Bug 493295

SUMMARY
As reported at https://bugzilla.redhat.com/show_bug.cgi?id=2312900 , I noticed a Fedora openQA test has been failing quite often since KDE 6.2 Beta landed. The test is of Firefox, and it fails because Firefox does not complete startup within the timeframe the test expects. Playing around with this manually, I found it seems to be because KDE 6.2 performs far worse with software rendering than 6.1 did. (The openQA tests run in VMs with virtio graphics and without 3D passthrough; the host systems have weak graphics cards and run 30 tests simultaneously, 3D passthrough is impractical in such a configuration).

I was able to distil this down to a pretty simple test: boot a Fedora live image on a bare metal system with 'nomodeset' to force software rendering, open a terminal, run 'top' in it, then drag it around in circles for a while and observe the CPU usage stats as you do it. On an image with the 6.2 Beta update - https://bodhi.fedoraproject.org/updates/FEDORA-2024-e78c579506 - the movement of the window is jerky, and kwin_wayland CPU usage spikes to 750-800% on my test system. On an earlier image with 6.1, the movement of the window is smooth, and kwin_wayland CPU usage peaks around 300-350%.

I tested with https://kojipkgs.fedoraproject.org/compose/rawhide/Fedora-Rawhide-20240914.n.0/compose/Spins/x86_64/iso/Fedora-KDE-Live-x86_64-Rawhide-20240914.n.0.iso (6.1-based image) and https://kojipkgs.fedoraproject.org/compose/rawhide/Fedora-Rawhide-20240917.n.0/compose/Spins/x86_64/iso/Fedora-KDE-Live-x86_64-Rawhide-20240917.n.0.iso (6.2-based image). There are other differences between these nightlies, of course, but the openQA test results make it fairly clear that the KDE 6.2 update is what triggered this problem. Before that update, it was not happening; as soon as that update landed, it started happening. The images are garbage-collected after two weeks, so the 6.1 one will no longer be available around September 28.

STEPS TO REPRODUCE
1.  Boot a KDE system, forcing use of software rendering (e.g. with nomodeset)
2. Observe performance with typical desktop tasks (navigating system menus, running applications, moving windows)
3. Check CPU usage in a top utility, especially kwin_wayland

OBSERVED RESULT
With a 6.2 image, all aspects of performance are significantly worse than with a 6.1 image

EXPECTED RESULT
6.2 should perform as well as 6.1 or better

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Fedora Rawhide
KDE Plasma Version: 6.1.90