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Micro-Stuttering And GPU Scaling In CrossFire And SLI

Features
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We've received many emails from readers asking about the phenomenon known as micro-stuttering and what it means to multi-GPU setups in CrossFire and SLI. After running plenty of benchmarks, we're ready to weigh in on what turns out to be a real issue.

Step 1: CrossFire With Two GPUs

One Radeon HD 6870 X2 Gives Us CrossFire With Two GPUs

Let’s revisit that PowerColor Radeon HD 6870 X2 for a moment. This card places two Barts GPUs on a single PCB and connects them through the use of a bridge chip from Lucidlogix.

The first graph gives us performance in Call of Juarez at 1920x1080 using the Radeon HD 6870 X2 board. It's easy to see that the average frame rate of the PowerColor card nearly hits two times one Radeon HD 6870 on its own. But we also see how choppy that graph is compared to the single-GPU setup rendering on its own.

While some of the peaks approach 200% of one GPU's performance, the frame rate drops dramatically in other spots, barely exceeding the speed of a single Radeon HD 6870. Gamers see the elevated average frame rate and assume a great experience, but then experience those valleys in the graph as brief (but perceptible) stuttering.

Two Radeon HD 6870s Also Give Us CrossFire With Two GPUs

Now, let's compare the behavior of the Radeon HD 6870 X2 with a pair of regular Radeon HD 6870s connected with a CrossFire bridge. That's a pair of boards totaling $360 facing off against a $470 dual-GPU card.

It turns out that the average frame rates are comparable. However, the setup without Lucidlogix's bridge chip demonstrates slightly smaller performance dips.

Despite the almost-negligible performance advantage and a more than $100 price difference, the inconsistent frame rate of two cards working cooperatively remains a potential issue for some folks.

With that said, it would only make sense to use PowerColor's card if you didn't have the four slots worth of expansion space to fit two Radeon HD 6870s. The Lucidlogix bridge's only advantage is one of convenience; it doesn't improve performance in any way.

108 Comments Comment from the forums
  • 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
    Reply
  • 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.



    Reply
  • 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.
    Reply
  • 1kbuild
    What happens with Vsync turned on?
    Reply
  • pirateboy
    why wasn't hybrid crossfire mentioned in this article?
    Reply
  • 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
    Reply
  • 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
    Reply
  • 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.
    Reply
  • 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).
    Reply
  • 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 ?
    Reply