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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.

Display all 108 comments.
Top Comments
  • 28 Hide
    1kbuild , August 22, 2011 5:40 AM
    What happens with Vsync turned on?
  • 13 Hide
    shoelessinsight , August 22, 2011 7:55 AM
    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.
  • 11 Hide
    compton , August 22, 2011 5:24 AM
    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.



Other Comments
  • 9 Hide
    thorkle , August 22, 2011 5:16 AM
    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
  • 11 Hide
    compton , August 22, 2011 5:24 AM
    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.



  • 28 Hide
    1kbuild , August 22, 2011 5:40 AM
    What happens with Vsync turned on?
  • 6 Hide
    pirateboy , August 22, 2011 7:00 AM
    why wasn't hybrid crossfire mentioned in this article?
  • -9 Hide
    bombat1994 , August 22, 2011 7:20 AM
    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
  • 8 Hide
    tmk221 , August 22, 2011 7:47 AM
    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
  • 13 Hide
    shoelessinsight , August 22, 2011 7:55 AM
    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.
  • 7 Hide
    boletus , August 22, 2011 8:11 AM
    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).
  • -1 Hide
    haplo602 , August 22, 2011 8:18 AM
    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 ?
  • 5 Hide
    cmcghee358 , August 22, 2011 8:18 AM
    Good question 1kbuild
  • 0 Hide
    Haserath , August 22, 2011 8:18 AM
    Could a frame be analyzed and split into certain pieces for each GPU to process? It would make each frame much more consistent if the load for each GPU was just about equal per frame even if there was overhead for splitting the load up and then having to transfer it to the first cards buffer.
  • -3 Hide
    damric , August 22, 2011 8:47 AM
    Disable ULPS you noobs.
  • -2 Hide
    SpadeM , August 22, 2011 8:50 AM
    I guess buying 3dfx was the way to go for Nvidia. If all midrange cards are equipped with only one link then I assume that we won't get to see exotic combinations of 3, 4 cards in the next SBM.
  • -3 Hide
    Samy0806 , August 22, 2011 9:38 AM
    Nice article, it was very usefull. BUT why Three-Way, and Quad SLI configurations aren't benchmarked ?
    AND i saw that Lucidlogix makes things worse on Radeon HD 6870 X2, what about the Lucidlocix Virtu, integrated in many motherboards, does it affects performance of your graphics card, and if it does how?
  • -2 Hide
    silverblue , August 22, 2011 10:55 AM
    Page 9, first Ungine Sanctuary set of results, the 590 is displayed using AMD Crossfire colours.
  • 2 Hide
    RazberyBandit , August 22, 2011 11:08 AM
    One reason single-GPU Nvidia boards may trail AMD boards in Mafia II is because Mafia II supports PhysX. With PhysX enabled, a single board can sometimes struggle.

    Based on the summary conclusion, does this mean Tom's has firmly gone back to recommending that users purchase "the most powerful single-GPU board you can afford" again?
  • 0 Hide
    nforce4max , August 22, 2011 11:34 AM
    shoelessinsightWhat 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.


    SFR was really good back in the day and it was enjoyable as well stable. Also it scaled well for two way setups but Nvidia killed it off because of quad and tri was becoming popular back in 08. ATI has other rendering modes as well such as tile but they went the afr rout as well. The best days of sli and crossfire are over but one can still try to optimize their way out of some micro shuttering.
  • 4 Hide
    BrightCandle , August 22, 2011 12:30 PM
    One of the members of XtremeSystems has come up with a program that analyses the amount of variance from the average framerate from a fraps frame time file. Have a look at http://www.xtremesystems.org/forums/showthread.php?258433-Microstutter-in-latest-gen-cards-examples-included&highlight=microstutter for a link and some details on how to use it and results.

    Toms could adopt this tool and use it to show the amount of MicroStutter along with their benchmark results. Many other sites like to show minimum fps in their graphs and I think showing the bottom 5% of frame times would be another way to show this problem up and compare the cards in your reviews.
  • -7 Hide
    greenrider02 , August 22, 2011 1:10 PM
    A great article, but of course a few stickler points:
    -It's not about Bulldozer
    -I would like to read some more detailed observations of problems you were seeing with crashing and driver issues
    -A suggestion in the conclusion about what YOU would do with your own setups, as many of us want to know what the pros think
    -It's not about Bulldozer
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