https://bugs.kde.org/show_bug.cgi?id=469777
Bug ID: 469777
Summary: KWin since 5.27.5 uses 1 CPU core at 100% whenever
something is using PipeWire to capture the screen
Classification: Plasma
Product: kwin
Version: 5.27.5
Platform: Archlinux
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: wayland-generic
Assignee: kwin-bugs-n...@kde.org
Reporter: putr...@gmail.com
Target Milestone: ---
SUMMARY
After updating to 5.27.5 whenever something uses PipeWire to capture the screen
(I tried OBS, Krfb and Firefox) KWin starts using 100% of one CPU core, even
when the app isn't doing anything with the screen capture (like having OBS open
without it streaming/recording or Krfb with no active connections). On my
system with 8 cores and 16 threads that translates to about 6-7% overall CPU
usage, but it also causes the CPU to use its turbo state constantly to keep up
as a result.
Reverting to KWin 5.27.4.1 makes the CPU usage go back down to more normal
levels, and building KWin 5.27.4.1 with this MR
(https://invent.kde.org/plasma/kwin/-/merge_requests/3970, screencast: Offer
the real framerate range we have available) applied to it causes the issue to
come back.
I guess that MR might just be exacerbating an existing issue though, I have
been getting `kwin_screencast: Dropping a screencast frame because the
compositor is slow` messages appear repeatedly in journalctl anytime a
screencast session is active, but with 5.27.5 as well as with 5.27.4.1 with
that MR applied I get way more of those (from like 20 per second to well over
1000 per second).
With Krfb in particular the issue is particularly terrible, even after closing
it the CPU usage remains at the same high level.
STEPS TO REPRODUCE
1. Update to KWin 5.27.5 or build KWin 5.27.4.1 with the aforementioned MR
applied
2. Start a screencast session (either from OBS, Firefox or Krfb)
OBSERVED RESULT
KWin's CPU usage is ridiculously high.
EXPECTED RESULT
KWin should not consume so much CPU.
SOFTWARE/OS VERSIONS
Operating System: Arch Linux
KDE Plasma Version: 5.27.5
KDE Frameworks Version: 5.105.0
Qt Version: 5.15.9
Kernel Version: 6.3.2-arch1-1 (64-bit)
Graphics Platform: Wayland
Processors: 16 × AMD Ryzen 7 6800H with Radeon Graphics
Memory: 14.9 GiB of RAM
Graphics Processor: AMD Radeon Graphics
Manufacturer: HP
Product Name: OMEN by HP Gaming Laptop 16-n0xxx
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