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PCI Express And SLI Scaling: How Many Lanes Do You Need?

The PCIe Bottleneck?

A mere seven months have passed since our most recent PCI Express scaling article showed modest performance differences between PCIe x8 and PCIe x16 slots. But it has been a very busy seven months!

The first salvo came when Nvidia’s much-delayed GeForce GTX 480 smoked AMD’s Radeon HD 5870 as the fastest single-GPU card on the market, and the mid-priced solution that followed showed the highest multi-GPU performance scaling we’ve ever seen.

Unfortunately, such an elevated degree of technological achievement is difficult to swallow for a motherboard reviewer, as it makes my earlier findings irrelevant to most users.

The focus of today’s question will center on you, the PC owner. Do you actually need an X58 platform to support the latest graphics technologies, or will something with fewer lanes suffice? MSI helped us to facilitate the answer with a single product, by producing an X58 motherboard that also has the x8 and x4 modes found on some P55 solutions.

We’ve already seen how X58 and P55 motherboards offer similar gaming performance when using a single x16 slot. And limiting ourselves to a single board allows us to focus exclusively on PCI Express lane width by eliminating every other variable. The name of that product is, of course, the Big Bang-XPower.

While it certainly doesn’t represent the P55 market’s moderate pricing, the XPower’s biggest liability becomes an asset for the purpose of today’s test. Its two PCIe 2.0 x16 slots are divided between up to three x16-length slots each, changing to x8-x0-x8-x8-x8-x0 modes when slots three and five are filled, and then to x8-x4-x4-x8-x4-x4 mode when slots two and six are filled. Thanks to MSI, we can now check x16, x8, and x4 transfer modes on a single motherboard, without using little fingers of tape to reduce the number of connections on the card itself.

  • Mousemonkey
    There are quite a few folk who have been waiting for this.
    Reply
  • zorky9
    This would even add more value to the i5-750 in your best CPU for the money article.
    Reply
  • amk09
    I love how people always bash on x8 x8 and how it sucks, when in reality x16 x16 is only 4% better.

    You spend unnecessary $$$ on a x58 platform while I save money that I can put towards a GPU upgrade with my p55 platform :)
    Reply
  • carlhenry
    i'm curious how other games are "dependent" on bandwidth while others are not... does that mean that the games that aren't dependent on bandwidth isn't using the full potential given the the size advantage of x16 over the x8? i wish every game would utilize every inch of your hardware in the future.
    Reply
  • sambadagio
    For all your fps hunters, I bet you only have a screen at home with 50 or 60Hz. So just for your information, everything above 50 or 60fps is just useless... In this aspect, a PCIx 4x is actually enough... ;-)

    Reply
  • luke904
    so a 4850 crossfire setup will hardly be bottlenecked by an 8 lane motherboard.

    anyone know if 4850's are going to be unavailable any time soon? You could get the 3000 series for quite awhile after the 4000's released so I'm crossing my fingers until i can afford a cpu upgrade and another 4850

    cpu is currently a 7750BE and so im pretty sure it would bottleneck the 4850's. I think it does with just one actually.
    Reply
  • jgv115
    @ carlhenry

    It's not the game's fault. The GPU can only go as fast as it was made to go. So in simple terms you could say that GPUs these days aren't "fast" enough to use all the bandwidth PCI Express offers.
    Reply
  • outlw6669
    Very nice review but I have to ask, why did you not test with 5970's?

    On a card for card basis they are still quite a bit more powerful than the GTX 480 and should require the most bandwidth of any current card for maximum performance.
    Reply
  • barmaley
    This review tells me that if you already have an i7 and at least 2xPCIe 16x lanes on your motherboard then in order to play modern games, all you are going to be upgrading for the next several years is your graphics.
    Reply
  • Aionism
    Even though I'm not interested in SLI I am glad to finally see a benchmark comparing PCI-E x16 and x4. My motherboard only allows me to use my video card in my x4 slot for some reason. I've been wondering how much performance I've been losing over that.
    Reply