The R7xx Architecture Rumours & Speculation Thread

[Silent_Buddha]

Can anybody answer how Crossfire would work if you have a card that is in crossfire mode that is in crossfire with another card in crossfire mode?


More intresting is that ATi is introducing tri fire and quad fire. So I'm confused even more with how R700 will react to that kind of set up.

If they are planning AFR all the way even with multiple cards with multiple GPUs, this is how I envisage it.

GPU-Core. So GPU1-Core2 would be 1-2... And I see rendering going something like...

1-1
2-1
3-1
1-2
2-2
3-2

Although I suppose there's no reason it couldn't just go

1-1
1-2
2-1
2-2
3-1
3-2

And would there be a way to hide the possible effects of user input changing things mid-stream? As far as I can see in any program that requires constant user input, performance is going to absolutely tank or latency is going to be unbearable with more than 2-3 cores/GPUs.

As noted, there's also a slight latency with SLI/Crossfire that I can noticed when it's enabled (another reason I'll never use SLI/Crossfire again, other than the obvious multi-monitor one).

Ever since the Rage Fury Maxx I haven't really been a huge fan of AFR. It just seemed rather clunky (and "sloppy" due to the input latency) in the face of 3dfx's SLI.

Also wouldn't the scaling penalty of fixed SFR be reduced with more cores/cards working on a scene. IE - even if 1 card out of 8 was only operating at 10% capacity due to drawing a blue sky or whatever, it doesn't impact performance as a whole as much as 1 out of 2 cards being mostly idle due to unbalanced load.

I keep hoping that someone will come up with a more clever way of doing multi-gpu rendering...but not really seeing it at the moment.

I guess it's too much to hope for that there will be some massive interconnects between GPU packages (multipe HT 3.0 links?) allowing the package as a whole to operate as one virtual GPU.

I suppose the engineering hurdles for something like this are fairly substancial, but the payoffs in terms of multi-GPU performance would, IMO, be huge.

If R700 ends up just being another Rage Fury Maxx, I'm going to be sorely disappointed.

Regards,
SB

 

[Zengar]

Does anyone know how multi-GPU setups handle offscreen rendering? This approach (render to offscreen buffer/postprocess) is getting more and more popular (HDR/deffered rendering). Will SLI/Crossfire accelerate this?

 

[armchair_architect]

AFR mode does accelerate this. Each card draws an entire frame, including any non-backbuffer rendering that's part of the frame.

This is one of the things that makes SFR scaling difficult. Most offscreen rendering is reused as a texture, which means each GPU needs the entire thing not just half of it (in general you can't predict which parts of a texture will be mapped to which rendered pixels). So you either have to render the entire thing on both GPUs (and get no performance scaling for that part of the frame), or render half on each GPU and copy those halves to the other GPU (using a lot of bus bandwidth). Which is faster depends on the relative cost of rendering a pixel (also difficult to predict) vs. copying it.

 

[Blazkowicz]

that's a really interesting thread. I'm guessing optical interconnects will be badly needed but they are a few years away.

I can also speculate that AMD first Fusion, said to be two dies on one package, will sport a R7xx derivate?

 

[AlexV]

that's a really interesting thread. I'm guessing optical interconnects will be badly needed but they are a few years away.

I can also speculate that AMD first Fusion, said to be two dies on one package, will sport a R7xx derivate?

Maybe, but do keep in mind that AMD isn't positioning fusion as a discrete-GPU replacement solution in the performance sector. It'll probably be a means to get rid of the IGPs.