AMD's CrossFireX: Tri & Quad GPU Preview
by Anand Lal Shimpi on February 21, 2008 3:00 AM EST- Posted in
- GPUs
Power Consumption
In our 3-way SLI review we saw power consumption figures close to 800W at the wall outlet, thankfully with cooler running GPUs the CrossFireX numbers aren't as bad:
| Number of GPUs | Idle Power | Load Power (Bioshock) |
| 1 x Radeon HD 3870 | 148W | 257W |
| 2 x Radeon HD 3870 (1 X2) | 181W | 361W |
| 3 x Radeon HD 3870 (2 X2 + 1) | 211W | 406W |
| 4 x Radeon HD 3870 (2 X2) | 240W | 538W |
With four GPUs we're over 500W of power consumption at the wall when running our Bioshock benchmark. There's still no clear need for greater than 1kW power supplies, but the better-safe-than-sorry mentality appears to be in full effect.
Final Words
We have to give AMD credit, there was no cherry picking of titles for this preview - for the most part, the benchmarks AMD itself selected showed no real need for 4-way CrossFireX over 3-way. We do appreciate the honesty, but it's clear that the world just isn't ready for a quad-GPU solution.
Due to the state of AMD's driver optimizations DX10 games currently only scale well to 3 GPUs and not much beyond (Crysis/Bioshock), while DX9 games will generally scale better all the way up to 4 GPUs. We expected the opposite to be true but AMD provided us with technical insight as to why it is the case:
"The biggest issue is DX10 has a lot more opportunities for persistent resources (resources rendered or updated in one frame and then read in subsequent frames). In DX9 we only had to handle texture render targets, which we have a good handle on in the DX10 driver. In addition to texture render targets DX10 allows an application to render to IBs and VBs using stream out from the GS or as a traditional render target. An application can also update any resource with a copy blt operation, but in DX9 copy blt operations were restricted to offscreen plains and render targets. This additional flexibility makes it harder to maximize performance without impacting quality.
Another area that creates issues is constant buffers, which is new for DX10. Some applications update dynamic constant buffers every frame while other apps update them less frequently. So again we have to find the right balance that generally works for quality without impacting performance.
We are also seeing new software bottlenecks in DX10 that we continue to work through. These software bottlenecks are sometimes caused by interactions with the OS and the Vista driver model that did not exist for DX9, most likely due to the limited feature set. Software bottlenecks impact our multi-GPU performance more than single GPU and can be a contributing factor to limited scaling.
We’re continuing to push hard to find the right solution to each challenge and boost performance and scalability wherever we can. As you can see, there are a lot of things that factor in."
From AMD's explanation it sounds like there's still a lot of work to be done on the CrossFireX driver. While we can expect to see its public debut in March, it seems like it'll be a while before we're anywhere close to ideal scaling. We've found ourselves in this position with many-GPU designs in the past, at least the players are taking things a bit more seriously this time around.
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PhantomKnight - Thursday, February 21, 2008 - link
Sorry getting used to the comment posting.This would have helped.
3 x Radeon HD 3870 (2 X2 + 1)
NicePants42 - Thursday, February 21, 2008 - link
I was also wondering what that meant. Looks like two 3870x2s plus one 3870.I was also a little confused about the system setup - looks like they were using nVidia hardware and drivers...
tynopik - Thursday, February 21, 2008 - link
obviously it's a typo and they meant 1 X2 + 1PhantomKnight - Thursday, February 21, 2008 - link
Am I alone in being confused, i thought that 2 times 2 plus 1 is equal to 5.Anonymous Freak - Thursday, February 21, 2008 - link
Previous 3870 CrossFire vs. 3870X2 reviews have shown that the X2 performs better than two standalone 3870s in CrossFire. So using "2 GPUs" as the X2 only might be deceptive. I want to see:One 3870
Two 3870s
Three 3870s
One 3870X2
One 3870X2 + one 3870
Two 3870X2s
(I don't think there is any board that could take four plain 3870s.)
Simply because from previous reviews, the "one to two" delta is much higher for X2 than for two plain 3870s. Another question is if the delta from one to two 3870s is the same as one to two 3870X2s. Based on previous X2 reviews, it looks like by all rights, the system really sees the X2 as a single double-fast GPU.
Goty - Thursday, February 21, 2008 - link
The MSI K9A2 Platinum can. =)Actually, I think that's the same motherboard used in this review.
Anonymous Freak - Tuesday, February 26, 2008 - link
Except when you use all four slots, they all drop to x8. And, yes, you would need a case with 8 back-panel slots, including one beyond the 'bottom' of the card. (Or you'd need to hack your cards to be single-slot.)strikeback03 - Friday, February 22, 2008 - link
How many cases have the openings on the back for 4 dual-slot cards?yyrkoon - Thursday, February 21, 2008 - link
"From AMD's explanation it sounds like there's still a lot of work to be done on the CrossFireX driver. While we can expect to see its public debut in March, it seems like it'll be a while before we're anywhere close to ideal scaling. We've found ourselves in this position with many-GPU designs in the past, at least the players are taking things a bit more seriously this time around."You guys, and many others have been talking about how well quad core CPUs scale(or dont) in a none server, are you really all that surprised to find that quad GPUs would be similar ?
I have been long suspecting that while the PCIe specification is absolute in how much bandwidth a number of PCIe lanes are supposed to have total, that motherboard manufacturers have been skimping as to how much they are allowing those lanes use. Maybe this is just now biting them in the behind ? Or maybe I am just being silly . . . ; ) Either way, system details are not exactly forthcoming here.
skiboysteve - Thursday, February 21, 2008 - link
... What?