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Old September 23, 2009, 09:23 AM
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Default AMD Radeon HD 5870 PCI-Express Scaling Review [TPU]

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Our bottom-line on this subject is that there is every reason to be optimistic when opting for two of these accelerators on motherboards with two PCI-Express 2.0 x16 (electrical x8) slots, because the performance penalty between that and PCI-Express 2.0 x16 (electrical x16) is just too small. Unless you're the quintessential enthusiast and every frame per second increase matters to you, there is no reason to worry about a performance drop on mid-range motherboards, although this is only one of the factors, a main one at that, to contribute to the performance drop. Some motherboard manufacturers are offering a third PCI-Express x16 slot that is electrically x4. The results show that the performance drop isn't as bad as one would imagine, so we will green-signal installing a third accelerator for some 3-way ATI CrossfireX action, or 2-way CrossfireX on entry-level Intel P55 motherboards with the second x16 slot electrically x4 (running in 1.0 mode). If you're crazy enough to mod a PCI-Express x1 slot (by carefully cutting its end to let it seat a PCI-Express graphics card), then the scores should really dishearten you. Buy one of these accelerators now, and give them a neighborhood when they become more affordable, and you will have secured yourself future-proofing for quite long.
Source: techPowerUp :: AMD Radeon HD 5870 PCI-Express Scaling Review :: Page 1 / 26

Seems to me like x8 2.0 is still plenty for the 5870 as it only drops performance by ~2%. Even x4 2.0 is fairly close to full speed giving a ~5% drop in performance. This also leads me to believe that 5870X2 CF (for the people that need 100FPS on 30in monitors!) would only have a small drop in performance with x8/x8 CF. Since each 5870X2 is essentially two 5870s sharing a single bus, a 5870X2 on a x8 would be about the same as two 5870s on two x4 buses. So even if you get 5870X2 CF and put it with P55, you would still only get about a 10% drop in performance compared to true x16/x16. But of course, if you can afford 5870X2 CF, you can also likely afford X58 and i9!
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Old September 23, 2009, 09:36 AM
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This also means that those of us on the AMD side with 790GX/790X boards (which are also 8x/8x in crossfire) will be happy as well! Nice article!
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Old September 23, 2009, 09:39 AM
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Yeah, I was just about to post this review as well. I don't like how they're using older drivers than necessary, but it's still an interesting look at things. One of the lesser talked things that have slowly been optimized over the years has been the PCI-E traffic management, with both Nvidia and ATI attempting to minimize the amount of bandwidth that needs to go over the PCI-E bus.

That said, I find their "green-lighting" of using a x4 slot on a P55 board to be a little stupid - it's going to have to deal with a DMI link bottleneck, and compete with everything else using the PCH as well. Two cards at x8, sure. Three? Buy a damn X58 board, cheapskate.
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Old September 23, 2009, 09:55 AM
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Originally Posted by MpG View Post
That said, I find their "green-lighting" of using a x4 slot on a P55 board to be a little stupid - it's going to have to deal with a DMI link bottleneck, and compete with everything else using the PCH as well. Two cards at x8, sure. Three? Buy a damn X58 board, cheapskate.
Although the x4 wont bottleneck the GPU much, if you can afford three 5870s, you could have easily enough purchased a X58 system for only a small amount more money. So while the x4 may be "fine" to use on P55, its much easier just to be smart about it and buy into X58. Some company could perhaps split the PCIe into x8/x4/x4 instead of x8/x8/x4 using the extra x4, the board will likely scale better with three GPUs instead of only two. As you said, the x4 is enough, but throw in the other things using the PCH and you will likely start getting a more significant drop in performance.
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