In Summary: The Frame Pacing Problem

Before we dive into the technical details of AMD’s frame pacing mechanism and our results, we’re going to spend a moment recapping the basis of the frame pacing problem. So if you haven’t been keeping up with this issue, please read on, otherwise feel free to jump a page.

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.

However due to the mechanisms of AFR, left unattended it can significantly impact the intervals between frames and consequently 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 poorly paced frames that will result in perceived 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. Consequently it’s impractical to fully solve micro-stuttering and achieve frame pacing consistency on level with single-GPU setups, but it’s still possible to improve on previous methods and achieve a level of frame pacing that is reasonably effective and “good enough” for most needs. This is what AMD has been focusing on for the past few months.

Moving on, how AMD ended up in this situation is effectively the combination of three factors. The first of course being the innate technical challenged posed by AFR, while the second and third factors have been a poorly realized position on lag vs. consistency and a failure of competitive analysis respectively.

On the former, AMD’s position up until now has been that they’ve favored minimizing input lag in their designs. If you need to hold back a frame to better pace it, then you are by definition introducing some input lag, a quality that is generally undesirable to a user base that usually avoids mechanisms like v-sync for that reason. AMD’s position hasn’t been wrong of course, but it has come at the exclusion of allowing a bit of input lag to better manage frame pacing. AMD’s decision then has been to lighten up on this position and dedicate the resources to deal with both approaches. AMD would introduce advanced frame pacing as an optional control, while leaving the simpler, less laggy approach as another option.

Meanwhile the story with competitive analysis is far less complex. Simply put, AMD wasn’t testing for frame pacing as part of their standard competitive analysis, so when these results first broke AMD was caught flat-footed. This is a business failure rather than a technical failure, which makes it easy enough to resolve. But it’s also the reason why AMD needed time to develop an advanced frame pacing mechanism, as they had never seen the need to develop one before.

Ultimately this is a problem that should have never happened, and it is unfortunate that AMD let it come to this. At the same time however we believe it’s never too late for redemption, and AMD has been making all of the right moves to try to achieve that. They have been clear about their failures and shortcomings, including their frustrations that they’ve left performance on the table by not looking for these issues, and they have been equally clear in laying out a plan for how they would go about fixing all of this. So today we will finally get to see first-hand whether AMD’s initial efforts for resolving frame pacing in multi-GPU setups has paid off.

AMD Frame Pacing Explored Catalyst 13.8 Beta 1: The First Multi-GPU Frame Pacing Driver
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  • chizow - Wednesday, August 7, 2013 - link

    There was discussions of microstutter on various forums associated with multi-GPU, but PCGH was the first site to publish it's findings in detail with both video evidence and hard data. From what I remember, they were the first to develop the methodology of using FRAPs frametimes and graphing the subsequent results to illustrate microstutter. Reply
  • BrightCandle - Friday, August 2, 2013 - link

    One of the most shocking revelations to me is that AMDs quality assurance did not include checking the output of their cards frame by frame. I had always assumed that both NVidia and AMD had HDMI/DVI/VGA recorders that allowed them to capture the output of their cards so they could check them pixel by pixel, frame by frame and presumably check they were correct automatically.

    Such a technology would clearly have shown the problem immediately. I am stunned that these companies don't do that. Even FCAT is a blatantly blunt tool as it doesn't say anything about the contents of the frames. We still don't have any way to measure end to end latency for comparison either. All in all there is much to left to do and I am not confident that either company is testing these products well, its just I couldn't believe that AMD wasn't testing theirs for consistency (it was obvious when you played it something was wrong) at all.
    Reply
  • krutou - Friday, August 2, 2013 - link

    AMD is in the business of being the best performance per price entry in every market segment. Technology and quality come second.

    How often does AMD introduce and/or develop technologies for their graphics cards? The only two that come to mind are Eyefinity and TressFX (100 times more overhyped than PhysX).
    Reply
  • Death666Angel - Saturday, August 3, 2013 - link

    I think ATI had tessellation in their old DX8 chips. nVidia bought PhysX, so that shouldn't count. But I don't really see how having exclusive technology usable by a single GPU vendor is anything good. We need standardization and everybody having access to the same technologies (albeit with different performance deltas). Look at the gimmicky state of PhysX and imagine what it could be if nVidia would allow it to be fully utilized by CPUs and AMD GPUs? Reply
  • krutou - Saturday, August 3, 2013 - link

    Because OpenCl and TressFX are doing so well right? Reply
  • bigboxes - Sunday, August 4, 2013 - link

    March on, fanboi. Reply
  • JamesWoods - Sunday, August 4, 2013 - link

    If you think that is all AMD/ATI has ever done for graphics then you sir, are ignorant. I was going to use a more degrading word there and thought better of it. Reply
  • Will Robinson - Friday, August 2, 2013 - link

    LOL...what a load of tosh.
    "NVDA had to take them by the hand"?
    You and Wreckage ought to post in green text.
    Reply
  • chizow - Friday, August 2, 2013 - link

    Agree with pretty much of all of this, although I would direct a lot of the blame on AMD's most loyal, enthusiastic supporters as well. Every time microstutter was mentioned and identified as being worst with AMD solutions, AMD's biggest fans would get hyperdefensive about it. If those most likely to have a problem were too busy denying any problem existed, it really should be no surprise it was never fixed.

    And this is the result. Years of denial and broken CF, finally fixed as a result of the scrutiny from the press and laughter of Nvidia fans which brought this to a head and forced AMD to take a closer look and formulate a solution.
    Reply
  • EJS1980 - Friday, August 2, 2013 - link

    "Truth favors not one side." Reply

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