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Sideport 101 with AMD's Eric Demers

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Interview with AMD's Eric Demers

This is old, but a good read nonetheless.

Quote:
R3D: Now that we've started touching sensitive topics, can we find out what's up with the interconnect? Why is it disabled? Are naysayers correct in stating that it's up to the AIB whether or not the traces for it get built into the PCB, and that it's likely to not be enabled?

Eric: The sideport interconnect is fully functional in the reference design. Though we've found that with the current AFR mode of multi-GPU support, the additional bandwidth brought by the interconnect does not translate to a significant improvement in performance. However, we are continuously working on optimizing how our ATI CrossFireXâ„¢ technology scales and trying different methods, and could decide to enable the sideport if a method is found which gives better results and benefits from it.

R3D: So what happened? How did a good idea stop being a good idea? Are you disabling it due to heat/power concerns?

Eric: It's still a good idea...you have to consider that we've been working on Crossfire and on our AFR implementation for quite a few years, optimizing it. It's actually gotten to a point where it's really quite good, and difficult to improve upon performance-wise. As for heat and power concerns, that's hardly the case...the interconnect is pretty much an extra PCIE2.0 link. The power draw for that is in the realm of a few watts at most. The only reason for which it's not enabled currently is, as I've already mentioned, that it does not significantly impact performance ... and if you're not using something there's no point in enabling it.

R3D: What kind of data can be exchanged via the interconnect? Is it a faster/lower latency avenue for transfering non-frame data like RTs that must be copied from frame n to frame n+1 and so on via a p2p write in normal CF configurations? Is this one possible application of it?

Eric: The interconnect offers the same features as the PCIe interconnect, plus it allows for a GPU to broadcast memory writes to its own memory and to the other GPU's. The data exchangeable is any data that the driver would desire. The latency versus the on-board PCIe interconnect used by the ATI Radeon HD 4800 X2 series isn't very different, but it's generally better than that offered by using the northbridge. Nevertheless, the static latencies I'm discussing here generally don't matter that much, and bandwidth is the key factor.

R3D: Could the broadcast writes function be used for avoiding swapping a persistent RT over the PLX switch? So instead of GPU2 waiting for GPU1 to finish working on it and then wait for it to be swapped via a peer-write, you'd have GPU1 updating the RT both in its memory and in that of GPU2 so GPU2 could start working right away?

Eric: Yes (pause for dramatic effect). That was one of the reasons we have it there. Keep in mind that it does not mean that this approach would be necessarily better than what we're normally doing in AFR.

R3D: Are you considering using a more extensive/granular SuperAA implementation, for example, 2X AA being done by having the 2 GPUs render the frame with no AA and jittering the frames in the compositing chip, 4X AA has the 2 GPUs render the scene with 2X AA etc. (an accumulation-buffer like approach, if you will)?

Eric: I'm not going to comment on future algorithms that we might implement for multi-GPU. Let's just say that we believe that we have the highest quality and performance available today, and that we have many options available to us down the stream. We already support SuperAA resolution when doing multi-GPU, which allows for higher quality and higher number of samples.
I read another article recently explaining AFR Sideport relationship, basically stating that AFR would not benefit from the sideport.
For the life of me I can't find it
Edited by grunion - 12/23/08 at 5:37pm
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post #2 of 2
I read somewhere that Sideport isn't a requirement in the 4870X2 spec, so it's likely that some, if not all, 4870X2 will not support Sideport. I'm really just waiting for ATI's response to the GTX 295.

If they do end up enabling Sideport for the 4870X2, I hope it doesn't disappoint.
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