Multi-GPU solutions with StarCraft II
SC2 is a little different than the other games. We recorded frame times with Fraps, but we played back a pre-recoded demo of a multiplayer match on each card rather than playing ourselves. We captured 33 minutes of play time for each demo, so we didn't bother with five runs per card. Since 33 minutes of frame time data is nearly impossible to reproduce on a graph, we've included just 6500 frames in the plots below. Some cards produced as many as 140,000 frames, so be aware that we're looking at a small snippet of the total data plotted below.
Well, that was unexpected. Look at the results for the fastest single-GPU cards, like the Radeon HD 6970 and the GeForce GTX 580. Why do those appear to have a bunch of jitter in them? Hmm.
Yep, both the GTX 580 and 6970 have quite a bit of variance over the span of three to four frames. The stair-step pattern on the GTX 580 is very regular. The pattern on the 6970 is a little different, but similar. I'm hesitant even to hazard a guess about what's going on here. It could have something to do with triple-buffering, the way the game engine's main timing loop works, or some interaction between the two. I suppose this is evidence that delivering consistent frame times isn't just a challenge for SLI and CrossFireX setups.
The SLI configs don't appear to be affected by the same brand of variance we saw in the single-GPU GTX 580 results. Instead, they show a small to moderate amount of garden-variety multi-GPU jitter. The CrossFireX configs, though, have that same regular three- to four-frame bounce we saw from a single 6970. I wish I could tell you more about what's happening here, but it's tough to say.