https://bugs.kde.org/show_bug.cgi?id=446478
Bug ID: 446478
Summary: Plasma desktop unusable due to stuttering with
compositing enabled on VRR display with gsync enabled
Product: kwin
Version: 5.23.3
Platform: Other
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: general
Assignee: kwin-bugs-n...@kde.org
Reporter: e.bachma...@gmail.com
Target Milestone: ---
Created attachment 144213
--> https://bugs.kde.org/attachment.cgi?id=144213&action=edit
perf at 145Hz for 10s on kwin_x11 when gsync-enabled
SUMMARY
On my single-monitor x11 setup, enabling (a.k.a. "not disabling") gsync in the
nvidia-settings causes everything to stutter. This is new since my distro
upgrade (see versions below). Happens both logged-in and when sddm is running.
It gives the impression that "processing speed" is coupled with the refresh
rate. For example, with an empty desktop/no visual movement, triggering krunner
takes multiple seconds ... every time. When I trigger it while quickly dragging
a window (thus raising the VRR) it happens ~instantly. (It's almost as if it
takes a fixed number of frames to show it.)
When disabling gsync or compositing, fps is at a locked 144 and no problems
occur.
This bug sounds somewhat overlapping with #445330 but they explicitly say "x11
is fine" and don't mention VRR.
STEPS TO REPRODUCE
1. Have plasma5 desktop running on X11
2. Open nvidia-settings
3. Toggle "Allow G-SYNC/G-SYNC Compatible" from off to on
OBSERVED RESULT
1. FPS counter (built into the display itself) unlocks from 144 to a variable
number roughly proportional to amount of movement on screen, as low as 1 FPS.
2. During "low FPS times" everything stutters, there are large delays (e.g.
krunner or kickoff)
EXPECTED RESULT
1. either locked max fps (windows does this)
2. or instant fps scaling without any delays (phones do that I believe)
SOFTWARE/OS VERSIONS
KDE Plasma Version: 5.23.3 (prev: 5.21.5)
KDE Frameworks Version: 5.87.0 (prev: 5.81.0)
Qt Version: 5.15.3 (prev: 5.15.2)
ADDITIONAL INFORMATION
Kernel: 5.15.4 (prev: same)
Distro: NixOS 21.11.333896.a640d8394f3 (prev: 21.05)
Nvidia driver: 495.44 (prev: 470.63.01)
I was curious as to how kwin is handling this internally so peeked at it a bit,
see attachments. [gsync-on variant seems to eat quite a bit more cycles and
spend a lot of time within destructors?]
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