In gaming, input lag is defined as the delay between the when a user does something with an input device and when that action is reflected on the monitor.

The definition is straightforward, but the reality of input lag is much more subtle than may readily be apparent. There are many smaller latencies that contribute to the overall whole of input lag and understanding the full situation may prove beneficial to gamers everywhere.

The first subtlety is that there will always be input lag. Input lag is an unavoidable reality that can only be minimized and never eliminated. It will always take some amount of time for input data to get to the software and it will always take some amount of time for the software to use that data to display a frame of animation on the monitor. Keeping this total time as low as possible is a key mission of hardcore twitch gamers out there.

This article will step through all the different contributors to input lag, and we'll give some general estimates on the impact of each different contributor. Exact numbers will vary widely with different hardware and software combinations. But knowing where to focus when optimizing for input latency should help those who are interested.

After drilling down into the causes of input latency, we will provide a few examples of different hardware and settings in our lab. The extra twist is that we will be evaluating actual input latency using a high speed camera to count frames between input activation and monitor response. We'll be looking at three different games with multiple settings on both CRT and LCD monitors.

Reflexes and Input Generation
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  • DerekWilson - Sunday, July 19, 2009 - link

    It was bound to happen wasn't it?

    This has been around for a few years now, but (for obvious reasons) never made it into the mainstream gaming community. And, really, now that high performance mice are much more available it isn't as much of an issue.
    Reply
  • Kaihekoa - Saturday, July 18, 2009 - link

    From the conclusion this point wasn't clear to me. Reply
  • DerekWilson - Sunday, July 19, 2009 - link

    at present triple buffering in DirectX == a 1 frame flip queue in all cases ...

    so ... it is best to disable triple buffering in DirectX if you are over refresh rate in performance (60FPS generally) ...

    and it is better to enable triple buffering in DirectX if you are under 60 FPS.
    Reply
  • Squall Leonhart - Wednesday, March 30, 2011 - link

    This is not always the case actually, there are some DirectX engines specifically the age of empires 3 engine as an example, that have hitching when moving around the map unless triple buffering is forced on the game. Reply
  • billythefisherman - Saturday, July 18, 2009 - link

    First of all I'd like to say well done on the article you're probably the first person outside of game industry developers to have looked at this rather complex topic and certainly the first to take into account the whole hardware pipeline as well.

    Sadly though there are some gaping holes in your analysis mainly focused around the CPU stage. Sadly your CPU isn't going to run any faster than your GPU (and actually the same is correct in reverse) as one is dependent on the other (the GPU is dependent on the CPU). As such the CPU may finish all of its tasks faster than the GPU but the CPU will have to wait for the GPU to finish rendering the last frame before it can start on the next frame of logic.

    No game team in the world developing for a console is going to triple buffer their GPU command list.

    I intentionally added 'developing for a console' as this is also an important factor I'd say around 75% (being very conservative) of mainstream PC games now are based on cross platform engines. As such developers will more than likely gear their engines to the consoles as these make up the largest market segment by far.

    The consoles all have very limited memory capacities
    in comparison to their computational power and so developers will more than likely try to save memory over computation thus a double buffered command list is the norm. Some advanced console specific engines actually dropping down to a single command buffer and using CPU - GPU synchronisation techniques because of CPU's being faster than GPU's. This kind of thing isnt going to happen on the PC because the GPU is invariably faster than the CPU.

    When porting a game to PC a developer is very unlikely to spend the money re-engineering the core pipline because of the massive problems that can cause. This can be seen in most 'DirectX 10' games, as they simply add a few more post processing effects to soak up the extra power - you may call it lazy coding, I don't, it's just commercial reality these are businesses at the end of the day.

    So both your diagrams on the last page are wrong with regards to the CPU stage as they will be roughly the same amount of time as the GPU in the vast majority of frames because of frame locality ie one frame differs little to the next frame as the player tends not to jump around in space and so neighbouring frames take similar amounts of time to render.

    Onto my next complaint :
    "If our frametime is just longer than 16.67ms with vsync enabled, we will add a full additional frame of latency (with no work being done on the GPU) before we are able to swap the finished buffer to the front for scanout. The wasted work can cause our next frame not to come in before the next vsync, giving us up to two frames of latency (one because we wait to swap and one because of the delay in starting the next frame)."

    What are you talking about man!?! You don't drop down to 20fps (ie two more frames of latency) because you take 17ms to render your frame - you drop down to 30fps! With vsync enabled your graphics processor will be stalled until the next frame but thats all and you could possibly kick off your CPU to calculate the next frame to take advantage of that time. Not that thats going to make the slightest jot of difference if you're GPU bound because you have to wait for the GPU to finish with the command buffer its rendering (as you don't know where in the command buffer the GPU is).

    As I've said on the consoles there are tricks you can do to synchronise the GPU with the CPU but you don't have that low level control of the GPU on the PC as Nvidia/ATI don't want the internals of thier drivers exposed to one another.

    And as I've said not that you'd want to do such a thing on PC as the CPU is usually going to be slower than the GPU and cause the GPU to stall constantly hence the reason to double buffer the command buffer in the first place.

    I've also tried to explain in my posts to your triple buffering article why there's a lot cobblers in the next few paragraphs.
    Reply
  • DerekWilson - Sunday, July 19, 2009 - link

    Fruit pies? ... anyway...

    Thanks for your feedback. On the first issue, the console development is one of growing importance as much as I would like for it not to be. At some point, though, I expect there will be an inflection point where it will just not be possible to build certain types of games for consoles that can be built on PCs ... and we'll have this before the next generation of consoles. Maybe it's a pipedream, but I'm hoping the development focus will shift back to the PC rather than continue to pull away (I don't think piracy is a real factor in profitability though I do believe publishers use the issue to take advantage of developers and consumers).

    And I get that with GPU as bottleneck you have that much time to use the CPU as well ... but you /could/ decouple CPU and GPU and gain performance or reduce lag. Currently, it may make sense that if we are GPU limited the CPU stage will effectively equal the GPU stage in latency -- and likewise that if we are CPU limited, the GPU state effectively equals the CPU stage (because of stalling) in input latency.

    Certainly it is a more complex topic than I illustrated, and if I didn't make that clear then I do apologize. I just wanted to get across the general idea rather than a "this is how it always is" kind of thing ... clearly Fallout 3 has even more input lag than any of my worst case scenarios account for even with 2 frame of image processing on the monitor ... I have no idea what they are doing ...

    ...

    As for the second issue -- you can get up to two frames of INPUT LAG with vsync enabled and 17ms GPU time.

    you will get up to these two frames (60Hz frames) of input lag at 30FPS ...

    I'm not talking about the frame rate dropping to 2 frames then 1 frame (20 FPS) ... I'm talking about the fact that, at best, your input is gathered 17ms before your frame completes on the GPU (1 frame of input lag) and (because it missed vsync) it will take another frame for that to hit the screen (for a total of two).
    Reply
  • billythefisherman - Monday, July 20, 2009 - link

    I have to re-iterate: well done on tackling this rather complex issue, I applaud you! (I just wish you hadn't whipped up your punters so much in the benefits of triple buffering!) Reply
  • Gastra - Saturday, July 18, 2009 - link

    For (quite a lot if you follow the links) of information on what an optical mouse see:
    http://hackedgadgets.com/2008/10/15/optical-mouse-...">http://hackedgadgets.com/2008/10/15/optical-mouse-...
    Reply
  • DerekWilson - Sunday, July 19, 2009 - link

    That's pretty cool stuff ... And it lines up pretty well with our guess at mouse sensor resolution for the G9x.

    It'd still be a lot nicer if we could get the specs straight from the manufacturer though ...
    Reply
  • PrinceGaz - Friday, July 17, 2009 - link

    "For input lag reduction in the general case, we recommend disabling vsync. For NVIDIA card owners running OpenGL games, forcing triple buffering in the driver will provide a better visual experience with no tearing and will always start rendering the same frame that would start rendering with vsync disabled."

    I'm going to ask this again I'm afraid :) Are you sure Derek? Does nVidia's triple-buffer OpenGL driver implementation do that, or is it just the same as what most people take triple-buffer rendering to be, that is having one additional back buffer to render to so as to provide a steady supply of frames when the framerate dips below the refresh rate? Have you got confirmation either from screenshots or something else (like nVidia saying that is how it works) that OpenGL triple-buffering is any different from Direct3D rendering, or how AMD handle it?.

    Because if you don't, then all you are saying is that triple-buffering is a second back-buffer which is filled to prevent lags when the framerate falls below the refresh rate. Do you know for sure that nVidia OpenGL drivers render constantly when in triple-buffer mode or are you only assuming they do so?
    Reply

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