[Win/Libretro] "Delay Frame Swapping ON" + "Threaded rendering OFF" make some games stutter heavily in DX11/GL

[Immersion95]

"Delay Frame Swapping" is now default as it's the intended behavior of the Dreamcast.

Delay Frame Swapping. originally flycast (and reicast) would display the frame as soon as it is rendered. However, on real hardware the frame is only displayed at the next vblank interrupt. This is what Delay Frame Swapping does, and it should be on by default.

It make some games stutter heavily, it is reproducible across all of of my computer setups (Windows 11/Latest RA/GL/DX11/Ryzen 7600/AMD 7900 XTX/Nvidia 4070 Super). Only Vulkan is perfect.

It's a stutter at a regular interval, as if the FPS synchronization is off. It doesn't seem to be a problem with performance.

Games that are stutttering : Metal Slug 6, Dolphin Blue, SFIII Double Impact. Need to test other games too

Example : Dolphin Blue - Shot

With Threaded rendering OFF :

• 2 frames without Delay Frame Swapping - Great but hacky as it's not what is supposed to be - Smooth FPS
• 3 frames with Delay Frame Swapping - Accurate and exactly as intended on real hardware - Stutter

With Threaded rendering ON :

• 4 frames without Delay Frame Swapping - 2 more frames than with Single core rendering - Smooth FPS
• 4 frames with Delay Frame Swapping - Same input lag with or without Delay Frame Swapping - Smooth FPS

The accurate behavior should be with Delay Frame Swapping, unfortunately it's only completely smooth with Threaded rendering ON and it adds input lag.

1. Related issue regarding threaded rendering adding input lag : Threaded rendering adds a frame of input lag libretro/flycast#738
2. Related issue regarding Delay Frame Swapping adding stuttering : https://forums.libretro.com/t/flycast-delay-frame-swapping/28521

[flyinghead]

Nice try vatnik:

[mfigueirido]

I have similar issues in many games in RA.
Threaded rendering OFF and Delay Frame Swapping OFF still produce stuttering for me.
Threaded rendering ON produces audio stalls in RA so the only way to get clean audio is this one documented here:
#1062 (comment)

[Immersion95]

This only affects DX11/GL, it's totally smooth in Vulkan. It would be great if this was smooth across all backends though :).