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The Difference Between PCIe x8 And x16

Multi-GPU Setups: The Basics Of CrossFire And SLI
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A motherboard doesn't always come with two fully-featured PCI Express x16 slots. Available PCI Express lanes are often distributed across two slots, meaning that the second PCIe port may only run eight lanes electrically. This also means that the speed of one physical x16 PCIe port decreases when you use expand out to a second slot. If you're running a P55-based platform, like our test system, this applies to you.

But don't worry yet. Our tests show that even with the fast Radeon HD 5870 and Radeon HD 5850 cards, the performance impact when switching from PCI Express x16 to x8 is only a few frames per second (remember to check out next week's coverage for more in-depth exploration of PCI Express scaling). Additionally, both cards are throttled at x8, even if you combine a x16 and a x8 slot. According to the GPU-Z tool, the CrossFire configuration is completely synchronized.

With Nvidia, you have a bit more flexibility in slot selection. Even the fastest graphics cards are recognized as an SLI configuration, even if you don't use the SLI bridge connector and instead let the PCIe interface handle all data transfers. However, this comes at the expense of some performance. Ideally, you should always use a bridge connector to link the two cards together. Interestingly, GPU-Z shows that when you combine Nvidia cards in x16 and x8 slots, each card in the SLI configuration still runs at the speed of its respective slot.

With a bridge connector, the difference in speed is within the measuring tolerance. Without a bridge connector, though, you can clearly see that the cards use different interface speeds, and you witness a performance hit of up to 13% in SLI mode.

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Top Comments
  • 25 Hide
    Maziar , August 5, 2010 6:23 AM
    Great review,however i wish you tested on resolutions like 2560x1600 and higher,because i think the main difference between 8x and 16x is in resolutions like that.
  • 21 Hide
    ramcrazy360 , August 5, 2010 6:06 AM
    It's great to know there's so little difference between 8x and 16x preformance even with a 5850! I was worried about having a bandwidth issue with two 5770s, but it seems that won't be a problem.
  • 20 Hide
    one-shot , August 5, 2010 8:44 AM
    Avatar? I didn't know people played that game.
Other Comments
  • 21 Hide
    ramcrazy360 , August 5, 2010 6:06 AM
    It's great to know there's so little difference between 8x and 16x preformance even with a 5850! I was worried about having a bandwidth issue with two 5770s, but it seems that won't be a problem.
  • 25 Hide
    Maziar , August 5, 2010 6:23 AM
    Great review,however i wish you tested on resolutions like 2560x1600 and higher,because i think the main difference between 8x and 16x is in resolutions like that.
  • 16 Hide
    wildeast , August 5, 2010 6:28 AM
    same as ramcrazy,
    i was going to get an i5 but that means a p55 mobo (x8+x8) not x58 (x16+x16), so i thought i should be getting x58 and i7 rig which is beyond my savings, thnx tom's am getting i5 + p55 now :) 
  • 10 Hide
    Tamz_msc , August 5, 2010 6:48 AM
    Very nice article.It cleared a lot of confusion in my case.I would have liked to see scaling with increase in the number of CPU cores.
  • 6 Hide
    ruffopurititiwang , August 5, 2010 6:48 AM
    Great article! Now we know what the deal is regarding multi-gpu and PCI-E lane speeds with mainstream setups.

    Now how about showing some love for top of the line setups? Can you apply the same tests to GTX 480/ATI 5870 Multi-gpu configuration + X58 mobo & 980X proc?
  • -7 Hide
    rohitbaran , August 5, 2010 7:30 AM
    On page 7, performance of Radeons in Alien vs Predator is not arranged properly with CPU speed.
  • 7 Hide
    lothdk , August 5, 2010 7:43 AM
    rohitbaranOn page 7, performance of Radeons in Alien vs Predator is not arranged properly with CPU speed.


    All the tests are arranged by GPU performance not by CPU speed.
  • 5 Hide
    TheStealthyOne , August 5, 2010 7:46 AM
    I'll definitely be using this article as a reference for a dual 5850 machine I'm building :) 
  • 20 Hide
    one-shot , August 5, 2010 8:44 AM
    Avatar? I didn't know people played that game.
  • -7 Hide
    hixbot , August 5, 2010 12:34 PM
    In every single TOMS article showing benches on CPU scaling, you use High resolutions with FULL AA and aniso! WHY?!

    I know thats how many would play the game, and it shows the performance that one would expect with different CPUs. It shows that we don't need to overclock or buy expensive CPUs, when the load (bottleneck) is on the GPU.
    BUT if you would also show the CPU scaling in low resolutions and no AA/aniso, we could see how the CPUs might stack up. This is valuable for knowing how the CPUs perform in the future, when one might have upgraded the GPUs, and the bottleneck dissapears.

    Yeah, yeah, this is not a CPU article... i know but still.. it shows CPU scaling, and every time I see CPU scaling demonstrated on toms, it's shown with a bottleneck on the GPU.

    Humour us next time and include a bench or two with no GPU bottlneck (low res, eye candy). Just becaause that's not how we'd play the game, and just because the FPS would be insanely high, doesn't mean the numbers wouldn't be valuable. I complain about this in every article that includes CPU scaling.
    It doesn't erk me that much here because this article isn't focused on CPUs. but for the love god, the next CPU article you do, when you test gaming, throw a bench or two in there with very low settings.
  • 3 Hide
    tolham , August 5, 2010 1:46 PM
    "The compact design looks great on a single card, but when putting several cards together as a CrossFire or SLI configuration, the closely spaced PCIe slots barely give the cards any room for sucking in cool air. "

    i've wondered about this. with graphics cards getting huge and sli/crossfire becoming more prevalent, and CPU heatsinks getting huge and nearly touching RAM sticks, it seems that the motherboard needs to be redesigned with more space between PCI slots and a buffer area around the CPU socket.
  • 5 Hide
    Sorax , August 5, 2010 1:53 PM
    @hixbot I don't think it irks me as much as you but I agree that a CPU benchmark would be useful. It demonstrates the limitations of the platform and defines an upper limit. I hope to see these metrics in future articles.

    Good article by the way.
  • -4 Hide
    cknobman , August 5, 2010 2:12 PM
    Just asking: Why compare crossfire of ATI current gen top of the line against Nvidia SLI last gen cards??? Especially when you have already done several SLI benching in other articles on the new Nvidia gen cards?

    Also the point of SLI is super high res so why are you benching at 1080p? A single graphics card can handle that resolution nowadays.
  • 1 Hide
    wolfram23 , August 5, 2010 2:21 PM
    Decent article, but I'm looking forward to the one on PCIe bandwidth that will be out shortly.
  • 2 Hide
    hixbot , August 5, 2010 2:42 PM
    Since this is a multi GPU article, it would be nice to see a blurb about microstuttering. The issue has still not been 100% resolved.
  • 4 Hide
    ta152h , August 5, 2010 2:51 PM
    I'm curious how the x8 and x16 was tested. How could you possibly test x16 and x16 on a motherboard that couldn't support it (since you said you had a P55 test system). Was the x16/x16 done on a different motherboard from the x8/x8? If so, this invalidates the claim, since it's a different chipset and processor, and while it could be related to the number of lanes, it could also be related to differences in the processors and chipset, so is less than conclusive.
  • -3 Hide
    mindless728 , August 5, 2010 3:21 PM
    Quote:
    In every single TOMS article showing benches on CPU scaling, you use High resolutions with FULL AA and aniso! WHY?!

    I know thats how many would play the game, and it shows the performance that one would expect with different CPUs. It shows that we don't need to overclock or buy expensive CPUs, when the load (bottleneck) is on the GPU.
    BUT if you would also show the CPU scaling in low resolutions and no AA/aniso, we could see how the CPUs might stack up. This is valuable for knowing how the CPUs perform in the future, when one might have upgraded the GPUs, and the bottleneck dissapears.

    Yeah, yeah, this is not a CPU article... i know but still.. it shows CPU scaling, and every time I see CPU scaling demonstrated on toms, it's shown with a bottleneck on the GPU.

    Humour us next time and include a bench or two with no GPU bottlneck (low res, eye candy). Just becaause that's not how we'd play the game, and just because the FPS would be insanely high, doesn't mean the numbers wouldn't be valuable. I complain about this in every article that includes CPU scaling.
    It doesn't erk me that much here because this article isn't focused on CPUs. but for the love god, the next CPU article you do, when you test gaming, throw a bench or two in there with very low settings.


    if you didn't notice, they were all the same cpu at different speeds, and they were just showing what speeds are needed without having a cpu bottleneck
  • -1 Hide
    silversurfernhs , August 5, 2010 3:49 PM
    perfect timing - thank you, i just ordered my second 480
  • 0 Hide
    coldmast , August 5, 2010 3:56 PM
    Superb article!

    I still would like to compare x16 bottlenecks on the ASUS ARES with two 5870s running both on x8 and x16.

    Also it would be nice to compare the 5870+5830 vs 5850+5850.
  • 0 Hide
    jomofro39 , August 5, 2010 3:57 PM
    I enjoyed the article, good to know my CPU isn't capable of bottlenecking my possible sli setup. Hurrrayyyyyy...
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