G-SYNC 101: G-SYNC vs. Fast Sync


The Limits of Single Frame Delivery

Okay, so what about Fast Sync? Unlike G-SYNC, it works with any display, and while it’s still a fixed refresh rate syncing solution, its third buffer allows the framerate to exceed the refresh rate, and it utilizes the excess frames to deliver them to the display as fast as possible. This avoids double buffer behavior both above and below the refresh rate, and eliminates the majority of V-SYNC input latency.

Sounds ideal, but how does it compare to G-SYNC?

Blur Buster's G-SYNC 101: Input Latency & Optimal Settings
Blur Buster's G-SYNC 101: Input Latency & Optimal Settings
Blur Buster's G-SYNC 101: Input Latency & Optimal Settings
Blur Buster's G-SYNC 101: Input Latency & Optimal Settings
Blur Buster's G-SYNC 101: Input Latency & Optimal Settings
Blur Buster's G-SYNC 101: Input Latency & Optimal Settings

Evident by the results, Fast Sync only begins to reduce input lag over FPS-limited double buffer V-SYNC when the framerate far exceeds the display’s refresh rate. Like G-SYNC and V-SYNC, it is limited to completing a single frame scan per scanout to prevent tearing, and as the 60Hz scenarios show, 300 FPS Fast Sync at 60Hz (5x ratio) is as low latency as G-SYNC is with a 58 FPS limit at 60Hz.

However, the less excess frames are available for the third buffer to sample from, the more the latency levels of Fast Sync begin to resemble double buffer V-SYNC with an FPS Limit. And if the third buffer is completely starved, as evident in the Fast Sync + FPS limit scenarios, it effectively reverts to FPS-limited V-SYNC latency, with an additional 1/2 to 1 frame of delay.

Unlike double buffer V-SYNC, however, Fast Sync won’t lock the framerate to half the maximum refresh rate if it falls below it, but like double buffer V-SYNC, Fast Sync will periodically repeat frames if the FPS is limited below the refresh rate, causing stutter. As such, an FPS limit below the refresh rate should be avoided when possible, and Fast Sync is best used when the framerate can exceed the refresh rate by at least 2x, 3x, or ideally, 5x times.

So, what about pairing Fast Sync with G-SYNC? Even Nvidia suggests it can be done, but doesn’t go so far as to recommend it. But while it can be paired, it shouldn’t be…

Say the system can maintain an average framerate just above the maximum refresh rate, and instead of an FPS limit being applied to avoid V-SYNC-level input lag, Fast Sync is enabled on top of G-SYNC. In this scenario, G-SYNC is disabled 99% of the time, and Fast Sync, with very few excess frames to work with, not only has more input lag than G-SYNC would at a lower framerate, but it can also introduce uneven frame pacing (due to dropped frames), causing recurring microstutter. Further, even if the framerate could be sustained 5x above the refresh rate, Fast Sync would (at best) only match G-SYNC latency levels, and the uneven frame pacing (while reduced) would still occur.

That’s not to say there aren’t any benefits to Fast Sync over V-SYNC on a standard display (60Hz at 300 FPS, for instance), but pairing Fast Sync with uncapped G-SYNC is effectively a waste of a G-SYNC monitor, and an appropriate FPS limit should always be opted for instead.

Which poses the next question: if uncapped G-SYNC shouldn’t be used with Fast Sync, is there any benefit to using G-SYNC + Fast Sync + FPS limit over G-SYNC + V-SYNC (NVCP) + FPS limit?

Blur Buster's G-SYNC 101: Input Lag & Optimal Settings

The answer is no. In fact, unlike G-SYNC + V-SYNC, Fast Sync remains active near the maximum refresh rate, even inside the G-SYNC range, reserving more frames for itself the higher the native refresh rate is. At 60Hz, it limits the framerate to 59, at 100Hz: 97 FPS, 120Hz: 116 FPS, 144Hz: 138 FPS, 200Hz: 189 FPS, and 240Hz: 224 FPS. This effectively means with G-SYNC + Fast Sync, Fast Sync remains active until it is limited at or below the aforementioned framerates, otherwise, it introduces up to a frame of delay, and causes recurring microstutter. And while G-SYNC + Fast Sync does appear to behave identically to G-SYNC + V-SYNC inside the Minimum Refresh Range (<36 FPS), it’s safe to say that, under regular usage, G-SYNC should not be paired with Fast Sync.



1620 Comments For “G-SYNC 101”

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BlurDawg
Member
BlurDawg

Hello again Jorimt, hope you’re having a great day.

I have a question less related to gsync and more to do with render latency. I’ve noticed as I turn the resolution down, the render latency reduces even at the same fps. In fact I have found that @60fps capped the render latency is around 11.5ms at 1080p compared to 16-18ms latency when capped at 60fps 1440p. At 720p the render latency dropped even further.

As far as I understand, render latency is linked to input latency. Does this mean that for example with console games that are capped at 60fps @4k, the input latency is much higher than it could be if the game was instead rendered at 720p? Since the GPU is doing less work per frame?

AidenJr
Member
AidenJr

Hi, this was exactly what I was looking for, but I still have one unanswered question in my mind.
I’m going to buy a 165hz G-sync monitor, and the game that I play runs around 200 fps. Will I necessarily get screen tear if I don’t cap at 162 ? and do you suggest capping the fps at 162, or playing it on higher graphic settings of the game to stay below the monitor’s refresh rate? like 150 fps or so. Since I play FPS games input lag really matters to me.
Regards

Zole
Member
Zole

After reinstalling windows recently, my G-Sync behaviour has changed.

As far as I can remember, my usual set up was:
League of legends played in Windowed Borderless Mode.
v-sync: disabled – in-game
v-sync: enabled (set to “Fast”) – in NVCP
g-sync: enabled for both windowed and full-screen – in NVCP
preferred refresh rate: Highest available – in NVCP
power management: Prefer maximum performance – in NVCP
Monitor technology: G-SYNC – in NVCP
frame rate: uncapped – in game

With these settings, my frame rate was capped by the fast v-sync to 1 frame below my monitors max refresh rate 164 (down from 165).

However, after reinstalling windows and reapplying these same settings the frame rate is no longer capped to 1 below the monitors refresh rate. Instead I get FPS anywhere from 200-600 and I notice stutters and tearing.

Is there any way for me to get back my previous system behaviour?

PS I know the recommended way to set up a system is for full-screen g-sync but I prefer windowed borderless for the rapid alt tabbing, as I do that frequently.

brdon209
Member
brdon209

I am looking to buy a 1440p 144hz monitor and from what I have read, I should set my FPS at 141 using nvcp, set vsync on in nvcp and off in game. However, many sources have told me to turn on null on and others have told me to completely turn it off. Should I put it on ultra, on or off? I play league of legends fyi and my cpu is usually around 16% usage and my gpu is around 30% uncapped 1080p. Sorry, I am new to th this subject .

georgi74
Member
georgi74

I have a System with a Geforce 3080 and as display I use a LG c9 OLED TV with 120Hz and GSync.

In Nioh 2, which can easily perform over 120 fps with DLSS on 4k with that System if I limit the game to 117 fps with RTSS I get Micro Stutter when just looking around. If I disable the RTSS limiter and let the game limit the fps to 120 (in game there is only an option to lock at 30/60/120) the game is buttery smooth. Is there any way to get the game running smooth with a limit to 117 fps?

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