FastSync can remove 1 frame of input lag in some games compared to V-Sync OFF

Discussion in 'nVidia Flavor' started by XoR_, Apr 26, 2018.

  1. XoR_

    XoR_ Limp Gawd

    Messages:
    243
    Joined:
    Jan 18, 2016
    Tested FastSync and RTSS at 1fps and at least in DX9 ganes loose 1 frame of input lag. This is easily visible, after frame I would move mouse and game would have certain frames of delay before input would be registered. DX9 games on UE3 would behave this way as some other DX9 games and games forced in DX9. In DX11 I tested Tomb Raider and Ryse and no change was observed. More testing is needed. There was however no added input lag from FastSync even in these games. OpenGL games do not seem to support FastSync at all.
     
  2. Archaea

    Archaea [H]ardness Supreme

    Messages:
    7,986
    Joined:
    Oct 19, 2004
  3. Absalom

    Absalom Limp Gawd

    Messages:
    448
    Joined:
    Oct 3, 2007
    Fast Sync basically decouples the rendering from the display synchronization. It's providing a compromise between classic no-vsync and classic triple buffered vsync. Where Fast Sync shines is when rendering framerates in great excesses of the target framerate (i.e. display refresh rate). Blurbusters states Fast Sync gets near GSync levels of input latency reduction when at 2-5x the max GSync range (i.e. display refresh).

    The key words here are excess framerates. Because when you are at or under the target framerate (ideally capped), GSync is simply better. There's more going on behind the scenes with GSync when under the target framerate condition, which I won't go into here. So Blurbusters takes the sensible stance that GSync + capped framerate is the better experience. In fact they go on to point out that Fast Sync actually introduces microstuttering and more input lag, and is basically no better than classic vsync methods when under this condition (capped Fast Sync doesn't make any sense anyway). And if you take a step back from this, you'll realize that GSync and Fast Sync work best at opposite ends of the spectrum. And that's pretty much the real take away from the BB article.

    Mixing GSync + Fast Sync together brings two arguments (or cons):

    1) Fast Sync within the GSync range only serves to introduce slightly more input latency, which ultimately robs GSync of one of its favorable capabilities.
    2) GSync is effectively disabled outside the GSync range, thus rendering in excessive framerates (a la Fast Sync) effectively negates the very thing you paid for.

    The fallacy here is that you get the best of both worlds with both enabled.

    If you don't have a GSync monitor, then one take away from this is that, assuming you can manage excessive framerates, you are getting near GSync input latency reduction. Again, there's more to the GSync experience than just input latency reduction; however, the topic here is simply that, input latency reduction.

    Also something that Blurbusters doesn't really go into is that decoupling the renderer a la Fast Sync brings other auxiliary benefits. Most game engines run a loop that process several inputs (variables) to determine the resulting output. The more iterations a loop has, the more "samples" it gets to interpret. This means an uncapped game engine gets to sample more of your mouse movements, key presses, etc. While these input devices have virtually no affect on the display latency, they are just as equally important to the overall latency. Someone such as a CS:GO player who needs that extra sampling to interpret their reflex action might actually find value in running (uncapped) Fast Sync.

    The counter argument to running things "uncapped", in the general sense, is that the hardware is generating unnecessary heat for otherwise diminishing returns. To that end, running things "uncapped" is always going to be subjective.
     
    Last edited: May 1, 2018
    kalston likes this.
  4. Archaea

    Archaea [H]ardness Supreme

    Messages:
    7,986
    Joined:
    Oct 19, 2004
    Perhaps I don’t understand but:
    1) fastsync wouldn’t turn on during the gsync range from what I’ve read in another thread - so there shouldn’t be s conflict in gsync range
    2)gsync is disabled outside of gsync range? Well yes! With or without fast sync - gsync won’t work outside its range.

    Finally Rivia Tuner seems unreliable against Nvidia drivers in Windows 10 to set a max FPS cap inside freesync range. I couldn’t get it to work recently and it would seem others couldn’t either so what tool do you recommend to cap frames? Don’t say game config or ini files - that’s a Copt out and it’s the minority of games that support that. The problem i haven’t been able to solve with gsync alone is that when you go above gsync range you get a FPS hiccup and a bad one — so as you go in and out of gsync range in a easily rendered area like a hallway - the hiccup is really bad and it happens often in games. The seeming fix to me was the suggestion to enable fast sync in the Nvidia control panel. Fast sync doesn’t seem to the obviously mouse lag introduction of vsync (leave vsync off) and I don’t sense any micro-stutter or hitch when going in and out of gsync at 120hz. I guess I’m saying I can’t sense a problem anymore with the combination of gsync and fastsync.

    So what recommendation do you have?
     
  5. Absalom

    Absalom Limp Gawd

    Messages:
    448
    Joined:
    Oct 3, 2007
    Yes and no. It depends if and where you are capping your fps.

    There's a period of overlap within the GSync Range near the display's refresh rate where Fast Sync is always active. It really depend on where the fps cap is imposed.

    For example, say the GSync range is 30-100Hz and you enable both GSync & Fast Sync. Additionally you also impose a 100 fps limiter. When your frame rate exceeds 97 Hz, Fast Sync is active, thus you experience the same issues you would if regular ol' VSync were enabled instead. When your fps exceeds 97 Hz, you experience the aforementioned frame pacing or microstuttering.

    Basically, if you cap your framerate near your monitor refresh with Fast Sync enabled, it's effectively turned into regular ol' VSync. If you're going to use Fast Sync, don't cap your framerate at all, or at least keep it capped at a 2-5x (ideally 5x) multiple of your display's refresh rate.

    But if you don't cap your fps anywhere near the GSync range, then yes you can claim Fast Sync is doing something (assuming you can maintain excessive framerates). Just keep in mind that as your excess framerate nears the GSync range (i.e. lowers), you're getting into a weird situation where input latency is not exactly optimal.

    Edit: I realize now that my 1st point was worded rather poorly. Hopefully this response clarifies that point a bit.
     
    Last edited: May 2, 2018