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Bonus Page: Does CrossFire Work At x16/x4?

GeForce And Radeon On Intel's P67: PCIe Scaling Explored
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Many P55 and P67 motherboards include three x16 slots. Most of those slots are stuck at x8 or x4 bandwidth. Because the CPU itself is able to address a pair of devices from its sixteen lanes (Intel's desktop CPUs actually include three PCIe controllers), many motherboards add automatic switches that allow its first two x16 slots to share those lanes in x8 mode whenever a card is inserted into the second slot. The slower, third slot gets four lanes from the chipset's available PCIe lanes.

There are, of course, less expensive P55 and P67 motherboards that, lacking the somewhat-costly PCIe lane switches, only have an x16 and x4 slots to begin with. This configuration also covers enthusiast-grade H55 and H67 motherboards, since Intel artificially limits the CPU’s dual-card capability when used on its integrated graphics platforms.

While Nvidia prevents SLI from functioning on PCH-hosted lanes, x16/x4 configurations are completely possible in CrossFire. But should they be? We tested our motherboard in both x8/x8 and x16/x4 configurations to find out.

At 1680x1050, the best result is a 1% loss in Crysis, while the “worst” result is a 26% loss in F1 2010. The average difference between x8/x8 and x16/x4 is fairly bad at 10%. However, a worst-case scenario this bad will stick with us for a long time.

Our worst case is reduced to “only” a 24% performance loss at 1920x1080, with the average difference staying at 10%.

We’d expect to see GPU computational limits exceed bandwidth limits at 2560x1600, yet the worst-case scenario still shows 23% lower performance for the x16/x4 configuration. Again, the average difference is stuck at 10%.

Display 3 comments.
  • 1 Hide
    Stupido , 11 April 2011 17:40
    Interesting article...
    To me would be also interesting to see how older tech (C2Q & P45, i5/7 & P55) scales...
    sort: CPU to multi-card scaling...
  • 1 Hide
    mi1ez , 11 April 2011 18:47
    Potentially adding a new graphics card to my PCIe1 motherboard wouldn't be too much of a performance hit...
  • 0 Hide
    archange , 12 April 2011 22:41
    Quote:
    "Unfortunately, we often see well-meaning enthusiasts leading less-experienced readers astray. Case in point: we have one pervasive poster who offers exceptionally-detailed explanations of why Nvidia graphics cards need no more than four lanes of PCIe 2.0 bandwidth, citing something about internal PCIe to PCI-X conversion. We've also seen presumed experts tell curious gamers that AMD relies so heavily on its CrossFire bridge that a four-lane slot is ideal for hosting a second card. Today’s test results prove both suggestions completely wrong. We’re hoping that today’s article will be the impetus other advice-givers need to chime-in when the community calls for suggestions on how to best configure multi-card configurations."

    @TH: It seems to me that this article is the right way to counter misinformation: conduct the necessary testing for the masses. After all, that's why we keep coming back to this site.
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