PCI Express And CrossFire Scaling: Is P55 Good Enough?

We hear from nearly everyone in the comments section that the LGA 1156 platform, which has 16 PCI Express (PCIe) 2.0 lanes provided by the CPU's on-board controller, is destined to be nothing more than a mainstream product for gamers using, at most, a single graphics card. A slow DMI interface linking the CPU to the P55 Express PCH at PCIe x4 bandwidth certainly doesn’t help bolster the platform’s performance credentials, causing many to question why Intel would add the Core i7-870—a $540 part—as one of only three launch-day Lynnfield processors. Certainly nobody would drop such an expensive component ontp a “mainstream” motherboard. But that was exactly the option Intel was hoping many builders would choose.

We’ve since found the Core i7-870 to be an excellent (albeit pricey) part, with clock-for-clock performance in a dead heat with LGA 1366 processors, better Intel Turbo Boost ratios, lower average power consumption, and superior overclocking of up to 4.3 GHz on air cooling. All of these great advancements cause us to ask whether the Lynnfield family of Core i5 and Core i7 CPUs might be wolves in sheep's clothing. 

Is the limit of 16 PCIe lanes really that much of a hindrance? Is the performance difference between eight and 16 lanes really big enough to dismiss CrossFire from the list of the platform’s capabilities, as some have suggested? Is the x4 slot often added to $200 X58 motherboards really better than those of similarly priced P55 products?

Those are all great questions, and we begin our journey with an examination of slot performance, comparing the performance capability of x4, x8, and x16 slots on both the P55 Express platform and the X58 Express that came before it. Great pains have been made to ensure that everything else is equal throughout each test, as described in our configuration page.

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  • coozie7
    Interesting, and it matches the results found elsewhere on the subject.
    With the i5 seeming to be a popular mid-upper bracket choice, why not pair it off with similar placed cards like the HD5770/HD5850?
    I would be very interested to see how these less potent cards perform with the popular x16/x4 or x8/x8 'boards out there rather than see a HD5970 crippled by an x8 'board-a configuration that seems pointless in the extreme.
  • damian86
    So if there is not much difference between x16 and x8,it doesn't really matter if a P55 system does't support 16x/16x eg.only performs 16x/8x-8x/8x or whatever?
    i found a similar report here http://www.hardware-revolution.com/p55-motherboards-crossfire-sli-performance-problem/
  • Anonymous
    But why bother about x16/x8 there are true high-end chipsets from AMD with full CF at lower price?
  • b82
    In a previous look at PCIe 2.0, Toms Hardware found variation in the impact of bandwidth reduction according to the application running. See this http://www.tomshardware.com/reviews/pci-express-2.0,1915-9.html

    A particularly difficult program was Microsoft's FSX flight simulator (scroll down on the linked page). Well you might say, FSX doesn't benefit from SLI or Crossfire, you'll be using a single graphics card, so why are you bothered?

    Well unfortunately a lot of people might be doing things like adding a USB 3.0 adapter card soon. That only requires PCIe 2.0 X1, BUT if you stick it in the other PCIe 16x slot it'll cut the graphics card down to PCIe 8x with P55.

    Or you could put the high bandwidth adapter in a PCIe x4 slot. However, in Intel's wisdom they created the P55 chipset with 2.5 GT/s PCIe generation 1.1. Even if you were happy with that (it'll limit a USB 3.0 adapter) I'm still stuck because I want a Micro ATX build. All Micro ATX P55 motherboards have the PCIe x16 slot positioned so that a dual slot graphics cooler will obstruct the PCIe x4 slot. Great.
  • Anonymous
    A useless comparison since the whole point of getting an X58 board over P55 is that it can do 16x/16x whilst a P55 can only ever get as good as 16x/8x so a real test of Crossfire performance would have been a better X58 board that is up to the Intel spec rather than one hobbled to a P55 spec. You might as well be testing two P55 boards.

    What a pointless article.
  • Solitaire
    Just when I thought it was safe to come back here... the smacktardian smell of payola wafts through the door as Intel and nVidia give Toms little brown envelopes for stating the blindingly obvious. Yet. AGAIN.

    So, from this test we draw the conclusion that P55 is EVIL and ATI are EVILLER!!! No, Toms, we're not as jack-**** retarded as Intel thinks we are, and will not rush out to buy X58-based systems and GTX295s to put on them just to avoid the EVIL of bandwidth bottlenecking. Your test is risible; who in HELL has the money to use three €400 graphics cards in CF these days?! I'll tell you who: someone who wouldn't buy a P55 system even if Hell froze over just for their pleasure. Where the hell are the HD5750/5770/5850 2-card comparisons?! The above article is just utterly pointless - of course three cards that could eat a HD4870X2 or GTX295 are going to overwhelm a P55 if you jam them all on together! That's just common sense!