Do Fullscreen Optimizations Affect Input Latency?

[Mark Fable]

For this post, FSO stands for Fullscreen Optimizations and FSE stands for Fullscreen Exclusive.

I have done a lot of reading lately on the topic and high-level implementation of FSO in Windows 10 (which I use) and I still have a few specific questions about how this impacts the latency from peripherals (from key press to display). I would greatly appreciate informed answers to these questions as opposed to speculation:

1. Does disabling FSO cause Windows to treat games like how it once did, before Windows 10, by allowing games to run in traditional FSE mode?

2. If the answer to (1) is yes, then is it correct to say that FSO does not affect the latency of peripherals?

3. If the answer to (2) is also yes, then what would be the biggest disadvantage while in-game (if any) to disabling FSO?

My goal is to make sure that by me disabling FSO in all my games, I am only helping and never hurting system latency. I do not believe that running games in traditional FSE mode should affect how Windows handles input from the mouse and keyboard.

[bumbeen]

1. I believe this works in windows 10, not sure about windows 11 though. It gives the GPU control of the display without DWM in the mix.

2. FSO can affect display latency if you do not have multi-plane overlay enabled because the frame is still handled by DWM, although the effect is supposed to be minimal. If you do have multi-plane overlay enabled AND the game is using flip presentation via either SpecialK, the game engine itself, or any DX12 game, then there is no latency difference between FSO and FSE.

3. The answer to 2 is no if you have multi-plane overlay disabled and/or do not have independent flip presentation mode in the given game. Regardless the biggest disadvantage is slower alt tab and no overlay support.

FSE is always going to be minimal latency anyway, but in windows 11 or with special k and MPO, you do not need to give up alt tab or (temporary) overlays to get equivalent to FSE latency, so there's little point in disabling FSO and using FSE.

i'll note some gpu have had issue with MPO causing flickering though, so if that's the situation then FSE is definitely better than flickering.

[Chief Blur Buster]

1. Does disabling FSO cause Windows to treat games like how it once did, before Windows 10, by allowing games to run in traditional FSE mode?

2. If the answer to (1) is yes, then is it correct to say that FSO does not affect the latency of peripherals?

3. If the answer to (2) is also yes, then what would be the biggest disadvantage while in-game (if any) to disabling FSO?

My goal is to make sure that by me disabling FSO in all my games, I am only helping and never hurting system latency. I do not believe that running games in traditional FSE mode should affect how Windows handles input from the mouse and keyboard.

As you so suggested, the answer is not always clear cut, so the automatic answer is use FSE whenever possible.

Historically, FSE is the most direct way for games to access the monitor. More sync technologies are more compatible with FSE with no effect.

FSO is getting better and better over time with more game and drivers behaving well with it -- and in many situations does feels equivalent (i.e. no difference), but it is not quite equivalent for all possible configurations (game, refresh rate, frame rate, sync technology in use, multimonitor configuration, presentation hooks like RTSS or SpecialK etc)

If you can vet that FSO works well, then FSO is fine. However, in a critical esports situation, where championship money is involved, I would err on the safe side and use FSE. That being said, if that's not your situation, experimenting with FSE vs FSO and their pros/cons, may be the rule of the day.

Bear in mind, the biggest problem appears to occur with multimonitor systems, and in this situation, FSE is still king in preventing a 2nd monitor from jittering/lagging the primary.

[OniiChan69]

So should I disable FSO if I have MPO disabled on Windows 10?

[bumbeen]

So should I disable FSO if I have MPO disabled on Windows 10?

sorry for the necro, we never replied to this and i found this on google so this is more of a reply for the people googling.

Yes you should, because without MPO, if you use FSO, then you're getting a latency penalty since DWM now has to composite the image for you instead of letting the GPU scan out the buffer straight to your display. Without MPO, FSE is always going to be faster.

[JimCarry]

So should I disable FSO if I have MPO disabled on Windows 10?

sorry for the necro, we never replied to this and i found this on google so this is more of a reply for the people googling.

Yes you should, because without MPO, if you use FSO, then you're getting a latency penalty since DWM now has to composite the image for you instead of letting the GPU scan out the buffer straight to your display. Without MPO, FSE is always going to be faster.

[RealNC]

sorry for the necro, we never replied to this and i found this on google so this is more of a reply for the people googling.

Yes you should, because without MPO, if you use FSO, then you're getting a latency penalty since DWM now has to composite the image for you instead of letting the GPU scan out the buffer straight to your display. Without MPO, FSE is always going to be faster.

Not in fullscreen mode. MPO is for windowed mode. FSO only activates for exclusive fullscreen games, and MPO is not required to bypass DWM.

In other words, FSO on vs off has no latency penalty.

In Widows 11 however, there is windowed mode optimization as well, which does require MPO, but this is a different matter and is independent of FSO.

[bumbeen]

sorry for the necro, we never replied to this and i found this on google so this is more of a reply for the people googling.

Yes you should, because without MPO, if you use FSO, then you're getting a latency penalty since DWM now has to composite the image for you instead of letting the GPU scan out the buffer straight to your display. Without MPO, FSE is always going to be faster.

Not in fullscreen mode. MPO is for windowed mode. FSO only activates for exclusive fullscreen games, and MPO is not required to bypass DWM.

In other words, FSO on vs off has no latency penalty.

In Widows 11 however, there is windowed mode optimization as well, which does require MPO, but this is a different matter and is independent of FSO.

Question: MPO does not alleviate the latency penalty created by FSO on w10? It's still going to get composited through dwm regardless? This is why I use windows 11, so I can alt tab easily without incurring latency penalty.

[RealNC]

Question: MPO does not alleviate the latency penalty created by FSO on w10? It's still going to get composited through dwm regardless? This is why I use windows 11, so I can alt tab easily without incurring latency penalty.

There is no extra latency with FSO and MPO is not needed. DWM is bypassed.

[bumbeen]

Question: MPO does not alleviate the latency penalty created by FSO on w10? It's still going to get composited through dwm regardless? This is why I use windows 11, so I can alt tab easily without incurring latency penalty.

There is no extra latency with FSO and MPO is not needed. DWM is bypassed.

by definition if the gpu is not scanning out the game to its own plane or display there will be additional latency, no? There's something between the gpu and the screen unless you're using FSE or MPO