SUMMARY STEPS TO REPRODUCE 1. Make sure compositor is enabled with OpenGL 2.1 and vsync set to auto. 2. Watch video on Firefox or start chat app with video support like Skype. OBSERVED RESULT Over some time one can see small white horizontal lines on the screen (with around 4-5 mm in width). I have tried different vsync options in composer section but the only thing that made the lines to go away is to set vsync off (with side affect that tearing appeared). EXPECTED RESULT No white lines should appear. SOFTWARE/OS VERSIONS Windows: macOS: Linux/KDE Plasma: Ubuntu 20/x64 (had the same issue with Ubuntu 18.04) (available in About System) KDE Plasma Version: 5.18.5 KDE Frameworks Version: 5.68.0 Qt Version: 5.12.8 ADDITIONAL INFORMATION OpenGL Information GL_VENDOR: Intel Open Source Technology Center GL_RENDERER: Mesa DRI Intel(R) Iris(R) Pro Graphics P5200 (HSW GT3) GL_VERSION: 3.0 Mesa 20.0.4
Created attachment 129024 [details] Visualization of the issue.
Additionally the white lines show briefly and go away very fast. They are noticeable when you play video with black background.
> 1. Make sure compositor is enabled with OpenGL 2.1 and vsync set to auto. If you switch to another render backend, e.g. XRender, or disable compositing altogether, are you still able to see the visual artifacts from time to time?
(In reply to Vlad Zahorodnii from comment #3) > > 1. Make sure compositor is enabled with OpenGL 2.1 and vsync set to auto. > > If you switch to another render backend, e.g. XRender, or disable > compositing altogether, are you still able to see the visual artifacts from > time to time? I think only OpenGL but not sure. I will try with xrender.
I remember now - I am not using xrender since tearing is very bad (with all possible vsync options).
I have also tried to disable all effects and that did not help. When I do dd if=/dev/sda of=/dev/null problem is more prominent. What should I try next?
Here are more findings - I definitely could see the problem when I execute update-grub. I see this only when video is playing in background (firefox). If I use Xrander there are no issues. If I disable the vsync no issues for all. So to sum up to reproduce: 1. Start firefox and start video on youtube. 2. Execute update-grub Result: The whole screen is flashing with white lines.
And in a twist of the events it turned out I can reliably reproduce with this command: dd if=/dev/random of=/dev/null bs=4096 count=1000000 oflag=sync
I have disabled haveged and things are much much better. I rarely see the lines. It seems the issue is with the kernel since I also test this with compton as well.
Unfortunately that was just mitigation - lines are there. The only way to fix this for me was to compile this mr https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/24/ which has TearFree code and turn off compositor vsync.
This bug was reported against an outdated version of KWin. We have made many changes since the. If the issue persists in newer versions can you reopen the bug report updating the version number.