App time variance about bevy_framepace HOT 6 OPEN

Since vsync is not always on, you'd need an additional framerate limiter just prior to the frame presentation step, which can absorb any remaining time to reach the target frametime.

FWIW, this was actually the initial design of this plugin.

0ms -----------------------
            Estimate how long the next frame will take, minus a small margin to give us space if it takes longer
            Sleep for this duration. (Forward estimation)
8ms         Start of event loop. Get input. Do stuff.
11.8ms      See how close our estimate was to the requested frame time, sleep if required to get the frametime just right
12ms        Send to GPU, present frame (input is 4ms out of date)
16ms ----------------------

That was taken from my comments in the initial discussion that spawned this plugin: bevyengine/bevy#3317

However, in practice I had a lot of trouble getting good results from it. This may have predated my use of spin_sleep, so it may be worth revisiting that approach.

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@MatthewScholefield a comprehensive rework of this crate is blocked by bevy's render code, which doesn't currently expose timing information for presenting frames.

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However, in practice I had a lot of trouble getting good results from it. This may have predated my use of spin_sleep, so it may be worth revisiting that approach.

Yeah you'd essentially need to duplicate the framepace PID controller for the framerate limiter. You need very accurate sleep times.

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Yup, I did play around with writing a PID controller, but I may have been limited by platform sleep time accuracy/variance. Definitely worth revisiting this, I appreciate your deep-dive.

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Appreciate the investigation into this. Would be great to get this improved. I can replicate frequent stutters on my machine under Wayland.

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Unfortunately that is the case. I can take another look at the PID impl if I get the time, but it's low on my priority list as I expect improvements will be minor until we can properly measure vblanks.

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