AMD Comments on GPU Stuttering, Offers Driver Roadmap & Perspective on Benchmarking
by Ryan Smith on March 26, 2013 2:28 AM EST
For as long as I can remember talking about video cards and GPU performance at AnandTech, there has been debate over the type of benchmarks used to represent that performance. In the old days, the debate was mostly manufacturer driven. Curiously enough, the discourse usually fired up when one manufacturer was at a significant deficit in GPU performance. 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. Even Intel advocated for a shift away from most CPU bound gaming benchmarks back during the early years of the Pentium 4 - again, for obvious reasons.
It’s a shame that these revolutions in gaming performance testing were always associated with underperforming products (and later dropped once the product stack improved in the next generation or two). It’s a shame because there has always been merit in introducing additional metrics in order to provide the most complete picture when it came to gaming performance.
The issue lay mostly dormant over the past several years. Every now and then there’d be a new attempt to revolutionize GPU performance testing, but most failed to gain widespread traction for one reason or another. Broad repeatability, one of the basic tenets of the scientific method, was usually cast aside in pursuit of a lot of these new attempts at performance testing - which ultimately limited acceptance.
A year and a half ago, Scott Wasson over at the Tech Report did something no one since Dr. Pabst was able to do: he actually brought about a revolution in the 3D game benchmarking scene.
The approach seemed ridiculously simple - we’ve all had the tools for so very long. Scott used FRAPS to record frame times, and would calculate how long every frame in a benchmark took to render. By focusing on individual frame latencies, Scott’s method could better characterize the little hiccups and stutters that would get smoothed out in an average frame rate. With the new method came a bunch of nifty graphs, and the world changed.
The methodology wasn’t perfect, as FRAPS lacks a holistic view of the 3D rendering pipeline, but it did reveal some surprising issues (in addition to spawning further work that uncovered even more issues on the multi-GPU front). Interestingly enough, many of the issues uncovered by this focus on frame times/latency seemed to primarily impact AMD hardware.
AMD remained curiously quiet as to exactly why its hardware and drivers were so adversely impacted by these new testing methods. While our own foray into evolving GPU testing will come later this week, we had the opportunity to sit down with AMD to understand exactly what’s been going on.
Although neither strictly a defense nor merely an explanation of what we’ve been seeing over the past year, AMD wanted to sit down and better explain their position. This includes both why AMD’s products have been impacted in the manner they were, and why at the same time (and not unlike NVIDIA) AMD is worried about FRAPS being given more weight than it should be. Ultimately AMD believes that it’s to the benefit of buyers and journalists alike to better understand just what is happening, why it’s happening, and just what the most common tools can and are measuring.
What follows is based on our meeting with some of AMD's graphics hardware and driver architects, where they went into depth in all of these issues. In the following pages we’ll get into a high-level explanation of how the Windows rendering pipeline works, why this leads to single-GPU issues, why this leads to multi-GPU issues, and what various tools can measure and see in the rendering process.
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Tuvok86 - Tuesday, March 26, 2013 - link
This is great victory for all of the tech press.When people started complaining about stuttering years ago we were only dreaming of getting so much attention from gpu brands.
I still remember someone constantly saying "micro-stuttering doesn't exist", I wonder how they feel now that they enjoy the fps and smoothness benefits.
In any case I praise constructive journalism that triggered a significant leap in the technology.
BrightCandle - Tuesday, March 26, 2013 - link
One important fact I feel is missing in your treatment of what it is fraps is measuring and why its more representative of problems than you and AMD think it is. For some reason everyone who makes this argument that fraps is isn't very useful seems to skip this one, but its really really important.Fraps measures at the present call and that isn't a random choice. Because the present call has a few different modes of operation, but all games use blocking mode. What that means is that if the context queue is full (which it normally is) then game thread is held up waiting for that present call to complete. Subsequent present calls are regulated by the GPU's driver in this case as the thread is held and when it chooses to accept the completion of that frame only then can the games thread continue. Since Fraps is measuring this it can see when the driver is accepting frames in an uneven fashion, so while you might see even frames presented to the monitor due to the buffering there is still a knock on effect.
Game simulations produce particular frames of their simulation, sometimes in the same thread as the present call and sometimes in a different thread. Regardless they use the release of the present call as the end of their rendering step and that allows another frame to be started or delivered. So if the present calls are coming back unevenly the game simulation itself will stutter as it tries to produce as many simulation steps as the rendering is producing. If the present calls are stuttering there is a feedback loop into the game simulation that is too causing it to stutter.
Its this feedback loop on the rendering and game simulation which causes much of the problem, and it starts in the GPU driver. It might very well be caused by Windows but the big difference we see in the manufacturers solutions tells us that its almost entirely the manufacturers fault when it happens and impacts on gameplay.
So quite rightly fraps does not measure stuttering out to the screen, it measures the GPUs regulation of the frame rate of the game rendering and its simulation and that does cause real stuttering, both of the subsequent present calls and the game simulation.
Of course pcperspective have now shown that AMD's SLI stuttering out the DVI port is considerably worse than Fraps, so much so they considered what they are doing is a cheat as the frames aren't real. But you need bothperspectives, the output and the input to the pipeline to see the impact on the game. Its not just the frames themselves that have to be regulated to be smooth its also the game simulation that must run smoothly, and it is regulated by the handling of the context queue.
JPForums - Tuesday, March 26, 2013 - link
There are two things you need to keep in mind:1) Nvidia also agrees with the limitation of FRAPS. In fact, IIRC they were the first to voice the issue that FRAPS recordings are in the wrong place and can only infer what actually needs to be recorded. The author is correct, when Ati and Nvidia agree, we should at least pay attention.
2) Though your your points are AFAIK correct and well articulated, they still point to the issue of FRAPS inferring, rather than recording the the targeted information. The difference is, rather than consistency of output frames, you are looking for consistency of simulation steps. I agree that this is a metric that really needs to be covered. In fact, I would even go as far as matching simulation steps to their corresponding frame times to expose issues when short steps are accompanied by long frames or vice versa.
Unfortunately, FRAPS can't measure any of this directly and even for your points proves to be limited to inference. That said, until a reviewer gets tools that can reveal this information, inference via FRAPS is better than no information at all. Pcperspective's comments on AMD's stuttering issues are related (as they state) to crossfire setups. I could see the differences between CF and SLI in blind tests (though SLI also has some microstutter) and this only confirms it. The runt frames only add fuel to the fire. I'm open to using AMD in single GPU builds, but only use Nvidia for multiGPU builds. Perhaps this will change in July, but I'm guessing there will still be plenty of work to do.
JPForums - Tuesday, March 26, 2013 - link
I should probably expand a little on what I consider a limitation of FRAPS for stutter caused by simulation steps. FRAPS inserts itself at the output of the render and is therefore subject to a variable delay between the simulator time step through the render. Important information can still be inferred, like simulation stutter in AMD's heartbeat waveform. However, I'd still rather get a timestamp directly at the output of the simulator rather than at the output of the renderer, if it ever becomes an option. Unfortunately, that would probably require cooperation with the game developer, so I'm not sure that will ever happen.tipoo - Tuesday, March 26, 2013 - link
The third page makes me wonder, just how much would a real time operating system improve performance? QNX on BB10 is real time, the PS4 OS may be too.juampavalverde - Tuesday, March 26, 2013 - link
Time to update the GPU review template guys... At least copy&paste PCPer and TechReport methods.cjb110 - Tuesday, March 26, 2013 - link
Sounds like there's a market for a tool then, something that does what GPUView does but in simpler manner (like Fraps presents).drbaltazar - Tuesday, March 26, 2013 - link
sadly the issue they find isn't exsactly caused by the gpu!it is at the os end!data fragmentation at various level is often the cause.and this happen everywhere,at the processor cache level to the server cache level!ms say it doesn't mather !they re wrong!it affect everything related to image quality.bufferbloat also is the main problem.mtu,udp fragmentation ,multithreading and rss fragmentation etc etc etc!oh they say they can reconstruct the data in the proper maner that wont impact performance or quality!again ms is either wrong or unknowing of the problem these various issue cause .I haven't event started on the gpu side yet!all that data manipulation etc is the main issue !how to fix it?mm!probably use official standard limit like the 1460 for mtu and add udp to that also so that it is also at 1460.(just a random exemple cause these will need to be tweaked ,why?so that packet don't get fragmented anywhere in the computer or the server.or they tell people how to make it happen ,because right now not many have 1080p quality even most have a 1080p monitor.so imagine if amd is using window idea to tweak their gpu?like .net4 etc !(yep it become a nightmare)hopefully they ll fix this but all side have been on a race for performance .(wouldn't want to sell a = performing w8 instead of w7 .it wouldn't sell!i am all for getting better performance but not at the expense of subpixel quality of graphic.nvidia is probably better because they noticed ms error and have worked to avoid the os mistake by using standard and proper ways .I aint saying ms is wrong maybe they can really fragment packet and have everything being fine and dandy looking in 1080p.but I will tell you this.in most area of computing it feels like this:os is saying 255.0.0 and at the other end for some reason its like our old phone game,at the other end what is being done isn't at all what the os said the beginning (and viceversa)hopefully these idea of new data mining and testing tool will go deeper and test what is actually going on in our computer,network and server datapath so they all can work together.cause right now?our game look 1080i even tho we are all set at 1080pmi1stormilst - Tuesday, March 26, 2013 - link
I love you guys, but this article comes off a bit like sour grapes. The Tech Report dove into this issue head first and admitted from the beginning the testing methods may not be perfect. They have continued to be clear on this and you made no mention of the high speed video tests that they performed. The bottom line is The Tech Report is primarily responsible for getting AMD to get on the ball with this issue. Regardless of AMD's bag of excuses and their sudden clarity on the best methods for testing we would not be where we are without the sold work of The Tech Report. I feel that if the FRAPS method of testing was sufficient for bringing these issues to light then a job well done. The situation will only improve from there and Scott Wasson and company deserve more praise than this sour attempt of an article to discredit the good work they have done. If that we not your intention then I apologize, but it comes off as such.brybir - Tuesday, March 26, 2013 - link
I did not see it this way at all. Instead, I read it as TechReport started a trend in evaluating stuttering that most were not looking for, and that while there is some merit to their methods, there are other better ways of evaluating the issue. I did not see any effort to hide, obscure, or otherwise show "sour grapes" to them for their testing.As to the merit of the article, if AMD, Nvidia, and Anandtech folks all agree that the methods used by TechReport are okay but could be improved upon with better tools, then the end result will be better for everyone. Much as standard bench-marking software has evolved a lot over the the last decade, the bench-marking for this type of testing will change dramatically as people find interesting and new ways to really get in depth with the issue and generate data that is easy to aggregate and report. I think that is a net benefit for all of us!