gnome-shell (via Mesa/LLVMpipe in a virtual machine) uses all CPU cores just to update the screen (on Wayland only)

[TomaszChmielewski]

This started to happen with Ubuntu 19.10.

When an animated gif is displayed in a browser, gnome-shell begins using 100% CPU on all cores.

To reproduce:

1) open Firefox, or Chromium

2) load an URL where an animated gif is displayed - you can also use this direct link to https://i.stack.imgur.com/h6viz.gif

3) gnome-shell will start using 100% CPU on all cores (as long as Firefox/Chromium window is in the foreground)
---
ProblemType: Bug
ApportVersion: 2.20.11-0ubuntu8.1
Architecture: amd64
DisplayManager: gdm3
DistroRelease: Ubuntu 19.10
GsettingsChanges:

InstallationDate: Installed on 2019-05-12 (172 days ago)
InstallationMedia: Ubuntu 19.04 "Disco Dingo" - Release amd64 (20190416)
Package: gnome-shell 3.34.1-1ubuntu1
PackageArchitecture: amd64
ProcVersionSignature: Ubuntu 5.3.0-19.20-generic 5.3.1
RelatedPackageVersions: mutter-common 3.34.1-1ubuntu1
Tags: eoan
Uname: Linux 5.3.0-19-generic x86_64
UpgradeStatus: Upgraded to eoan on 2019-10-16 (15 days ago)
UserGroups:

_MarkForUpload: True

[Daniel van Vugt (vanvugt)]

Hmm, gnome-shell is mostly single threaded so it shouldn't be able to use "all cores". It should saturate only one core at most.

Please provide a screenshot of 'top' running in a Terminal window, showing that gnome-shell (or anything) is using the CPU.

[TomaszChmielewski (mangoo-wpkg)]

In "ps -eLf":

tomasz 1410 1265 1410 1 10 Oct31 ? 00:33:23 /usr/bin/gnome-shell
tomasz 1410 1265 1421 0 10 Oct31 ? 00:00:00 /usr/bin/gnome-shell
tomasz 1410 1265 1423 0 10 Oct31 ? 00:00:02 /usr/bin/gnome-shell
tomasz 1410 1265 1424 0 10 Oct31 ? 00:00:00 /usr/bin/gnome-shell
tomasz 1410 1265 1425 7 10 Oct31 ? 02:33:40 /usr/bin/gnome-shell
tomasz 1410 1265 1426 7 10 Oct31 ? 02:32:22 /usr/bin/gnome-shell
tomasz 1410 1265 1427 7 10 Oct31 ? 02:32:50 /usr/bin/gnome-shell
tomasz 1410 1265 1434 0 10 Oct31 ? 00:00:02 /usr/bin/gnome-shell
tomasz 1410 1265 1435 0 10 Oct31 ? 00:00:02 /usr/bin/gnome-shell
tomasz 1410 1265 1436 0 10 Oct31 ? 00:00:02 /usr/bin/gnome-shell

So it's definitely using multiple threads.

[Daniel van Vugt (vanvugt)]

Wow, yes. That is unusual. Multiple threads exist in gnome-shell but they're pretty well hidden from the main code, mostly handling asynchronous IO etc...

Next we need to see more information about the machine. Please run this command to send it automatically:

  apport-collect 1850874

[TomaszChmielewski (mangoo-wpkg)]

apport information

[TomaszChmielewski (mangoo-wpkg)]

apport information

[TomaszChmielewski (mangoo-wpkg)]

apport information

[TomaszChmielewski (mangoo-wpkg)]

apport information

[Daniel van Vugt (vanvugt)]

Thanks. The above attachment suggests to me that this is a virtual machine or something unusual, because:

[ 34.403367] tomasz gnome-shell[1410]: Failed to initialize accelerated iGPU/dGPU framebuffer sharing: Do not want to use software renderer (llvmpipe (LLVM 9.0, 256 bits)), falling back to CPU copy path
[ 34.918603] tomasz gnome-shell[1410]: glamor: 'wl_drm' not supported
[ 34.918603] tomasz gnome-shell[1410]: Missing Wayland requirements for glamor GBM backend
[ 34.918603] tomasz gnome-shell[1410]: Failed to initialize glamor, falling back to sw

So that's where all the CPU is going... It's Mesa trying to use all the CPU cores to emulate a GPU in software. Probably.

Next, please:

1. Run: lspci -k > lspcik.txt
   and attach the file 'lspcik.txt'

2. Run: glxinfo > glxinfo.txt
   and attach the file 'glxinfo.txt'

3. Tell us if you have any gnome-shell extensions installed.

[Daniel van Vugt (vanvugt)]

4. Run: dmesg > dmesg.txt
   and attach the file 'dmesg.txt'

[TomaszChmielewski (mangoo-wpkg)]

Yes, it is a virtual machine (and hence 3 CPUs!) - though it didn't show these symptoms with Ubuntu 19.04 (noticed by laptop fans going on as soon as I visit a page with animated gifs, i.e. showing build status and such).

There are no gnome extensions installed.

[Daniel van Vugt (vanvugt)]

I'm not sure what changed to make 19.10 heavier than 19.04, but I can explain what you are seeing.

What you are seeing is an OpenGL-based application (Firefox or Chromium) that is being forced to update its entire window constantly. Since it is OpenGL-based, gnome-shell does not have the opportunity to identify just the small area of the screen that is changing, but has to update the entire application window at full frame rate. To do that in software without a GPU is expected to use a lot of CPU and multiple threads, sorry.

[TomaszChmielewski (mangoo-wpkg)]

There is one more change I did after upgrading from 19.04 to 19.10.

This is a KVM VM - and for some reason, the GUI (X) sometimes freezes after the VM is paused/saved/resumed, while SSH still works (this was also before 19.10 and behaviour did not improve in 19.10).

I've changed to Wayland to see if it this kind of freezing happens there as well (so far, it doesn't).

High CPU usage on all cores happens with Wayland only. I've just checked, it doesn't happen with regular X; CPU usage is normal/low there.

[Daniel van Vugt (vanvugt)]

I think the Gnome Shell developers have been working on how that code (efficient screen updates for VMs) works in Wayland sessions. But I don't know the details. Let's hope mutter 3.36 performs better in that area.

[Daniel van Vugt (vanvugt)]

Comment #16 is confusing because it sounds like you're now talking about multiple unrelated issues. It is also confusing because comment #16 suggests this bug occurs in Wayland only but comment #13 looks like it's from Xorg.

Is this bug affecting Wayland sessions only?

[TomaszChmielewski (mangoo-wpkg)]

This bug only is present on Wayland.

[Daniel van Vugt (vanvugt)]

Thanks. I've now added this bug to the performance tracking list: https://trello.com/c/pe5mRmx7

[Daniel van Vugt (vanvugt)]

Thank you for reporting this bug to Ubuntu.
Ubuntu 19.10 (eoan) reached end-of-life on July 17, 2020.

See this document for currently supported Ubuntu releases:
https://wiki.ubuntu.com/Releases

We appreciate that this bug may be old and you might not be interested in discussing it any more. But if you are then please upgrade to the latest Ubuntu version and re-test. If you then find the bug is still present in the newer Ubuntu version, please add a comment here telling us which new version it is in and change the bug status to Confirmed.