Bug 509924 - High CPU usage in kwin_wayland while Firefox is open due to KWin::SubSurfaceInterface::parentApplyState()
Summary: High CPU usage in kwin_wayland while Firefox is open due to KWin::SubSurfaceI...
Status: RESOLVED UPSTREAM
Alias: None
Product: kwin
Classification: Plasma
Component: performance (other bugs)
Version First Reported In: 6.4.5
Platform: openSUSE Linux
: NOR normal
Target Milestone: ---
Assignee: KWin default assignee
URL:
Keywords: efficiency-and-performance
: 513066 (view as bug list)
Depends on:
Blocks:
 
Reported: 2025-09-25 12:30 UTC by Jure Repinc
Modified: 2025-12-08 00:44 UTC (History)
3 users (show)

See Also:
Latest Commit:
Version Fixed/Implemented In:
Sentry Crash Report:

Attachments
perf data (2.95 MB, application/x-xz)
2025-09-25 12:30 UTC, Jure Repinc 		Details
flamegraph SVG (514.39 KB, image/svg+xml)
2025-09-25 13:46 UTC, Jure Repinc 		Details
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Description Jure Repinc 2025-09-25 12:30:44 UTC 
Created attachment 185247 [details]
perf data

SUMMARY
While I have Firefox web browser open I can see high CPU usage (about 35% of one core of AMD Ryzen 7 7840HS) in kwin_wayland process even when I don't do anything in Firefox (e.g. no video is playing and nothing unusual shows up in about:processes) and it is in the background. I have a lot of tabs open in several windows of Firefox. I tried to close all windows but one and have not yet managed to find out if some specific tab in the last open window is causing this. I have profiled the kwin_wayland process with perf, the profile info is attached.

STEPS TO REPRODUCE
1. Have Firefox wit a lot of tabs/windows open
2. Observe CPU usage of kwin_wayland

OBSERVED RESULT
CPU usage is quite high (about 35 % of single CPU core)

EXPECTED RESULT
CPU usage should be much lower when not doing anything in Firefox.

SOFTWARE/OS VERSIONS
Operating System: openSUSE Tumbleweed 20250923
KDE Plasma Version: 6.4.5
KDE Frameworks Version: 6.18.0
Qt Version: 6.9.2
Kernel Version: 6.16.8-1-default (64-bit)
Graphics Platform: Wayland
Wayland version: 1.24.0
Firefox version: 143.0.1
Mesa version: 25.2.2
Processors: 16 × AMD Ryzen 7 7840HS w/ Radeon 780M Graphics
Memory: 32 GiB of RAM (30.7 GiB usable)
Graphics Processor: AMD Radeon 780M
Manufacturer: SLIMBOOK
Product Name: EXCALIBUR-16-AMD7
Comment 1 Zamundaaa 2025-09-25 13:16:35 UTC 
Perf data is not portable, you have to look at it yourself. You can use hotspot to generate a flamegraph for it, if there's anything specific, it should immediately stand out in the image.

It is somewhat likely that Firefox is constantly requesting repaints though, we had that issue in the past. Does the CPU usage go away if you minimize the window?
Comment 2 Jure Repinc 2025-09-25 13:46:13 UTC 
Created attachment 185258 [details]
flamegraph SVG
Comment 3 Jure Repinc 2025-09-25 13:47:32 UTC 
I attached the flamegraph exported as SVG.

And indeed when I minimize all Firefox windows the CPU usage drops to almost 0 %
Comment 4 Nate Graham 2025-09-25 15:46:10 UTC 
Looks like it's mostly in KWin::SubSurfaceInterface::parentApplyState()
Comment 5 Zamundaaa 2025-09-26 14:35:42 UTC 
Yeah, that means Firefox is doing a huge amount of commits, and more specifically setting the subsurface position every time. Please report this to Mozilla.
Comment 6 leo 2025-12-08 00:44:48 UTC 
*** Bug 513066 has been marked as a duplicate of this bug. ***