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.



347 Comments For “G-SYNC 101”

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dotyerts
Member
dotyerts

I’ve been reading your stuff for years. Appreciate all of it.
Let me get this summarized in one quick writeup.

I have a freesync compatible monitor viotek gft27db 144hz. If i enable G-SYNC properly with my DP cable on an RTX 2070, and have VSYNC ON, as well as + NULL, then I should
1. be all good in terms of enabling GSYNCwithout the issues of VSYNC off causing tearing sometimes.
2. NOT be required to Limit my fps to 141 by using RTSS? I’m currently using it limited to 141 to stay under and avoid the buffer flip interval.
2.b. wondering if this is the case, so I can turn off RTSS and retrieve that minuscule amount of input lag it causes.

nuk_won
Member
nuk_won

Tell me if I am wrong. But I think I figured it out. G-sync it self don’t ad input lag so if i have it I might as well turn it on to get stutter/screen tearing free game play. So as I understand u turn g-sync on + vsync off find the fps by caping it 165 then 164 ect where there is no screen tearing at bottom of the screen let’s say it would be 158 then u go Nvidia control panel turn g-sync on vsync off set low latency to ultra and cap ur fps at 158 and this is the way u get lowest input lag + as it would be running a monitor who don’t have g-sync and just caping fps at 165fps but this way u don’t get the benefits as smooth game play no stuttering. This would be best only for GPU bound games like apex legends other games that use CPU this could increase input lag? Please correct me if I’m wrong or maybe I need to turn g-sync off set fixed refresh rate null set to ultra fps cap at 165. Main game is apex legends. This I how I run the game currently.

Specs pc: 87k at 4.5gh 1080ti oc. Monitor: AOC 24″ 2k rez 165hz g-sync.

Ownsin
Member
Ownsin

Hey there. Can I ask If I should turn on V-Sync in-game or from the Nivida Control Panel for my G-Sync 144hz monitor? I recently watched a video for Battlenonesene where he says turning on V-Sync inside of games is better than turning it on from the NVCP because some engines have optimizations for their V-Sync. What’s your take on this matter?

Another thing I want to bring up. Can you tell me what’s the difference between turning on V-Sync and Fastsync from the NVCP? as far as I know, Fastsync eliminates tearing, leaves the frame rate unlocked and doesn’t add nearly as much input lag as regular V-Sync, so why is it recommended that we use V-Sync instead of Fastsync?

P.S: which frame limiter do you suggest I should use.

Thank you.

gzmm
Member
gzmm

Hello, i have a samsung C24FG73, with 70~144hz freesync range.
for me, gsync+vsync in driver with 138 fps limiter seens to be working very well.
but, if a game is locked at 60 frames, the gsync will be ‘disable’, and only vsync will work, correct?
this will introduce alot of input lag, or the FLC helps?

buddersnaps
Member
buddersnaps

Hi when running a game that GPU is using 100% resources do i still want NVCP v sync set ON and low latency set to Ultra. i have g sync monitor thats 144hz. i have games capped at 141 but get about 100 – 120 most of the time. are my settings still correct when getting frames below monitor refresh rate and GPU bound using NVCP v sync ON, low latency ULTRA, Gsync ON, and in game fps capped to 141 and vsync off?

then to clarify when not gpu bound just use the “on” option instead of ultra for low latency?

also monitor has ULMB or should i just stick with gsync? if i used ULMB what would i do for vsync and latency options?

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