https://bugs.kde.org/show_bug.cgi?id=493295
Bug ID: 493295
Summary: Substantially worse software rendering performance
with KDE 6.2 beta compared to 6.1 (Fedora Rawhide)
Classification: Plasma
Product: kwin
Version: 6.1.90
Platform: Fedora RPMs
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: performance
Assignee: kwin-bugs-n...@kde.org
Reporter: ad...@happyassassin.net
Target Milestone: ---
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
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