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G-Sync requiring very low FPS cap?

Talk about NVIDIA G-SYNC, a variable refresh rate (VRR) technology. G-SYNC eliminates stutters, tearing, and reduces input lag. List of G-SYNC Monitors.

Re: G-Sync requiring very low FPS cap?

Postby jorimt » 22 Apr 2017, 15:02

@Glide, is that Dishonored or Dishonored 2? If it's the latter, again, don't use it as a baseline for this issue; the Void engine is a mess.

If it's Dishonored 1, I own the game, and I can do a quick test of the in-game and RTSS framerate limiter at 100 Hz to check for additional input latency when the spikes occur, though my previous tests in this thread showed no additional sync-induced input latency in CS:GO, even during readout spikes.

As for the differences in numbers between the RTSS counter and the display's counter, RTSS, to the best of my knowledge, has never been able to read its own frametimes when it is the limiting fps factor, so that's another reason you're probably seeing what you're seeing.
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Re: G-Sync requiring very low FPS cap?

Postby Glide » 22 Apr 2017, 15:56

Dishonored 1
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Re: G-Sync requiring very low FPS cap?

Postby jorimt » 23 Apr 2017, 15:16

@Glide, thanks for the clarification.

Do you still need me to do a test to confirm whether G-SYNC is adding any sync-induced input latency in Dishonored with either the in-game or RTSS fps limiter (which adds 1 frame input latency no matter what, in 99% of games) VS unsynced when the built-in display refresh rate meter spikes to 100 Hz, or were my previous tests convincing enough?

Because while the spikes and the discrepancy between readouts is obnoxious and unnerving, my previous tests lead me to believe that this issue isn't increasing input latency with an appropriate fps limit in place.
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Re: G-Sync requiring very low FPS cap?

Postby RealNC » 23 Apr 2017, 15:49

It occurred to me that one possible explanation for this issue involves gsync's high frame time protection. Even if you cap your FPS, if you don't have another frame to fallback to (like NVidia's limiter does, which is why it adds 2 frames of lag instead of 1), a temporary spike in frame time will result in gsync temporarily doubling the frame rate.

There's no information on this, but it is likely that gsync will not just do frame rate doubling to the closest multiple, but rather to the closest multiple that is CLOSER to the current refresh rate (in an attempt to keep overdrive looking good.) 100Hz is closer to 90Hz than 75Hz for example, so if a frame needed repeating, gsync might choose to refresh sooner, completely bypassing the frame cap.

I could be totally wrong here, but that would explain why the nvidia limiter doesn't have this issue. It keeps one frame buffered, so if there's a 1-frame stall, it uses the buffered frame. Thus, gsync does not need to repeat the previous scanout, and that means the max refresh of the monitor is not reached.
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Re: G-Sync requiring very low FPS cap?

Postby Chief Blur Buster » 23 Apr 2017, 16:26

Interesting -- you're saying that the variable overdrive requires some prediction on when the next refresh cycle will occur.

This would make sense, as overdrive knowledge of the subsequent refresh cycle, can help overdrive.
Just, predicting is hard.
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Re: G-Sync requiring very low FPS cap?

Postby RealNC » 23 Apr 2017, 16:35

Chief Blur Buster wrote:Interesting -- you're saying that the variable overdrive requires some prediction on when the next refresh cycle will occur.

NVidia's Tom Petersen confirmed that gsync does frame time prediction for the sake of better overdrive in a PC Perspective interview. Don't know which interview exactly anymore. PCPer has lots of them :D

He didn't mention exactly HOW they do prediction, but I guess that's for obvious "it's a secret" reasons. But prediction was needed, otherwise overdrive is going to either overshoot or undershoot. Overdrive needs to know when the next frame will come in order to feed the pixels with the correct voltage. You need to apply the voltage BEFORE you know when the next frame comes, so by the time the next frame comes, the pixels have transitioned appropriately. If the next frame comes too late, your pixels will have overshooted by then. If it comes too soon, the pixels will only have half-transitioned, having wrong colors and retaining information from the previous frame (huge ghosting issues.)

For fixed refresh, that's very easy. You always know that the next frame will come in 16.7ms when running 60Hz for example. So you use a voltage that gets the pixels in their new state at the correct time. Not too soon (or they'll overshoot), not too late (they'll undershoot.)

For VRR, you need to be able to be smart and predict. Which for gsync could mean hitting the refresh rate max even when using a frame limiter.

Testing this theory would be rather involved though, to say the least :P You'd need a high frame rate camera, an application you write yourself that does flipping and is controlled by you, and a monitor that has a built-in frame time "FPS" OSD.
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Re: G-Sync requiring very low FPS cap?

Postby Chief Blur Buster » 23 Apr 2017, 18:44

We have the programming means to test this out, so we may develop some GSYNC motion test apps that display test patterns at predictable/ramping/unpredictable variable framerates -- albiet not after a few priorities first.

Unfortunately, this would not be TestUFO, but an executable, because web browsers don't (yet) support VRR, even though I've brought a VSYNC 5.2 API suggestion to W3C.
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Re: G-Sync requiring very low FPS cap?

Postby jorimt » 23 Apr 2017, 22:03

Hm @RealNC, that is all plausible, but it would take further research to figure out exactly what is going on in this instance, or if anything is going on at all.

Again, it's possibly a built-in refresh rate meter glitch, because it did not behave like this before the Creators update, at least not that I recall. So either the update allowed the meter to somehow output more accurate readouts than ever before, and it is exposing this behavior for the first time, or the update caused something to glitch out.

Time will tell...

But I must repeat, this quirk does not appear to have a negative impact on input latency with G-SYNC + appropriate fps limit.
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Re: G-Sync requiring very low FPS cap?

Postby RealNC » 03 May 2017, 15:42

This just in: Battle(non)sense found that the Creator's update is to blame, and that the fix is to use high performance mode in the power settings of windows:

https://www.youtube.com/watch?v=mVNRNOc ... u.be&t=287
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Re: G-Sync requiring very low FPS cap?

Postby Glide » 03 May 2017, 16:11

I'm not really seeing a difference between the Ryzen Balanced and High Performance power plans. At least from a quick test.
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