Bug 2404262

Summary:	slitherer causes very high CPU usage without 3D acceleration support (qemu VM with no 3D passthrough, bare metal with nomodeset)
Product:	[Fedora] Fedora	Reporter:	Adam Williamson <awilliam>
Component:	slitherer	Assignee:	Neal Gompa <ngompa13>
Status:	NEW ---	QA Contact:
Severity:	high	Docs Contact:
Priority:	unspecified
Version:	43	CC:	kde-sig, kparal, lruzicka, ngompa13, nicolas.fella
Target Milestone:	---	Keywords:	CommonBugs
Target Release:	---
Hardware:	All
OS:	Linux
Whiteboard:	https://discussion.fedoraproject.org/t/170291
Fixed In Version:		Doc Type:	---
Doc Text:		Story Points:	---
Clone Of:		Environment:
Last Closed:		Type:	Bug
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:

Description Adam Williamson 2025-10-15 19:03:32 UTC

While testing something else, I noticed that when running the installer (anaconda webUI) from a current Fedora 43 KDE live image on a typical qemu VM, there is very high resource usage.

On a VM with 6 vCPUs and 4G RAM, and a virtio video card with no 3D acceleration passthrough enabled, top shows kwin_wayland using 150-200% CPU, and a process called 'slitherer' using another 50-100%, even when the UI is idle.

I don't see the same on Workstation live. When I interact with the anaconda UI, gnome-shell kicks up to ~30-50% and firefox to ~10-30%, but that's all. When the UI is idle, they show very low CPU usage.

If I run Firefox on the KDE live and go the local system's Cockpit (closest equivalent to running the installer), I don't see the same as the slitherer+installer behaviour, I see something more like the Workstation Firefox+installer behaviour - smaller resource spikes, low usage at idle. I also see the same if I run something else that uses qt6-qtwebview - Discover - and browse around a bit (though I'm not 100% sure if just looking at package info pages actually triggers Discover's use of qtwebview). So far the only thing that behaves like slitherer+anaconda-webui is slitherer+anaconda-webui.

On bare metal with a 3D adapter, I see kwin_wayland and slitherer using about 10-15% CPU combined, which is obviously much better. Still feels like *kind of* a lot for an installer UI, but who knows.

On bare metal with basic graphics mode, I see the bug - very high kwin_wayland and slitherer CPU usage. So it looks like this is possibly tied to the availability of 3D acceleration.

Comment 1 Adam Williamson 2025-10-15 19:04:50 UTC

Proposing for a final FE in case we slip and the fix for this proves easy and safe (maybe it's just a tweak when initializing qtwebview or something like that?)

Comment 2 Neal Gompa 2025-10-16 08:53:09 UTC

Is this an issue still in Rawhide with Qt 6.10 and Plasma 6.5 pre-release?

Comment 3 Adam Williamson 2025-10-16 21:43:05 UTC

Yes. Same behaviour with today's Rawhide.

Comment 4 Kamil Páral 2025-10-17 08:52:01 UTC

I can confirm that I also noticed very high CPU usage in KDE VMs when anaconda is active.

Comment 5 Lukas Ruzicka 2025-10-20 19:04:04 UTC

Discussed at the blocker review meeting on 20th Oct. 2025

AGREED AcceptedFinalFreezeException

This is accepted as it affects the usability of the live installer on unaccelerated configurations, and obviously cannot be fixed with a post-release update.

https://meetbot.fedoraproject.org/blocker-review_matrix_fedoraproject-org/2025-10-20/f43-blocker-review.2025-10-20-16.02.html

Comment 6 Adam Williamson 2025-10-21 06:06:18 UTC

With a current snapshot of anaconda-webui that includes https://github.com/rhinstaller/anaconda-webui/commit/9b3a42a63b01983e04c14187e6c72bcfb92b1816 , this is even worse. I'm now seeing 240% CPU use by slitherer and 160% by kwin_wayland.

Comment 7 Adam Williamson 2025-10-21 17:28:13 UTC

Interestingly, I don't see the same thing with the LXQt image from the Final RC - https://kojipkgs.fedoraproject.org/compose/43/Fedora-43-20251016.1/compose/Spins/x86_64/iso/Fedora-LXQt-Live-43-1.4.x86_64.iso . It uses slitherer, but there's no resource thrashing. CPU usage briefly spikes to 60-70% when actively using the installer, but when it's sitting idle, CPU usage is under 1%. I'll test a few other images and see how they behave.

Comment 8 Neal Gompa 2025-10-21 19:14:18 UTC

That's because the issue is in KWin and only affects KDE Plasma. KDE Plasma does *not* like running without 3D acceleration, and there are all kinds of bumps related to not having it.

Comment 9 Adam Williamson 2025-10-21 23:10:23 UTC

What exactly is "the issue", then? Is there a reference? Do we re-assign this to kwin?

slitherer does seem genuinely worse than firefox in this context, though. I tweaked /usr/libexec/anaconda/webui-desktop to use firefox instead of slitherer, and it doesn't spike the CPU anywhere near as bad as slitherer. If I leave anaconda idle, CPU usage stays under 1%. When I do stuff it spikes, but much lower than with slitherer; it's more like it briefly goes to 50-60% then quickly drops under 5% again.

Comment 10 Adam Williamson 2025-10-27 16:38:27 UTC

F43 is done, dropping metadata.

Comment 11 Neal Gompa 2025-11-10 11:11:17 UTC

I wonder if this is similar to bug 2399673 where the breeze QtQuick style is doing something to cause this.

Comment 12 Neal Gompa 2025-11-21 13:57:07 UTC

Now that this has landed in Rawhide and F43, it might be worth checking again.

Comment 13 Neal Gompa 2025-11-21 13:58:00 UTC

"this" being a fix for the other bug, that is.

Comment 14 Adam Williamson 2025-11-21 16:08:54 UTC

Thanks, I'll check again. When did this fix land? Is it in today's Rawhide?

Comment 15 Neal Gompa 2025-11-21 19:05:12 UTC

Two days ago: https://bodhi.fedoraproject.org/updates/FEDORA-2025-c15dc96c66

But I don't think a compose has published since...

Comment 16 Adam Williamson 2025-11-21 19:37:55 UTC

Eh? Sure it has. https://kojipkgs.fedoraproject.org/compose/rawhide/Fedora-Rawhide-20251121.n.0/ is FINISHED_INCOMPLETE and https://dl.fedoraproject.org/pub/fedora/linux/development/rawhide/COMPOSE_ID shows that it's that compose.

Comment 17 Adam Williamson 2025-11-25 00:21:53 UTC

Doesn't seem noticeably different with today's Rawhide nightly. I confirmed it has kwin-6.5.3-1.fc44 , booted it on my test box with nomodeset , ran the installer. Still slitherer and kwin_wayland thrashing the CPU even when the installer is idle.