https://bugs.kde.org/show_bug.cgi?id=448496
Bug ID: 448496
Summary: latency reduction can negatively affect fullscreen
gaming, apparently despite direct scan-out
Product: kwin
Version: 5.23.90
Platform: Archlinux Packages
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: wayland-generic
Assignee: kwin-bugs-n...@kde.org
Reporter: tempel.jul...@gmail.com
Target Milestone: ---
SUMMARY
Setting latency reduction to "force lowest latency" introduces stutter by
dropped frames for Counter-Strike 1.6 (native Linux OpenGL XWayland)
fullscreen. It doesn't stutter (at least not more than it should due to the
unfortunate mailbox vsync) when setting latency reduction to "force smoothest
animations".
STEPS TO REPRODUCE
1. On a really slow Gemini Lake system, run Counter-Strike 1.6 via Steam inside
Plasma Wayland (Sorry, I don't have access to faster devices that aren't with
Nvidia GPU.).
2. Hold down left or right arrow key to continuously pan the camera.
3. Test with "force lowest latency" vs. "force smoothest animations".
OBSERVED RESULT
"force lowest latency" affects smoothness of presentation. According to
mangohud, the game's rendering performance is fine (~150fps on 60Hz display),
so stutter seems to be happening at presentation level.
EXPECTED RESULT
I think ideally, latency reduction shouldn't affect presentation for fullscreen
gaming? The game seems to trigger direct scan-out, as fps are the same as on
Xorg with modesetting DDX and compositing suspended. I also updated to latest
mesa git-master to make sure dma-buf feedback hopefully is available, but
situation didn't change vs. stable Mesa from Arch repo. With Plasma 5.23
Wayland, performance was much lower (~110fps instead of ~160), which should be
another indicator for direct scan-out being active with 5.23.90.
SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Linux 5.16
KDE Plasma Version: 5.23.90 via kde-unstable Arch repo
KDE Frameworks Version: 5.90
Qt Version: 5.15.2
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