AMD & Multi-GPU Stuttering: A Work In Progress

When it comes to stuttering there’s really two classes of stuttering that need to be discussed. The first is single-GPU stuttering as we’ve discussed in the previous pages, where driver issues, application issues, and the context buffer all interact to influence the pace for how frames are doled out before finally being rendered and presented. The second type of stuttering, micro-stuttering, is endemic to multi-GPU configurations. In micro-stuttering on top of all of the other issues encountered with single-GPU stuttering, there are also further variance and frame interval issues introduced due to how multi-GPU configurations split their workloads.

In brief, in multi-GPU setups, be it single-card products like the GTX 690 or multiple cards such as a pair of 7970s, the primary mode of splitting up work is a process called Alternate Frame Rendering (AFR). In AFR, rather than have multiple GPUs working on a single frame, each GPU gets its own frame. This method has over time proven to be the most reliable method, as attempting to split up a single frame over multiple GPUs (with their relatively awful interconnect) has proven to be unreliable and difficult to get working. AFR in contrast is by no means perfect and has to deal with inter-frame dependency issues – where the next frame relies in part on the previous frame – but this is still easier to implement and more consistent than previous efforts at splitting frames.

Moving on, due to the mechanisms of AFR, it can further impact the frame intervals and as a result whether stuttering is perceived. To do AFR well it’s necessary to pace the output of each GPU such that each GPU is delivering a rendered frame at as even a rate as possible; not too soon after the previous frame, and not too late such that the following frame comes up quickly. In a 2 GPU setup, which is going to be the most common, this means the second GPU needs to produce a finished frame when the first GPU is roughly half-way done with its current frame. Should this fail to happen then we have micro-stuttering.

Micro-stuttering has been a longstanding issue on multi-GPU setups. Both NVIDIA and AMD have worked on the issue to various degrees, but at the end of the day multi-GPU setups have never proven to be as reliable as single-GPU setups, which is why our editorial position on the matter has been to always favor single powerful GPUs over multiple GPUs when at all possible. To that end, just as FRAPS has ignited an interest in single-GPU stuttering issues, it has also ignited an interest in multi-GPU stuttering issues.

The bulk of AMD’s presentation – and consequently our own article – has been focused on single-GPU issues. AMD is working on multi-GPU issues too, but the team that is handling microstuttering is not the team we were speaking to. The team we were speaking to is the team that has been handling single-GPU issues. As such AMD’s statements on the matter are meaningful, but brief.

Multi-GPU stuttering has become an important issue for AMD just as single-GPU stuttering has, and AMD is working on a resolution for it. That resolution will come in or around a July driver drop, at which point AMD will introduce some new driver options to control how their cards deal with the issue. In the meantime however micro-stuttering and how AMD’s multi-GPU technology compares to NVIIDA’s multi-GPU technology is likely to become a bigger issue before AMD can push out their new driver. So AMD may be spending the next couple of months on the defensive.

AMD’s current position on micro-stuttering is that they are favoring latency (and not frame intervals) above all else. By keeping their latency low and as even as possible the resulting input lag from multi-GPU setups is reduced, making the experience more responsive for the user. This is a position that’s essentially in alignment with how they’re handling single-GPU stuttering too, but in the single-GPU world there isn’t a deliberate frame pacing aspect to take into consideration since they merely need to render frames as fast as they receive them.

In any case, AMD’s position on the issue has been one where they clearly still think they’re right, but also one where they’re going to lighten up on their position regardless. The alternative approach to favoring latency is to favor the frame interval, which in the case of multi-GPU setups means focusing on frame pacing. By deliberately delaying frames AMD can ensure they arrive more evenly, but in doing so they would increase the latency in the rendering pipeline, and ultimately the latency the user experiences. AMD already does this to some degree, but today it’s not being done explicitly and favored over latency concerns, which is what’s going to change.

In a typical AMD move, AMD will ultimately be leaving this up to the user. In their July driver AMD will be introducing a multi-GPU stuttering control that will let the user pick between an emphasis on latency, or an emphasis on frame pacing. The former of course being their current method, while the latter would be their new method to reduce micro-stuttering at the cost of latency.

We don’t have any more details on this driver or AMD’s frame pacing method at this time, but in our conversation with AMD they didn’t sound like they were worried about having any problems implementing explicit frame pacing, and that it was merely a matter of the process taking a while. Frame pacing itself can cause its own stutter issues – holding back one frame but not another can sometimes make the frame display evenly, but from a simulation step only a few milliseconds after the previous step – but ultimately the pacing process will cause the simulation to try to match the GPU and pace itself accordingly, so it would be a transient issue.

On a final note, more so than even single-GPU stuttering, multi-GPU stuttering is something that unfortunately FRAPS is poorly prepared for. By looking at Present calls it’s completely blind to how the GPU is doing any frame pacing, which means it’s currently difficult to see the impact of frame pacing short of a high-speed camera. As further tools are developed that let us analyze the end of the rendering chain, this will allow us to more properly analyze how frame pacing works and what its true impact on the user is.

AMD & Single-GPU Stuttering: Causes & Solutions Final Words
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  • mi1stormilst - Tuesday, March 26, 2013 - link

    All of us will benefit from the light shed on the subject with better testing and companies paying closer attention to issues and work arounds related to the subject. Still we would not even be talking about better testing methods right now without the attention it got from The Tech Report. I look forward to more sites implementing some type of real world testing methods that results in a true user experience evaluation. I reread the article and still standby my original conclusion. The Tech Report gets credit, but rather then stopping there this article seems to attack their methodology when they themselves had already admitted that it was less then perfect. To date there are still not better tools being used for reviews and The Tech Report still got the point across with what was available. I am a huge fan for what they did over there as I could not pinpoint why my AMD experience was less than optimal. It forced me to early retire my 6950 grab a very affordable 660 OC and enjoy a much smoother game experience. This is my first nVidia card since my trusty 4200ti and I am not looking back until AMD is on par with nVidia in the stuttering was literally making me motion sick )-:
  • SPBHM - Tuesday, March 26, 2013 - link

    "holding back one frame but not another can sometimes make the frame display evenly, but from a simulation step only a few milliseconds after the previous step"

    wouldn't this also happen with the single GPU "heartbeat stuttering"?
  • BrightCandle - Tuesday, March 26, 2013 - link

    Yes it would, which is exactly the problem with the heartbeat pattern that AMD's problem causes. You can deliver the frames evenly out to the monitor but their contents has a noticeable stutter due to the graphics driver accepting the frames unevenly. The heartbeat is a sign of a real problem without a doubt, all non smooth frame time captures are. What they are not is a sign that the DVI monitor is seeing frames at those periods, but then no one ever said that was what was being measured anyway.

    The best way to think about it is that this is the problem going into the pipeline, measuring the output also needs to be done to get the smoothness on the output. Only with both can you understand the impact. We have half the picture, and that half is accurately measured by fraps.
  • Gunbuster - Tuesday, March 26, 2013 - link

    Design and launch a product. Ignore user feedback.

    Did we forget about those people with $2000 laptops sporting AMD mobile card drivers that didn't work correctly for over a year due to some bug with the graphics switching MUX? This seems to be a pattern that revolves around AMD software people being wholly out of their depth, overworked, or just not caring. They don’t even seem to be able to figure out when they have a fix. The laptop GPU story here on AT was presented as AMD sending over beta drives and asking “Did we fix it this time?”
  • rootheday - Tuesday, March 26, 2013 - link

    One minor correction to the description of the submission of commands through the stack - the DirectX runtime under Windows Vista and later does NOT accumulate a frames' worth of draw calls before sending them to the UMD. I believe it sends state and draw calls to the UMD immediately.

    The UMD accumulates commands in the command buffer and flushes them to the KMD either when a present call occurs, when the command buffer is full, or when the application requests to read back the results of enqueued rendering (Map/Lock/read Query result).

    It used to be true under Windows XP that the dx runtime accumulated calls and dispatched them to the driver - but that is because in XP, the driver ran in kernel mode and it was too expensive to make the user mode->kernel mode transition on every "SetState", etc call.
  • tynopik - Tuesday, March 26, 2013 - link

    "frame latter than it would have" -> later (pg 3)
  • cactusdog - Tuesday, March 26, 2013 - link

    As a long time ATI/AMD fan this report doesn't fill me with confidence. It appears AMD is using anandtech for their public relations spin on the stuttering issue. I don't blame anandtech for running the story, AMD's comments are newsworthy and anandtech deserves credit for being honest about AMD's intentions. On the negative side, the explanation about fraps not being an effective tool only need to be said once, it seems (by the number of times it was mentioned) that AMD's message is to make sure everyone knows Fraps its not accurate, but doesn't explain why Nvidia performs better.

    On the issue, it sounds like AMD is conceding and preparing us for much of the same. No where in the explanation do they mention why Nvidia performs better in the latency tests, other than to say its not what the end user is seeing. Well I disagree, users have been complaining about stuttering for years. I just don't believe that AMD have never looked into this issue before. Also with the multi-gpu stuttering. It has been an issue since crossfire/SLI first appeared and nothing has really happened there.

    Im a fan of AMD cards but I use both brands and personally I have noticed Nvidia do a better job with latency and general responsiveness in game, whereas ATI/AMD has the edge with image quality. Its subtle, and probably not something the average user notices but a lot of people do notice.. If AMD can solve this issue they would sell many more cards but by the sounds of this article, its too big and complex for them to solve completely without major work. Hence the excuses. Nvidia has to play by the same rules, the same OS etc and they do a better job at latency/stuttering, hopefully AMD can fix it enough to at least perform as good as a NVidia card.
  • WaltC - Tuesday, March 26, 2013 - link

    "NVIDIA made a big deal about moving away from timedemos and average frame rates during the early GeForce FX (NV30) days, when its cards might have delivered a decent gaming experience but were slaughtered in most benchmarks."

    Well, that's not really what happened at all...;) The chip "slaughtering" everything nVidia made in those days was the ATi R300. Seems rather strange to tell just half of that story. And the problem nVidia had with benchmarks wasn't technical--it was that nVidia was found to be actively cheating in 3dMark (camera on rails), among other cheats/shortcuts/optimizations in their drivers. The benchmarks told a story nVidia couldn't abide, and that was how much better the R300 was than anything nVidia had at the time. R300 was in every sense a revolution in the 3d gpu markets, blowing everything else away. All gpus on the market today are descended from R300 (just as all Intel and AMD x86 cpus are descended from AMD's original 64-bit Opterons.) nVidia did eventually own up to all of it, right before cancelling the nV30 after a month or two in production, however. People kept publishing proof after proof of what nVidia was doing until finally the company said "uncle." nVidia has been a better company since, imo. At least, its products are certainly better.

    I'm using a single ATi gpu and over the last few years I have to say that I haven't seen any stuttering worth mentioning. Whenever I have seen stuttering it is usually due to some software condition or other, and rectified by the appropriate patch. I do appreciate your pointing out that Fraps isn't perfect and I think TR should stop pretending that it is. Fraps as you point out was never intended to measure this kind of latency and so using it to produce data other than frame-rate data is an "off-label" use of the program, imo. And also as you point out, I use vsync more often than not.

    Really, though, I would loathe seeing AMD optimizing its drivers just to look better in TR's off-label Fraps usage...!...;) Let's hope that doesn't happen as I got quite a belly full of that sort of thing back in the nV30 days--enough to last me a lifetime.
  • beginner99 - Tuesday, March 26, 2013 - link

    How can FRAPS detect any vendor-specific stuttering if it injects itself before the gpu-driver is called?
    The second thing is that v-sync is just crap. I'm not a professional gamer, not even close but in certain games turning it off made me a much better player and the difference is huge. even more annoyingly it was not directly noticeable. I did not "feel" anything changed. Except that my stats were better. Tearing and stuttering: no issue for me so far.
  • DanNeely - Tuesday, March 26, 2013 - link

    The timing at the point it's measuring is normally blocked until the queue the GPUs feeding from has an open slot?

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