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Micro-Stuttering And GPU Scaling In CrossFire And SLI

Micro-Stuttering And GPU Scaling In CrossFire And SLI
By , Greg Ryder

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.

Single GPU or multi-card setup? That's a question we face every time we start a System Builder Marathon series or evaluate the worth of a flagship GPU. 

Do you want a just one high-strung racehorse or a pair, trio, or quartet of draft horses? Can a team of inexpensive cards perform the work of a pricier one and still come in at a lower cumulative cost?

Welcome to Groundhog Day. Due to recurring forum questions and direct requests by our readers, we decided that it's time to go beyond the usual performance-oriented benchmarks of CrossFire- and SLI-based systems, and shed some light on the underlying principles. Frame rates in and of themselves do not necessarily translate into a high-quality experience.

Yes, we're going to tackle the issue of micro-stuttering, which seems to keep so many sensitive eyes from investing in multiple cards running in concert. We'll also look at the scaling of two, three, and four GPUs. Where is the benefit? And at what point is actual added value really realized, or is churning out high (but ultimately useless) frame rates a self-defeating exercise? As we're sure you can imagine, at some point, the pursuit of performance can become a money pit and a power hog. At the same time, we've seen multi-card setups yield incredible value.

What Has Changed?

When it comes to measuring the scaling of mid-range cards, we run into a little problem. AMD and Nvidia try to limit the amount of expansion you can achieve with those less expensive boards by only exposing one bridge connector, limiting us to two-card configurations. Conversely, cards with two bridge connectors (enabling three- and four-way arrangements) are only found at the high end of the price spectrum. 

For most folks, the prospect of anything beyond two high-end cards simply gets too expensive. Enter PowerColor's Radeon HD 6870 X2. It features two mid-range GPUs on a single PCB, consequently allowing us to set up and test four-way CrossFire with mid-range chips. As a result, we have the opportunity to compare a typical mid-range card like the Radeon HD 6870 (a £160 card) in a two-way setup to a single Radeon HD 6870 X2, and then add a three-way setup (Radeon HD 6870 X2 + 6870) and the four-way results.

Additionally, we compare those three configurations to a number of current cards, right up to the GeForce GTX 590, in a comprehensive set of benchmarks. Our goal is to evaluate performance, assess the seriousness of micro-stuttering, and finally rank the cards based on raw performance and their price/performance ratios. Are we due for a surprise? Nothing can be ruled out; we invite you to read on!

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  • 3 Hide
    Stupido , 22 August 2011 15:58
    Just an idea: maybe stuttering can be described as standard deviation of the mean frames per second?

    In this way you can have some quantitative measure of the stuttering...
  • 1 Hide
    aje21 , 22 August 2011 18:01
    Will future SBM competitions take this into account? Multi-card setups seem to be very common. Perhaps the frame rate reported should be based on the low-point seen with micro-stuttering?
  • 1 Hide
    Anonymous , 22 August 2011 18:01
    Great work , this has cleared up alot of questions i had regarding micro stuttering , TBH i think im just gonna buy a gtx 580
    (was thinking of duel ) but after seeing this artical i have changed my mind, your frame rate is only as good as it's lowest point . Ave/top frame rate realy dont matter if you drop from 60 fps to 20fps and stutter. Thanx to all at toms.
  • 0 Hide
    technogiant , 22 August 2011 23:51
    Nice article....uhhmm...me wonders if the lower power consumption/ heat production of the next gen 28nm gpu's will allow for the production of X3 triple mid range gpu cards with a nice large lump of GDDR ram to produce their flagship rather than a microstutery X2 card?
  • 0 Hide
    pantsu , 24 August 2011 05:08
    Microstuttering is also heavily game dependant, some engines stutter more than others. The selection of games in the article is limited so I wouldn't take it at face value. Still, in the games tested it seems like Nvidia does a better job of reducing stutter. I'm personally considering buying a second 6950, but I might end up selling it if the stuttering is too noticeable.

    A good way to measure microstuttering is to use Fraps and its frametime log. You can create a graph from the values that shows the frame speeds for every single frame instead of one second avarages that mask the stuttering in normal fps graphs. ( = 1000ms/frametime-previousframetime)
  • 2 Hide
    alangeering , 24 August 2011 06:19
    I'd like to first thank Igor and Greg for a very insightful article and for

    discussing the not often talked about phenomenon of stuttering.

    There's one thing I'd like to expand upon.

    A few times in the article the observation is made that while dual GPU scaling is

    good, the stuttering effect is bad.
    No real point is made that when scaling is poor, stuttering is less pronounced.

    It's precisely because three cards aren’t as efficient that stuttering is reduced.

    Bear with me and I'll explain.


    For the following thought experiment I've used the data from the Call of Juarez

    graph on the page called "Step 2: Crossfire with three GPUs"

    Three situations:
    A: 1 card @ 70 fps average
    B: 2 cards @ 135 fps average
    c: 3 cards @ 160 fps average

    In other words:
    A: The card takes an average of 14.3 ms to produce the frame.
    B: Each card has 14.8 ms to produce the frame to maintain the average.
    C: Each card has 18.8 ms to produce the frame to maintain the average.

    Look again at the data from Call of Juarez.
    The lowest frame rate recorded for the single card is 60fps or 16.7 ms per frame.

    This is well below the 14.8 ms required to not delay/stutter the pipeline in

    situation B but...
    This is well within the 18.8 ms time frame for the 3 card set up in situation C.

    As frames are now arriving in time for use, the evidence of stuttering is reduced.

    So efficiency is good; but inefficiency in scaling allows each card a little

    longer to provide its frame, and the eventual combined frame rate is less

    variable.

    A quote from the article:
    "This phenomenon manifests itself even more seriously in CoJ. While CrossFire

    scales well under load, it becomes even more susceptible to micro-stuttering."

    And another:
    "For some reason, the third GPU almost always eliminates micro stuttering and has

    a less-pronounced effect on performance."

    You got so close; it just needed another jump of statistical thinking. Efficiency

    correlates with stuttering (NVIDIA and AMD) and there is a logical reason why.
  • 0 Hide
    alangeering , 24 August 2011 06:37
    The above post isn't trying to explain why microstuttering occurs - only why it's more pronounced as multi-gpu scaling increases. (and less so as scaling efficiency decreases)
  • 1 Hide
    technogiant , 24 August 2011 15:27
    @alangeering......that's an interesting theory, following it through to its logical conclusion if you were to artificially cap the maximum frame rate below that which the graphics setup is capable of producing then this should also reduce stuttering...does imposing vsync of say 60Hz on a system capable of producing say 80fps reduce stuttering?
    Is it possible to impose various frame rate caps by editing game.ini files to test this?
  • 1 Hide
    alangeering , 25 August 2011 03:30
    @ technogiant
    This is exactly why some people have had success in using V-sync to hide the effect.

    In the case of this benchmark set-up: if you could reduce the frame rate to around 18 ms per frame per card for two cards then you should see the same smoothness as was achieved with three cards.

    Unfortunatly this means you get a 'smooth' frame rate of 111 fps - which doesn't win any benchmarks. (It's a lot less than 135 fps).

    If we were to reevaluate crossfire/sli setups based on a frame rate with acceptable microstuttering it would not nearly look as good for a 2-way setup vs a single card.
  • 0 Hide
    chronicbint , 25 August 2011 06:33
    Well this has just put me off going for my first crossfire setup.....
  • 0 Hide
    picture_perfect , 28 August 2011 22:00
    i appreciate you guys bringing attention to this problem, since i can't stand judder. i'd like to know how v-sync worked.

    comment; since microstuttering is most obvious at low fps, dual gpu systems still seem to have communication problems. i avoided sli/cf in my last build a couple years ago because of this (and the cost/efficiency thing). looks like i'll be doing the same thing again this year. thanks for the heads up.
  • 0 Hide
    Anonymous , 3 September 2011 01:04
    Just one question: why no review for 3-way/4-way SLI setups?
  • 0 Hide
    Gublo , 15 October 2011 21:15
    Thanks for the great article! I was on the 580/590 track for my new build but this article got me very interested in the 3x 6870. How would they handle being put in sandwich crossfire? Would it be to hot? And wouldn't that 1 gb vram be a problem with future (or Battlefield 3) AA and post efx requirements?
  • 0 Hide
    markdj , 20 March 2012 05:18
    Quote:
    Just an idea: maybe stuttering can be described as standard deviation of the mean frames per second?

    In this way you can have some quantitative measure of the stuttering...



    I don't think the standard deviation of the usual frames per second measurements would give a good representation since the stuttering is a result of the variation on much lower than the second level.

    If you took the standard deviation of the pie chart of metro 2033, which was the time in milliseconds for each frame to be rendered over a one second period, I believe that would give a much better representation of micro-stutter since it is looking at a much smaller increment of time in much more detail.

    Using this std dev along with subjective viewing of the games I think you could come up with a way to judge it numerically. Have multiple people all view various game footage and see where microstutter becomes noticeable, then looking at that standard deviation could give you a number to judge future benches.

    This of course would leave some subjectivity for finding that number, does anyone have an idea to remove that subjectivity?