NVIDIA Fast Sync

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NVIDIA Fast Sync

Postby Glide » 17 May 2016, 10:01

NVIDIA are introducing a new feature with Pascal GPUs: Fast Sync
As I understand it, this is basically true triple-buffering implemented at the driver level for all games.
As far as the game is concerned, V-Sync is off, but the GPU will only present fully rendered frames to the display.


https://www.pcper.com/reviews/Graphics- ... mers/SLI-C

FastSync splits render and display pipelines

I sure hope you weren’t tried of technologies that end with “sync” because NVIDIA is releasing another one with the GTX 1080. FastSync is an alternative to Vsync On and Vsync Off states, but is not variable like G-Sync or FreeSync. The idea is straight forward: decouple the render pipe from the display pipe completely and there a lot of interesting things you can do. FastSync tells the game engine that Vsync is OFF, allowing it to render frames as fast as possible. The monitor is then sent frames at its maximum refresh rate but only completely rendered frames, avoiding the tearing artifacts usually associated with Vsync Off states.

Image

FastSync creates a virtual buffer system that includes three locations. Front buffer, back buffer and the last rendered buffer. The front buffer is the one that is scanned out to the monitor at the same speed as the display refresh rate. The back buffer is the one that is being rendered to by the GPU and cannot be scanned out until it’s complete. The last rendered buffer will hold all new frames just completed in the back buffer, essentially saving a copy of the most recently rendered frame by the game. When the front buffer is finished scanning to the display, then the last rendered buffer would be copied to the front buffer and scanned out.

Interestingly, because buffer copies would take time and add latency, the buffers are just dynamically renamed. In high frame rate games the LRB and BB would switch positions concurrently at the render rate of the application, and when the FB had completed its most recent scan out, the current LRB would be renamed to the FB, immediately starting its scan out.

The usage model for FastSync is games that are running at very high frame rates (competitive gaming) and thus have to decide between the high input latency of Vsync On or the screen tearing of Vsync off. For CS:Go gamers that are used to hitting 200 FPS, you’ll be able to play the game tear-free with only a very slight increase in latency, about 8ms according to NVIDIA.

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This is definitely something that should only be enabled in the NVIDIA Control Panel for those games that are running at frame rates well above the maximum refresh rate of your display. FastSync will be its very nature introduce some variability to the smoothness of your game, as it is “dropping” frames on purpose to keep you time with the refresh of your monitor while also not putting backpressure on the game engine to keep timings lined up. At high frame rates, this variability issue isn’t really an issue as the frame times are so high (200 FPS = 5ms) that +/- 5ms is the maximum frame to frame variance you would see. At lower frame rates, say 45 FPS, you could get as much as a 22ms variance.

FastSync is a cool new feature to improve the experience of FAST games, but don’t think NVIDIA has found a free alternative to variable refresh rate technology.

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NVIDIA did state that FastSync was coming to Maxwell as well, and possibly even Kepler graphics cards.
Glide
 
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Re: NVIDIA Fast Sync

Postby Sparky » 17 May 2016, 10:23

This is basically just fullscreen windowed in fullscreen mode. It's still going to have judder proportional to frametime. The advantage over g-sync is you can use it with low persistence displays.
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Re: NVIDIA Fast Sync

Postby Glide » 17 May 2016, 10:46

Well it's still going to be full-screen exclusive mode with the GPU talking directly to the display instead of passing through the desktop compositor.

But yes, it's a latency-focused feature, rather than one focused on smoothness.
Good point about low-persistence displays though.

I wish that NVIDIA would enable G-Sync even with ULMB active, as that would be amazing for emulation, or older games where you can sustain a 100% constant framerate with no drops ever.
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Re: NVIDIA Fast Sync

Postby Sparky » 17 May 2016, 11:14

I'll admit I was a bit disappointed, I was hoping it was a refresh synchronous framerate cap, so you can get low latency vsync AND smooth animation, provided you have consistent frametimes.
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Re: NVIDIA Fast Sync

Postby Glide » 17 May 2016, 12:45

I think that would be front-buffer rendering, which NVIDIA are restricting to VR Works. :(
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Re: NVIDIA Fast Sync

Postby Sparky » 17 May 2016, 13:23

Watched a video of the nvidia presentation on it, and it's mostly what I expected. They did confirm that it can be enabled alongside g-sync, as the fallback case when framerate>refresh rate.

I think that option is still inferior to g-sync with an in-game framerate cap in terms of both latency and animation smoothness.
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Re: NVIDIA Fast Sync

Postby Glide » 17 May 2016, 13:41

Well no-one has suggested that it is a replacement for G-Sync.
It's another option for those of us without G-Sync.
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Re: NVIDIA Fast Sync

Postby Trip » 25 May 2016, 13:18

Just so everyone knows you can enable fast sync through nvidia inspector already with their latest official driver 368.22. If you go to a game profile it is under the vertical sync tab labeled as 0x1888888. I tested it on a kepler card (gtx 780) on call of duty 4 using unlocked fps. It feels pretty good actually but you have to get very high fps (at least twice refresh rate) to get a noticeable difference in perceived input lag. The usual stutter associated with vsync and this particular game is also not present. Seems like a very good way to decrease the stuttering and the latency especially for low persistence monitors. Although I doubt it will ever be used in a competitive setting like they advertised it. But it certainly works and I'm pretty happy that they included this after all these years.

I found about this from somewhere else but cant find the source anyway props to that person.
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Re: NVIDIA Fast Sync

Postby Glide » 25 May 2016, 14:52

Tested a few games so far and the only one it actually worked in was Half-Life 2.
It doesn't seem to do anything for OpenGL, and older DirectX games had severe graphical corruption.

Edit: For OpenGL I was testing the new Doom, and Quakespasm.
It may be pre-DX9 games that aren't working properly, such as Max Payne 2.
Last edited by Glide on 25 May 2016, 16:53, edited 3 times in total.
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Re: NVIDIA Fast Sync

Postby Trip » 25 May 2016, 16:08

Dunno I only tested it in cod4 because thats pretty much the only game I have installed at the moment where I can get very high framerates. Worked without glitches atleast.
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