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.



1149 Comments For “G-SYNC 101”

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AlianosFN
Member
AlianosFN

Hi, I have a question.

I play Fortnite competitively on a 165 Hz 1440p monitor with a GTX 1070 Ti using pretty much all the settings you recommended in this thread (gsync ON, vsync ON, game on fullscreen, fps limited in game files to 162, and finally low latency mode ON).

Now since the nvidia driver update from today got released the new nvidia Reflex Low Latency setting was added to the game. With the setting come 3 options: OFF, ON or ON + BOOST. A thing i noticed when i turned it to either ON or ON + BOOST was that instead of my fps cap being 162, my fps got capped at 158 (so 4 frames lower than the limit i set myself).

This raised a question;
Does turning the setting ON (or ON + BOOST) impact the way gsync works (also in combination with the other recommended settings in this thread)?
If so, what would you recommend me to do? Should i keep the nvidia reflex setting off? Or should i keep it on (since according to nvidia it reduces system delay) and change my fps cap to make up for the 4 frames i lost?

Ultimately i think my question is; how does the nvidia reflex low latency setting fit in the picture with gsync and the recommended settings in this thread?

Thanks in advance for taking the time to read/answer to my questions!

tomato884
Member
tomato884

does this apply to the new 30xx series with all the new reflex technology?

JediBrysen
Member
JediBrysen

should competitive players for any game but for my situation fortnite use this? I really just want low input delay while it being smooth

ankurszn
Member
ankurszn

i have the same question

Avean
Member
Avean

Could LLM = On with G-Sync cause frametime spikes in cases where the game uses its own frame queue? Cause in the end of the article you say you recommend testing this on per-game basis to see if worse or not. And if at best youre improving latency by 1 frame, its kinda best to leave it off i feel? Unless you want to fire up RivaTuner and monitor each game to see if improvements. But really the biggest impact is more framerate than whatever setting your LLM is at, feel people are kinda hung up on this setting 🙂

Glarbstintenford
Member
Glarbstintenford

Amazing guide and information, thank you so much!

I have a questions or two, sorry this ends up being so long but I wanted to also share the information I found and make it as clear as possible:

1 – Does the use of the smoothframes feature found in Unreal Engine 4 have any effect? I’m not sure how smoothframes & in game Vsync relate to each other with or without G-Sync and thought you could shed some light.

The game I play is online multiplayer only, and their servers currently run 30Hz tickrate, which in the near future will be increasing to 60Hz tickrate.

It has smoothframes and vsync enabled by default, using the default 22-62FPS range for smoothframes, with no in-game option to disable either or set an FPS limit. It also has what I am assuming is an in-game FPS limiter that you can manually adjust via editing the ini files. This line is FrameRateLimit= and the default is =0.000000.

Also just so you know this game has no in-game settings other than a resolution scale slider that’s it, and only runs in borderless fullscreen.

However you can edit GameUserSettings.ini and disable/enable vsync & smoothframes.

Now with or without G-Sync enabled I have discovered the following:

– If I use the games default settings of vsync and smoothframes enabled and FrameRateLimit=0 I am limited to 62FPS regardless of desktop refresh rate provided that my desktop refresh rate is higher than 63Hz **However if I set FrameRateLimit to any number up to and including 120 then my FPS limited to that number unless my desktop refreshrate is lower than the value set in FrameRateLimit= in which case it is limited to desktop refresh rate.

– If I disable in-game vsync and leave smoothframes enabled I am still limited to 62FPS regardless of desktop refresh rate if FrameRateLimit=0

– If I keep in-game vsync enabled and only disable smoothframes my FPS is capped to desktop refresh rate but only to 120 any refresh rate above that I’m still limited to 120FPS. Also worth noting this is when leaving FrameLimit=0.000000

– If I disable in-game vsync & disable smoothframes and FrameRateLimit=0 then my FPS is uncapped.

Wonderinig if smoothframes should be enabled or disabled, as well as if using FrameRateLimit should be used and am curious what your thoughts are about it all.

Thank you! 🙂

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