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CrossFire, SLI, And Micro-Stuttering

Micro-Stuttering And GPU Scaling In CrossFire And SLI
By , Greg Ryder

Drivers or Driven Ones?

Getting the best possible results often comes down to driver installation. What is usually a fairly routine process when you use one GPU on its own can quickly turn into a test of patience when multiple cards are involved.

For example, it is rather difficult to get two Nvidia cards obtained from different vendors, which deviate from the reference specification, to cooperate in SLI. From a pool of six different GeForce GTX 580 cards, only two would work together consistently. From a pool of five different GeForce GTX 570s, only three would. We had problems mixing clock rates, BIOSes, supporting platforms, and so on.

AMD is one step ahead in the interoperability game because it only places priority on the GPU you're using. But that advantage is lost when you try to get two dual-GPU cards working together in four-way CrossFire, which led to crashes several times for us. Also, CrossFire profiles are not user-accessible; they cannot be configured or assigned freely. If you do decide to run multiple GPUs, be prepared to spend extra time battling the idiosyncrasies of more complex graphics configurations.

CrossFire With Two Cards

If you're only judging based on average frame rates, two cards seems like a great deal for the price. We've had several readers write in, though, complaining about this micro-stuttering issue, which simply cannot be seen in the context of normal benchmarks.

Even at frame rates above 50 FPS, micro-stuttering rears its ugly head, pronounced enough to significantly detract from the gaming experience. A paradigm shift seems necessary, at least until both AMD and Nvidia are able to prevent or mask the artifact. Right now, if you asked us whether it'd be smart to "go cheap" on an inexpensive card and double-down later with another one, we'd have to suggest against it if you're the sort to be bothered by micro-stuttering. The improvement in performance would be negated by the phenomenon's impact. Currently, it seems like cards less powerful than the Radeon HD 6950 are not well-suited for dual-card CrossFire. Even if the frame rates look decent, the slower the GPU, the more pronounced you'll see micro-stuttering during gameplay.

At the same time, not everyone is equally sensitive to time-skewed frame sequences, and quite a few cheap TFT LCD displays help hide the effect. Even so, AMD has a major undertaking ahead of it in order to really improve the dual-card experience.

Three- And Four-Way CrossFire

The performance of three- and four-way CrossFire setups not only surprised us, but also managed to utterly convince us that micro-stuttering doesn't have to affect your multi-GPU configuration. For some reason, the third GPU almost always eliminates micro stuttering and has a less-pronounced effect on performance. At least in the games we ran, the fourth GPU is more eye candy than anything, and its contribution is minimal.

If AMD considered putting two CrossFire connectors on its Radeon HD 6870, along with adding some more memory, a trio of them would absolutely be a price/performance leader with minimal micro-stuttering. Currently, the only way to achieve this is by going off the beaten path with PowerColor's Radeon HD 6870 X2 with an extra 6870 on the side. You'd end up paying The cost for this setup, $645, is significantly less than what you'd pay for a GeForce GTX 590. Nvidia's flagship would lose a head-to-head performance battle and demonstrate worse micro-stuttering.

We wish that AMD put more emphasis on balancing its dual-GPU setups. Despite a potential impact on average frame rates, some improvement to playability should be achievable while still demonstrating overall faster performance. The three-way setup shows how this should be done.

SLI

Nvidia's approach is much more consistent, though its own SLI technology doesn't side-step micro-stuttering entirely. At least the company's high-end cards work together the way we'd expect. However, sporadic micro-stuttering brings us back to reality time and again. Cards like the GeForce GTX 550 Ti seem to be wasted on a SLI setup for this reason alone, achieving decent frame rates in the charts and nasty micro-stuttering in the real world.

Closing Thoughts

Frankly, there haven't been any revolutionary developments in the fields of frame rate consistency and micro-stuttering, even though we have seen improvements from Nvidia's drivers. At this point, neither competitors can claim to deliver a 100% stutter-free gaming experience with two GPUs working cooperatively.

Bearing in mind that vendors purposely try to price two mid-range cards similarly to a faster single-GPU board wherever possible (generally, when the competitive landscape allows for it), we’d have to pick the single-GPU card every time. The three-way setup based on a trio of mid-range cards is the pièce de résistance. But AMD and Nvidia also know this, and purposely handicap their less-expensive boards with just one bridge, limiting configurations to two boards. The way around this, PowerColor's Radeon HD 6870 X2 bears its own significant price premium. It's also not a quiet board, and it requires a bit of faith on your part to trust that CrossFire profiles will continue to incorporate support.

We learned one other thing from our experimentation: the faster the linked cards are, the less you see side effects from teaming them up. This precludes using two low-end cards. CrossFire and SLI only make sense from the mid-range and higher, with a slight advantage for SLI. That makes both technologies a lot less interesting for upgraders and bargain hunters. Again, given comparable pricing, we'll take the single-GPU card any day. And even then, not having to worry about micro-stuttering would compel us to pay a little more.

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