Multi-GPU Setups: The Basics Of CrossFire And SLI

The Difference Between PCIe x8 And x16

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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  • Redsnake77
    Good article. Totally agree with difference in ease of setup having gone from a pair 8800GTX OC2's in SLI to 5850's in crossfire, I miss my 8800GTX monsters they were great cards. Also agree about processor speed. I'd noticed in a few games (some are more noticeable than others) the framerate would slowly decrease then spring back up again, it was bugging me hugely. It was most noticeable in Dirt and Grid, two games I hadn't played since my old E6850 with the 8800's. Ran a fraps benchmark and you could see it cyclicly droping down through numbers 120, 118, 114, 110, down to 94fps, then jumping back upto 120+ and decreasing again. I ran Core Temp and straight away I could see the multiplier starting at 16 then dropping increntally down to 12 then jumping back up to 16. So I kept the voltages all stock and increased the baseclock to 160MHz, Loaded up the game, the multiplier now stays at 12, but the framerate is stable at 120-124fps with a processor speed of just under 2GHz. But at stock clocks why wouldn't the multiplier not just stay at 16?
  • Redsnake77
    This is on an i7 930, GA-EX58-UD5, 6GB 1600MHz DDR3, 2 Sapphire 5850 Toxic GPUs, and Win 7 64bit.
  • nesters
    ATI doesn't have terrible drivers, nVidia does.

    Well, ATI has some problems with Crossfire but remember that you can mix up different cards but with SLI you can only use the same card from same manufacturer. This might explain why drivers fail with Crossfire setup.
  • redkachina
    cmon what's with all these spam?..
  • Redsnake77
    I know, it's getting stupid, and the mods don't seem to be doing anything about it.