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


The big question today was whether or not we needed more than 16 lanes to feed multiple high-end graphics card in SLI, and the answer is a solid “perhaps not.”

OK, let’s call it a conditional "no.”

While we did see a fairly large difference between x8 and x16 slots when a single card was used, adding a second card shifted our limit to CPU performance. That is to say, for most of today’s tests, a faster CPU would be far more important than dual x16 slots in achieving the ultimate SLI performance.

That answer presents its own set of questions, since our high-flying Core i7 CPU was already pushed to 4.00 GHz. Most builders simply can’t go much higher with a daily-use gaming machine.

Another part of that conditional answer pertains to test resolution. GPU dependence increases with resolution, to the point that two cards eventually become a “bottleneck” far tighter than the CPU. Yet, that level of GPU dependence outweighs even PCIe x8 bottlenecks.

In the end, we simply needed a faster CPU to apply everything we learned about single-card bandwidth to multi-GPU configurations. This finding should come as some comfort to owners of “high-end” P55-based systems who might be considering an SLI upgrade for their GeForce GTX 480 graphics cards. If you have a high-performance processor in that motherboard, and you're overclocking to 4 GHz+, the extra CPU horsepower will have a more profound impact than an upgrade to an X58-based machine. Making X58 truly worthwhile requires an even faster processor and resolutions beyond 2560x1600.

Thomas Soderstrom
Thomas Soderstrom is a Senior Staff Editor at Tom's Hardware US. He tests and reviews cases, cooling, memory and motherboards.
  • Mousemonkey
    There are quite a few folk who have been waiting for this.
  • zorky9
    This would even add more value to the i5-750 in your best CPU for the money article.
  • amk09
    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 :)
  • carlhenry
    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.
  • sambadagio
    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... ;-)

  • luke904
    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.
  • jgv115
    @ 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.
  • outlw6669
    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.
  • barmaley
    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.
  • Aionism
    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.