Micro-Stuttering And GPU Scaling In CrossFire And SLI

Micro-Stuttering: So Subtle, Yet So Annoying

What Is Micro-Stuttering?

Let’s recap how successive frames are generated. Since the ideal "perfect balance" isn't achievable in the real world and the system cannot render, say, 30 frames in advance, the following scenario is typical:

What we have here is a close-up of one second of Metro 2033 game play, rendered on a pair of Radeon HD 6850 GPUs. We chose this title and hardware combination because it yields a frame rate very close to 30 FPS. That's where the micro-stuttering phenomenon, if it exists at all, is rumored to be most aggravating.

Looking at the diagram, we immediately spot the dramatic variation in the number of milliseconds it takes to render each frame. The pie chart shows all 30 frames within the one-second period that we analyzed. It's easy to notice the delay of some frames, which is, in turn, perceived as stuttering. And of course it doesn’t help when the next frame catches up to help keep the average frame rate high enough.

You end up with what feels like a stuttering engine. Yes, it's going 30 MPH just like a smooth inline-six. But this one hits the same speed and feels like it has one cylinder out of whack. .

When Can Micro-Stuttering Be Seen?

In a nutshell, all of the time. The lower the average frame rate, the more the frame rate is perceived as being lower than the actual average frame rate. Thus, as bad luck would have it, a frame rate of 30 FPS may be perceived as merely 20 to 25 FPS. The human eye does, however, still notice differences in when frames show up on-screen beyond 60 FPS.

This is one of the reasons why we prefer testing with higher frame rates in the GPU scaling tests on the following pages. It continues to amaze us how, even beyond the generally-accepted target of 40 FPS, you can still see the impact of micro-stuttering once rendering becomes imbalanced.

  • thorkle
    This is a very interesting article, I have wondered about this issue myself many times in the past. I was always curious why I would see strange lag like anomalies while still achieving perfect frame rate. Bravo Toms
  • compton
    Well, I'm a little surprised that three cards in Crossfire seem to eliminate visible microstuttering -- I would have guessed that triple cards would increase stuttering. But it also seems like there must be other factors at work. Unfortunately, there really isn't a good way to test for other factor -- if you even could know what to test for. In some circumstances, it seems like my monitor is causing some issues. If I play a game (lets use Fallout New Vegas for example) at a Synced 60FPS, you can look at FPS and it never deviates. It only uses 1/3 of my GPU cycles. But on one monitor, at the same resolution, it micro stutters. On another monitor, it looks perfectly fine. I thought it was some lag variance -- but then I've been told lag is always constant, that the reason lag varies in monitor testing is improper test methods. What ever the reason, it's actually really annoying. And I'm not anything approaching a competitive FPS player. Thanks for helping to track this issue down.

  • iam2thecrowe
    so will you now change your best gpu for the money from 2 x 6850's, since they obviously suck. I already bought one 6850 thinking it would be great to crossfire later and that was the best choice according to you toms........now i will have to throw it in the bin come upgrade time and buy a better single card. Oh, and AMD/Nvidia, if you cant get dual card configs to work properly, don't offer them, your wasting our money. Please fix this microstuttering crap, im sure it would be possible with a driver tweak.
  • 1kbuild
    What happens with Vsync turned on?
  • pirateboy
    why wasn't hybrid crossfire mentioned in this article?
  • bombat1994
    i think the MSI Radeon 6870 Hawk is the best graphics card on the market right now in terms of heat, power, performance and price
  • tmk221
    in most games you can limit maximum frame rates. So maybe if you limit max FR to let say 10 to 15 above min FR then the Micro-Stuttering effect would be gone ? anyone tried this? please share
  • shoelessinsight
    What is performance like using other load-balancing methods, like the split frame rendering that SLI originally used, or ATI's Scissor mode? Are these modes still available to those that choose them?

    Obviously, they won't reach frame rates as high as those attained through AFR, but if the frame rate loss is small enough, those modes might still be justifiable if they eliminate micro-stuttering altogether.

    I'd be curious if these alternate methods could justify the cost of an additional card through added performance without coming with the drawback of micro-stuttering.
  • boletus
    Regarding the decreased stuttering with 3 or 4 cards: could this be a similar effect to superimposing sine waves? Two waves a half cycle apart show visible peaks and valleys, while three waves at evenly staggered cycles form a much smoother band (on a graph or a scope).
  • haplo602
    I ma confused ... you are using 2 identical cards, so the frame rendering times as show on the metro 2033 second would be THE SAME on a single card as on a dual card configuration. the only difference is when each card starts to render right ?