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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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  • 25 Hide
    amk09 , August 9, 2010 6:38 AM
    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 :) 
  • 14 Hide
    zorky9 , August 9, 2010 6:34 AM
    This would even add more value to the i5-750 in your best CPU for the money article.
  • 13 Hide
    Mousemonkey , August 9, 2010 6:22 AM
    There are quite a few folk who have been waiting for this.
Other Comments
  • 13 Hide
    Mousemonkey , August 9, 2010 6:22 AM
    There are quite a few folk who have been waiting for this.
  • 14 Hide
    zorky9 , August 9, 2010 6:34 AM
    This would even add more value to the i5-750 in your best CPU for the money article.
  • 25 Hide
    amk09 , August 9, 2010 6:38 AM
    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 :) 
  • -8 Hide
    carlhenry , August 9, 2010 6:51 AM
    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.
  • -4 Hide
    sambadagio , August 9, 2010 7:01 AM
    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... ;-)

  • -9 Hide
    luke904 , August 9, 2010 7:39 AM
    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.
  • -4 Hide
    jgv115 , August 9, 2010 7:46 AM
    @ 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.
  • 5 Hide
    outlw6669 , August 9, 2010 8:15 AM
    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.
  • -4 Hide
    barmaley , August 9, 2010 8:38 AM
    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.
  • 0 Hide
    Aionism , August 9, 2010 8:48 AM
    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.
  • 5 Hide
    wa1 , August 9, 2010 8:56 AM
    I'm not an enthusiast, so a single powerful card would be more than enough for me... :) 
  • 13 Hide
    Crashman , August 9, 2010 8:57 AM
    outlw6669Very 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.

    The first article tested CrossFire scaling with three 5870's:
    http://www.tomshardware.com/reviews/p55-pci-express-scaling,2517-2.html
    1.) It appears that the GTX 480 runs out of CPU faster than the HD 5870. 2.) It also appears that the biggest difference between games is how hard they hammer the GPU, based on details, lighting effect, etc.
    3.) The result is that you're seeing an FPS cap from either the board or the CPU as the load shifts away from the GPU to other components. The good news is that this "cap" is higher than the "minimum playable" frame rate most people can tolerate, in most games.
    So, what does this have to do with your question? The HD 5970 uses a PLX Bridge: http://www.tomshardware.com/reviews/radeon-hd-5970,2474-2.html

    Both GPUs get the same data, and the PLX Bridge simply doubles it from one set of lanes to two GPUs. So, an x16 slot turns into two identical x16 sets, or an x8 slot turns onto two identical x8 pathways. The PCIe "bottlenecking" data you get for two 5870's should therefore be identical to the PCIe data you get from one HD 5870 x2, such as the Asus ARES, which is actually a faster card than the HD 5970.
  • 2 Hide
    outlw6669 , August 9, 2010 9:00 AM
    Ah, that does make sense now.
    Thank you for the explanation Crashman :) 
  • 1 Hide
    iam2thecrowe , August 9, 2010 9:24 AM
    would the same pcie scaling performance differences still be there with a slower CPU?
  • 0 Hide
    th_at , August 9, 2010 10:00 AM
    I'd be interested in seeing more games tested. In this test, only CoD seemed to show any noteworthy decrease in performance for a single VGA card in the 4x PCIe slot and even that at FPS rates where it didn't matter.
    I'm considering using the 4x on my mainboard for airflow reasons in my case and as of now, nothing seems to speek against it. I'm only using a lowly GTX 460 anyway.
  • -2 Hide
    Tamz_msc , August 9, 2010 10:03 AM
    I would have liked to see what would happen if the i7 wasn't overclocked.
  • 5 Hide
    Crashman , August 9, 2010 10:23 AM
    iam2thecrowewould the same pcie scaling performance differences still be there with a slower CPU?

    The slower your CPU, the more the limit shifts from other components to the CPU. That means the maximum FPS will get dragged down even farther, making the 1680x1050 results look closer to the 1920x1200 results.
    th_atI'd be interested in seeing more games tested. In this test, only CoD seemed to show any noteworthy decrease in performance for a single VGA card in the 4x PCIe slot and even that at FPS rates where it didn't matter. I'm considering using the 4x on my mainboard for airflow reasons in my case and as of now, nothing seems to speek against it. I'm only using a lowly GTX 460 anyway.
    Is it a PCIe 2.0 slot? Please read the CrossFire article to see how bad PCIe 1.1 x4 is, and don't use it.
  • 3 Hide
    eddieroolz , August 9, 2010 11:37 AM
    Very educational article. I usually recommend against the X58 platform when I'm asked for help, and seems like this article validates my recommendations.
  • 0 Hide
    feeddagoat , August 9, 2010 11:41 AM
    Seems that by the time PCIe 2.0 x16 is needed we will be using PCIe 3.0 anyway. This is also good in the sense that not you can look for x8 - x8 boards with other features such as USB3 and SATA6GB/s rather than having them gimped in favour of x16 - x16. I can guess the general result of this but how much performance drop is there using one of the p55 x16 - x4 links? Im assuming the article was using both cards at x4 -x4?
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