GeForce And Radeon On Intel's P67: PCIe Scaling Explored


We found that Nvidia’s high-flying GeForce GTX 570 suffers around 2% performance loss when using an x8 slot, while the Radeon HD 6950 gives up around 4%. While noteworthy in respect to the negative impact on system value, those losses probably wouldn’t be noticeable in actual game play.

More significant are the losses when using a x4 slot, as expected. The GeForce GTX 570’s relatively significant 9% performance loss is far exceeded by the Radeon HD 6970’s 18% loss. And this is where some builders will hit an advice snag, otherwise known as self-proclaimed experts analyzing an effect and incorrectly deducing the cause.

Tom’s Hardware readers often rely on each other to provide the best technical advice, and the advisor offering the most technically-detailed description often gets the nod (hey, if it sounds fancy, it's more likely to be right, right?). 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.

Putting aside our concern for the honest knowledge-seekers, we can conclude that minimal performance losses in x8 mode set us up for the perfect logical next-step in our three-part series, where we compare X58 and NF200 solutions to today’s board. Will that 2% to 4% difference remain in SLI? Does the NF200 really hurt graphics performance by adding latency? Will the smaller performance gains of three-way CrossFireX mitigate the performance deficits of its x4 slot? Stay tuned to find out!

Thomas Soderstrom
Thomas Soderstrom is a Senior Staff Editor at Tom's Hardware US. He tests and reviews cases, cooling, memory and motherboards.
  • geofelt
    These tests were done with a single card, on X16/X8/X4 slots. Fine.
    But... Who would use anything other than a X16 slot if they had one?
    The only real use for a X8 slot would be for sli/crossfire where the addition of a second card should result in an Increase of performance, not a decrease.
  • carlhenry
    it would be nice if you included the GTX 570 in the x8/x8 and x16/x4 test. the 570 flies over the 6950 on the single card config but i was curious how it would do since i think the AMD's scale better than nvidia's. would the 570 still lead because of its advantage? or would AMD even it out because of its scaling (if any) "advantage"
  • classicaxe
    ^They already did an article on that man
  • joytech22
    Can you guy's do an article on how performance is affected if you SLI/Xfire using PCI-E 16x slots running @ 4x?

    3 way would be preferable because if performance is still adequately faster I'll consider it.
  • dalauder
    Good comments. Can we please see 8x/8x and 16x/4x since that comparison is relevant? I get the impression that somehow SLI/crossfire reduces the performance hit of x4 lanes but I'd like to see numbers.
  • Crashman
    joytech22Can you guy's do an article on how performance is affected if you SLI/Xfire using PCI-E 16x slots running @ 4x?I think you missed a page then!,2887-10.html
    The numbers were there all along!

    carlhenryit would be nice if you included the GTX 570 in the x8/x8 and x16/x4 test.Well, you should probably read the linked page too then. There's no point in artifically creating a configuration (by taping lanes or whatever) that doesn't exist in real life, is there?

    "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."
  • dalauder bad.
  • Crashman bad.I didn't mean to call you out to that extent, here's a link to the forum part of this thread:
    I'm going there to delete your quote from my response.
  • rolli59
    Looking forward to see the third article!
  • cats_Paw
    Im guessing that 8x lanes are mostly enought. I do belive that it would depend on how fast an actual gpu is, as well as how much ram it has, and how big is its bandwidth.
    I means, its logical, but mayb not true :D. Would be nice to see this test on a GTX560 Ti, since it has a lot of headroom for OC, then compare oced version vs non oced. Also this might be interesting in GPUs that have diffrent versions with more and less RAM.
    Just my 2 cents :D.