DJ_PAPA
Power Member
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.
http://www.anandtech.com/video/showdoc.aspx?i=3232&p=1
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PCGH: exclusive benchmarks from AMDs Quad Crossfire
http://www.pcgameshardware.de/aid,6...xclusive_benchmarks_from_AMDs_Quad_Crossfire/
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CrossFireX preview
http://www.hexus.net/content/item.php?item=11874&page=1Summary
We've seen first-hand that CrossFireX, in its three- and four-GPU incarnation, works well enough in DX9 titles run at ultra-high resolutions and image-quality settings.
A single-GPU Radeon HD 3870 is no slouch but performance can be up to 3x better with dual Radeon HD 3870 X2s. That will only really matter if you're playing fast-paced games and want all the eye candy turned on to maximum, however.
Scaling, though, is comparatively limited with the DX10 API and Crysis - which is the most talked-about game of 2007 - gains are virtually non-existent at high resolutions.
Given what we've seen thus far, the optimum combination, assuming the underlying hardware is in place, appears to be a three-GPU graphics sub-system run at 1,920x1,200 and above. NVIDIA, we note, already has three-way SLI for its GeForce 8800 GTX and Ultra SKUs.
It's difficult to give firm buying advice with such a nascent technology. What we can say for now is that CrossFireX opens up new, flexible avenues for potential upgrades in the HD 3800-series family.
However, we reckon that the majority of popular gaming titles will need to scale to 3x before four-GPU operation becomes compelling enough to invest in.
Further analysis will be presented as we review the CrossFireX driver before its launch with the CATALYST 8.3 drivers.
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