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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

    In the "general case" we mean /when triple buffering is not available/ (real triple buffering is not available in the majority of games), in order to reduce input lag, vsync should be disabled.

    Here's a better break down of what we recommend:

    Above 60FPS:

    triple buffering > no vsync > vsync > flip queue (any at all)

    Always EXACTLY 60 FPS (very unlikely to be perfectly consistent naturally)

    triple buffering == vsync == flip queue (1 frame) > no vsync

    Below 60FPS (non-twitch shooter):

    triple buffering == flip queue (1 frame) > no vsync > vsync

    Below 60FPS (twitch shooter or where lag is a big issue):

    no vsync >= triple buffering == flip queue (1 frame) > vsync

    ... to us the fact that at less than 60 FPS you start on the exact same frame with either triple buffering, no vsync or with a 1 frame flip queue is enough -- you will get less than 1 frame of difference in the lag and the difference you get with no vsync is only on part of the frame anyway.

    ...

    And ...

    NVIDIA has absolutely CONFIRMED that they do OpenGL triple buffering in the way that I described it in the previous article. This confirmation is directly from their OGL driver team.

    I was just able to sit down with AMD two days ago and, while they don't do things in exactly the way I describe them, their OpenGL triple buffering does do something quite interesting at > 60FPS ... which I'll talk about in an upcoming article. They also said they are looking into what it would take to do what I was talking about -- but that they haven't yet because their workstation customers tend to care more about what happens at < 60fps (which is a case where a 1 frame flip queue == triple buffering).

    The situation with DirectX is a little more restrictive with both NVIDIA and AMD... I will also address this issue in an upcoming article.
    Reply
  • BoFox - Friday, July 17, 2009 - link

    Anandtech does yet another ground-breaking article, thanks to Derek Wilson and the use of a high-speed camera this time!

    It is rather interesting how you mention the human reaction time! To detect and process the image signal in your brain, and to react to it (usually faster if through reflex), are all part of the entire reaction time of 200-300 ms, correct? Well, we should be testing it on the professional gamers like Jonathan 'Fatal1ty' Wendel, to see how much he has tuned his reaction time in making it so reflexive that it's as low as 150ms or even lower. Why not try interviewing him and use your high-speed camera to test that on him, and on an average gamer?!? That would be a SUPER-awesome article here to read!!! :D

    About the rendering ahead, some games are designed to render ahead by as much as 3 frames, and changing that to 0 could incur a large performance hit--sometimes by as much as 50%. I have experienced this with some games a while back, cannot remember which ones exactly, and that was on a single video card (not when I was using SLI). The performance penalty would usually bring down the frame rates to below the refresh rate, thus making it very undesirable. There are some other articles cautioning against it, but as long as it does not make a difference in the game that you are playing, then great. At least, as long as it does not drop the frame rates to below the refresh rate, then great!

    Reply
  • DerekWilson - Sunday, July 19, 2009 - link

    If your framerate is above refresh rate (except for the multiGPU case) you should definitely not use anything more than 0 frames of render ahead. Triple buffering as I described in my previous article (and as NVIDIA does in OpenGL) will be a good option, but double buffered w/ vsync or no vsync will definitely be better than any sort of render ahead.

    If your performance is below refresh rate, you'll want to either use no vsync, exactly 1 frame render ahead, or triple buffering. double buffered vsync (0 flip queue/pre-rendered frames with vsync) will give you a significant drop in performance and increase in input lag when performance is below refresh. This is as you say -- by as much as 50%. But only when performance is already below refresh.
    Reply
  • nvmarino - Friday, July 17, 2009 - link

    Hey Derek, thanks for the great article. Nice job addressing many of the BS theories and comments about lag going on in the threads of your triple buffering article...

    And now I've got even more fuel for the fire of my lust for someone to release a 1920x1080 resolution display with support for 120Hz refresh rate at the INPUT and good black levels. If only I could set my PC to spit out 120Hz to my display, my 24Fps Blu-ray and 60Fps TV content would play without judder, AND my games would have zero tearing (well as long as they don't run above 120Fps) and minimal lag. Not only that, but I'd also have an ideal setup for shutter-based steroscopic 3d, and it would open the door for some of the new stuff like smoothing video via frame interpolation of 24P content to 120P currently going on in the PC videophile community. Can you say HTPC nirvana! Where's my 120Hz!!!!
    Reply
  • DerekWilson - Friday, July 17, 2009 - link

    yeah, i want a true 120hz 1920x1080 display too :-) mmmm that would be tasty. Reply
  • Belinda fox - Friday, July 17, 2009 - link

    If someones reaction times are up to 220ms and your results are around the 100ms mark surely this says something about the validity of results and conclusion.
    Sorry i never read all the article just first and last one, due to reaction times being mentioned as part of lag.
    Reply
  • BoFox - Friday, July 17, 2009 - link

    Just read the article first.

    If you had better common sense, you would figure out that the human response time of >200ms comes AFTER seeing what is on the monitor, which takes 100ms to display in the first place.

    Let's say that I appear just around the corner right in front of you, in a FPS game, like Unreal Tournament 3, for example. First, it takes like 50ms ping time for your computer to receive that information. Second, it takes your monitor 100ms to display the information. Third, it takes you 200ms to react to what you're seeing.
    Reply
  • lyeoh - Sunday, July 19, 2009 - link

    And AFTER you react to what you are seeing you press a key/button or move your mouse and the relevant portion of the input lag takes into effect (input device, game, network).

    With a 100+ms input lag handicap even on the LAN you could be "dead" before you even see anything or be able to do anything about it.

    So would be good if have some figures on wireless keyboards and mice vs wired (usb and ps2).

    I personally think wireless stuff will add a lot more lag due to the modulation/demodulation, and other "tricks" they have to do.
    Reply
  • DerekWilson - Friday, July 17, 2009 - link

    That is definitely an issue ...

    i didn't get into network play as it gets really sticky though ... the local client does do limited prediction based on last known info about other players -- if this prediction data is accurate enough at small intervals and if ping isn't bad then it isn't a huge issue ... but at the same time, servers may resolve hits (as apparent to the client) as hits even if they are misses (in reality as per what the player did) or it may not (the difference results in either increased or reduced effectiveness of techniques like bunny hopping). but it is way more complex and depends on how the client predicts things between server updates and how the server resolves differences between clients and all sorts of funky stuff.

    that's why i wanted to stick with mouse-to-display issues here ...

    but you are right that there are other factors (including human reaction time) that add up to make it so input lag can be an important slice of a whole chain of events and can affect the gaming experience.
    Reply
  • snarfbot - Friday, July 17, 2009 - link

    i have a crt and a plasma tv, both essentially lag free.

    my friends have tn monitors, also no perceivable lag.

    the worst offender is my friends samsung tv, its noticeably laggy.

    but no where near as bad as wireless mice. they gotta be in the 30+ms region.
    Reply

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