PCI Express And CrossFire Scaling: Is P55 Good Enough?


Today’s analysis began by questioning whether Intel's Lynnfield-based Core i5 and Core i7 processors, along with the P55 platform that complements them, is good enough for CrossFire, and the answer is probably “sometimes.” 

Test results showed that a PCIe x8 slot provides 4% less performance when using the fastest possible single GPU, and those results did translate into a 4% performance deficit in CrossFire. That 4% loss isn’t horrific, and less powerful cards would likely show less performance difference. However, these results shouldn’t dissuade anyone from using an LGA 1156 processor to host only one dual-GPU card, since the card’s x16 interface limitations perfectly match those of LGA 1366-based processors.

Less surprising was the horrific performance of the x4 slot available on many P55 motherboards through the chipset’s PCIe hub. These lanes offer only PCIe 1.1-class bandwidth regardless of their PCIe 2.0 labeling, dropping 26% behind even the x4 slot available on similarly-priced X58 motherboards. A slot that slow might be useful for adding a graphics card, but only if that card is used for lower-performance applications, such as a dedicated PhysX processor or 2D desktop expansion.

Perhaps the biggest question answered here is whether we should have forgone the P55 solution in our most recent $2,500 PC build and the answer, of course, is yes. A 4% performance difference might not make or break a real-life build, but it’s a huge obstacle in any competition such as our SBM.

So what’s left? We still haven’t seen how an X58 motherboard with x16/x16/x4 slots compares to today’s x16/x8/x8 sample, whether or not a PCIe hub such as the NF200 can help the P55 to cope with additional cards, or how badly a P55 motherboard in x8/x8 mode can hurt a pair of dual-GPU cards, such as the Radeon HD 5970. We don’t have the parts in place to test all those variables, but will continue to consider your suggestions while we assemble components to put at least some of these questions to rest.

Thomas Soderstrom
Thomas Soderstrom is a Senior Staff Editor at Tom's Hardware US. He tests and reviews cases, cooling, memory and motherboards.
  • kirvinb
    I wonder does the same go for 790gx vs fx
  • gilbertfh
    I just ordered myself an ASUS P7P55D-Pro and didn't understand the difference... If I spent $1200 on video cards to go in this bad boy expecting it to test at higher performance that I will never be able to see by the naked eye it may have been a bigger deal(but it isn't).
  • gkay09
    It is pretty logical to go the X58 route if you are planning for more than 2 cards...
    Only 2x HD 5970s would have significant bottleneck with x8x8 config of the P55...for rest of the cards, the x8x8 still would suffice...
  • jsc
    For most of us, it looks like the P55 is the way to go for a gaming machine. I was a little surprised at how well two X8 channels did.
  • neiroatopelcc
    Does it make a difference for the motherboard wether or not all slots are populated when we talk energy? Each pcie plug is supposedly able to deliver 75W to the cards right? so at 3 cards that's quite a bit of power. In addition it has to deliver what, 140W? to the 4ghz cpu .... remembering an earlier article detailing failed boards because of inadequate vrm's I just have to wonder if a higher power draw on the pcie connectors would make a difference in sustainability with such an oc? I mean at some point I would assume a board can draw more power than the atx connector or the power plane can handle?
  • a4mula
    pcie 2.0 supports up to 300w (1.1 was 75w). The 5870 draws about 170w at load. This was the reason the 5970 was scaled back to 5850 speeds. Overclocked 920 @ 4.0 is going to draw about 250w. i5 750 (@ 4.0) on the other hand draws about 150w.

    For awhile now it's been known that the p55 will run an 8x8x crossfire within close proximity of it's x58 16x/16x even on 5870's. The only real question that hasn't been seen is the quadfire 5970's and a trifire 5970+5870. Publish that and you'll garner my attention at least.
  • neiroatopelcc
    300w ? but why then would new cards still ship with pcie6 and 8 connectors if the cards actually could make do with the pcie supplied power? I mean, sure for pcie1 support, but won't they still bitch if one was to not plug em in under pcie2?

    ps. 150W is also a lot for a processor designed for two digit numbers. Is there some kind of list somewhere of which motherboards support how much vrm power, or how many phases generate how much wattage or whatever can be used as a guideline?

    Gonna upgrade the p35 to p55, and I've never been one for stock speeds...
  • jennyh
    The gap is just going to increase with more and more powerful graphics cards. We're talking 1-2 years and 8x will be performing just like 4x is now.

    8x Pci-e lanes should now be considered a drawback when purchasing new hardware.
  • notty22
    "8x Pci-e lanes should now be considered a drawback when purchasing new hardware."

    They just proved it was NOT a drawback with two 400 dollar top tier gpu's.
    Someone that advocates not upgrading the rest of their motherboard specs to the current high performance components such as DDR3 and the fastest hypertransport speed should not worry about THEORETICAL pci-e bandwidth.
  • baracubra
    Yeaaaaah! I've been waiting for this article for quite a while now, this clears up a lot of speculation! I hope you guys do a follow-up article to test whether two 8x,8x are enough to support 2x 5970's....

    If this article had come out 2 weeks ago, I would have upgraded to an i7-870 and 2x 5870's, but now that we know Fermi is arriving in Q1 2010 I'm afraid i'm gonna have to wait. . . again -.-

    Oh well, that's why we love the PC market, always advancing :)