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

PCI Express And SLI Scaling: How Many Lanes Do You Need?
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Are the most elaborate platforms really required to host the fastest GPUs, or can you get away with P55's lane-splitting scheme? As Nvidia’s latest graphics processors push 3D performance to new heights, we examine the interfaces needed to support them.

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.

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  • 1 Hide
    ronanh , 9 August 2010 20:53
    Interesting article... but proves little. The dual x16 crowd will swear that the CoD benchmark proves that x8 cripples your hardware, whereas if you're running a dual x8 setup you'll be more limited by cpu than anything else.

    The question must be asked though, if you can afford to go dual 480s why would you be running anything but a x58 mobo.
  • 0 Hide
    Silmarunya , 10 August 2010 00:08
    RonanHInteresting article... but proves little. The dual x16 crowd will swear that the CoD benchmark proves that x8 cripples your hardware, whereas if you're running a dual x8 setup you'll be more limited by cpu than anything else.The question must be asked though, if you can afford to go dual 480s why would you be running anything but a x58 mobo.


    The question must be asked though, if you are crazy enough to run dual 480's, why wouldn't you be crazy enough for a x58 build?

    Honestly, SLI/Crossfire with high end cards only pays off at the highest resolutions (think 30" monitors, or Eyefinity setups) and at the very highest details (who needs 32x AA anyway?).

    Anyone with the brains to earn a lot of money should have the brains to know it's not worth spending that cash on a x58 board with 480's in SLI (at least not as a gamer, not talking about professional usage).
  • 0 Hide
    darksai , 10 August 2010 03:38
    All in all.. You dont need x58 for anything less than 2560x1600, and even there its questionable. What i would like to tho, which is probably more relevant to p55 owners, is how mid-range cards scale on 3 way setups (like 5770/460 on x8 x8/x4 x4). Seeing these articles limited to such high end configs is moot, cos ultimately every setup is gonna get >> 30 min fps on reasonable res anyway.
  • 0 Hide
    Silmarunya , 10 August 2010 03:49
    darksaiAll in all.. You dont need x58 for anything less than 2560x1600, and even there its questionable. What i would like to tho, which is probably more relevant to p55 owners, is how mid-range cards scale on 3 way setups (like 5770/460 on x8 x8/x4 x4). Seeing these articles limited to such high end configs is moot, cos ultimately every setup is gonna get >> 30 min fps on reasonable res anyway.


    That'd be intresting, but even 5770's are crippled by those electrically underperforming x4 slots. A x4 slot is physically every bit as good as a x8 or x16 slot, but electrically it performs about as good a x2 slot - not good at all in other words.

    Crossfiring 5770's at 16x/4x has showed worse results in some games than a single 5770 - crossfire still implies a minor reduction in performance, and the extra performance from a 4x slot can't even make up that bump...
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