken up by the system reading the textures into memory from the HDD). When the GPU needs a texture that is not in VRAM, it has to go looking for it in system RAM, which is much slower. Now on the larger card, it may be possible that the GPU still uses the extra vram so it can store the entire texture file and not have to go to system memory at all. This would account for the higher memory usage you are seeing.

Because the system is in Crossfire, you aren't going to really see the advantage of that extra memory because, while one card may be able to quickly access all of the texture fire, it still has to wait for the next frame coming from the other GPU - who does have to go to sys RAM. So even though the game is using over 2GB of VRAM, you might still only get the performance of 2GB VRAM.

Anyone else want to chime in? I'm kinda hypothesizing as I typed this out.

[BlackZero]

Crossfire/sli require memory being mirrored so you will always be limited to the minimum amount available to either gpu. Msi afterburner considers memory in apooled fashion so will read both card's memory usage and for a 'real' usage figure you'd need to divide by the number of GPUs. In that sense seeing over 2GB vram is quite strange and though it's absolutely impossiable for the cards to use more than 1GB vram for 3D while in crossfire.

Perhaps take the matter up with Unwinder?

[automaticman]

Is there anything other than texture and the frame buffer that is stored in VRAM?

And I still don't think it's right to say that VRAM between the two cards is absolutely mirrored. After all, the frame buffers themselves don't store the same information because they are storing different frames of video.