Micro-Stuttering And GPU Scaling In CrossFire And SLI

Step 3: CrossFire With Four GPUs (Vs. SLI)

Two Radeon HD 6870 X2s Give Us Four-Way CrossFire

Now we're pulling out all of the stop and harnessing four mid-range GPUs together for moderate excess. Paying PowerColor's premium on a dual-GPU board twice means you drop somewhere around $940 bucks for the privilege of rocking a highly-parallelized graphics subsystem. Let’s compare the three- and four-way CrossFire setups first:

Because that X2 costs so much more, you're forced to pay dearly for a 15 to 20% improvement over the three-way arrangement. Four-way CrossFire simply doesn't make sense given the Barts GPU and resolutions of 1920x1080. We were already seeing more than 150 FPS, after all.

Ratcheting the resolution up to bring performance down would require more on-board memory. One gigabyte per card really isn't enough for a single 30" screen at 2560x1600 or a trio of 24" displays at 5760x1080. At that point, you'd need to consider Radeon HD 6950 2 GB boards, all lined up.

CrossFire Versus SLI

We benchmarked the GeForce GTX 590 and Radeon HD 6990 anyway, just to see how they'd measure up. We would have liked to measure the performance of two GeForce GTX 590s, but we weren't able to find a second card that'd run reliably in SLI mode. We'll talk about that on the next page.

Here are all the graphs for comparison: 

Interim Assessment

It's amazing how easily Nvidia's GeForce GTX 590 is vanquished by what many folks would consider a trio of value-oriented Radeon cards. Higher frame rates and minimal micro-stuttering surprised and excited us at the same time. It almost makes sense why AMD doesn't include a second CrossFire connector on these cards now; doing so would decimate any reason to spend extra on the company's highest-end (and highest-margin) cards.

We have to curb our enthusiasm a little bit, though. The drawback of AMD's setup is, almost always, inconsistent driver performance. Three-way CrossFire is very compelling in most games and benchmarks. However, driver issues reduce its performance to the level of a dual-GPU setup in a few applications. In those cases, the only remaining benefit of three-way CrossFire is better frame rate consistency in each of the tests we ran (admittedly, no small gain).

The Radeon HD 6990 is barely faster than a Radeon HD 6870 X2. All else considered, its only advantage is less micro-stuttering than the two-chip solutions. In turn, it gets destroyed by the three-way setup.

Now we can have a look at the rest of the benchmark results, bearing in mind the configurations that best minimize the micro-stuttering issue.