Skip to main content

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

Bonus Page: Does CrossFire Work At x16/x4?

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 a 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%.

  • 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.
    Reply
  • 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"
    Reply
  • classicaxe
    ^They already did an article on that man
    Reply
  • 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.
    Reply
  • 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.
    Reply
  • 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!
    http://www.tomshardware.com/reviews/pci-express-scaling-p67-chipset-gaming-performance,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."
    Reply
  • dalauder
    Yeah...my bad.
    Reply
  • Crashman
    dalauderYeah...my bad.I didn't mean to call you out to that extent, here's a link to the forum part of this thread:
    http://www.tomshardware.com/forum/2887-56-geforce-radeon-intel-pcie-scaling-explored
    I'm going there to delete your quote from my response.
    Reply
  • rolli59
    Looking forward to see the third article!
    Reply
  • 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.
    Reply