Multi-GPU Setups: The Basics Of CrossFire And SLI

Connectors

Each card has a number of physical connections determining the number of cards that can be coupled. The CrossFire and SLI solutions are pretty much identical in this aspect.

While the current Nvidia drivers allow for SLI to work without the bridge connector, this puts some additional stress on the PCIe bus. If the cards have one connection each, two cards can be connected. If they have two, three or four cards can be teamed up using a three-way bridge connector or a four-way bridge connector.

The advantages of connecting cards with bridges have to be considered separately for AMD and Nvidia. You could easily leave the bridge connector off when using mainstream graphics cards, since the PCI Express bus is quick enough for the data traveling over it. This is only an option for certain AMD-based configurations, from a pair of Radeon HD 4650s down to two Radeon HD 4350s. Dip down into the Radeon HD 3000-series or up to Radeon HD 4670s, and you'll need to use bridge connectors.

If the graphics card has two CrossFire bridge connections, it doesn't matter if you use the left or the right one. Nor are there any advantages to using multiple bridge connectors between the cards just because you can. One is enough to synchronize the data stream and make the driver recognize the CrossFire configuration.

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    Top Comments
  • Maziar
    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.
    25
  • ramcrazy360
    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.
    21
  • one-shot
    Avatar? I didn't know people played that game.
    20
  • Other Comments
  • ramcrazy360
    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.
    21
  • Maziar
    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.
    25
  • wildeast
    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 :)
    16
  • Tamz_msc
    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.
    10
  • ruffopurititiwang
    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?
    6
  • rohitbaran
    On page 7, performance of Radeons in Alien vs Predator is not arranged properly with CPU speed.
    -7
  • lothdk
    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.
    7
  • TheStealthyOne
    I'll definitely be using this article as a reference for a dual 5850 machine I'm building :)
    5
  • one-shot
    Avatar? I didn't know people played that game.
    20
  • hixbot
    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.
    -7
  • tolham
    "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.
    3
  • Sorax
    @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.
    5
  • cknobman
    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.
    -4
  • wolfram23
    Decent article, but I'm looking forward to the one on PCIe bandwidth that will be out shortly.
    1
  • hixbot
    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.
    2
  • ta152h
    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.
    4
  • mindless728
    Anonymous said:
    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
    -3
  • silversurfernhs
    perfect timing - thank you, i just ordered my second 480
    -1
  • coldmast
    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
  • jomofro39
    I enjoyed the article, good to know my CPU isn't capable of bottlenecking my possible sli setup. Hurrrayyyyyy...
    0