chenifa wrote: ↑12 Oct 2023, 07:48
Do you know how long it takes between the gpu finishing render and the first real visible line of pixels being scanned out? Is it just the time it takes to send the line over the cable or is there more latency added from vrr?
Most of the time (at least with G-SYNC native), there is no added latency for VRR except the scanout (that VSYNC OFF bypasses).
Within VRR range, Present() VRR scanline #1 is usually same latency as the first pixel row underneath a VSYNC OFF tearline (whatever position the scanout is currently at). The render latency in both situations is identical, it's just a matter of mandatorily starting the latency stopwatch at top edge (for VRR), where VSYNC OFF allows you to start the latency stopwatch at any position in scanout (at the cost of tearline).
Latency gradient during VRR: Pixel row #1 is lowest lag (Zero basis), and increases to the bottom of the screen.
Latency gradient during VSYNC OFF: Pixel row underneath tearline is lowest lag (Zero basis) and increases to the next tearline (including wraparound to next refresh cycle).
So the zone between two tearlines are individual latency gradients.
Now, that said, monitors and drivers vary a bit. There can be some overheads (on some displays and some driver technologies, like generic FreeSync displays) of just merely starting to scan out. But on many esports display, the latency of the zero-basis scanline is identical for VRR and VSYNC OFF -- it's just that the zero-basis scanline is far away from crosshairs (screen center) when it comes to VRR, which means you've got a necessary increasing-latency gradient from top edge of screen towards bottom edge. But at 360Hz, your VRR-penalty for crosshairs latency is as low as 0.5/360sec, but you've got near-zero latency volatility unlike for VSYNC OFF (where VSYNC OFF can jitter in latency from [0....1/360sec] because of random positions of tearlines). It's a lagfeel tradeoff, and different people prefer different.
But one thing for sure, scanouts are getting faster. Yesterday, we had 1/60sec=16.7ms lag difference between top/bottom edge. Today we have 500Hz, so that's 1/500sec=2ms lag difference between top/bottom edge. That means during VRR on a 500Hz display, your VRR lag penalty is only 1ms, but you've also (hopefully) concurrently eliminated jitter/stutter/lag variances. So you've traded off a [0..2ms] latency variance during VSYNC OFF for a constant +1ms latency offset during VRR, since you've locked your zero-basis latency scanline position by virtue of VRR, with zero busywaiting needed (like software based Scanline Sync technologies requires)
In both cases, latency of rendering is the same, so we ignore that difference. And then, latency of VSYNC OFF and VRR is the same for their specific zero-basis scanlines.
However, reviewers usually standardize their photodiode tester position (e.g. screen center), so they don't make this observation.
Not all pixels have the same latency, so you get different lag numbers from different websites, and/or from different sync technologies. Even the decision where in the GtG curve to stop the lag stopwatch at (GtG2%? GtG10%? GtG50%? GtG100%?), can affect lag results, and the differences in VRR overdrive on/off can affect the lag results synthetically. (NVIDIA's G-SYNC premium is very good here when you want the cleanest, most consistent, and fastest VRR overdrive).
If you use VRR in esports, the moral of the story is to:
(A) Purchase more VRR range than you need, so your framerate range is completely within VRR range without worrying about the capping compromises; and
(B) Don't worry about VSYNC ON nor VSYNC OFF, since your framerate stays within VRR range and never activates the fallback sync technology.
(C) Get good overdrive. In this case, sometimes the G-SYNC premium can be helpful, or one of the much better FreeSync implementations, or using OLED VRR, etc.
So that means a 360Hz+ LCD may be preferred by those who insist on using VRR during esports; to avoid the problems of capping, or the lagfeel changes of enter/exit VRR ranges.
I am currently pretty excited about upcoming 360-480Hz OLED monitors too -- compromises-free esports-quality VRR.