It also sounds like fullscreen direct scanout might have worked in jammy
but doesn't in noble. You can test that theory by comparing the windowed
performance between jammy and noble. Or by checking the scanout method
using MUTTER_DEBUG=kms and grepping the log for "Post" (Wayland only
though).
** Changed in: gnome-shell (Ubuntu)
Status: New => Incomplete
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Packages, which is subscribed to gnome-shell in Ubuntu.
https://bugs.launchpad.net/bugs/2052913
Title:
Large Performance Regression in gnome-shell from Ubuntu Jammy to
Ubuntu Noble
Status in gnome-shell package in Ubuntu:
Incomplete
Bug description:
Test system: ARM64 Ubuntu Jammy (gnome-shell 42.9-0ubuntu2) and Ubuntu Noble
(gnome-shell 45.3-1ubuntu1)
Default Ubuntu GNOME desktop environment used with no user added extensions.
GPU driverstack identical between the two distros (eliminated as a variable).
Linux kernel identical between distros (eliminated as a variable)
CPU, GPU, and RAM clocks pinned to max.
Test case: Minecraft Java Edition (shared drive with identical setup
between Ubuntu versions). Fullscreen 1080p. Xorg display server.
FPS (uncapped):
Ubuntu Jammy: 177fps
Ubuntu Noble: 140fps
Observations:
Ubuntu Jammy:
gnome-shell process cpu% when camera is still: 0-1%
gnome-shell process cpu% when camera is moving: 10-20%
Memory Bandwidth (CPU and GPU shared): 40%
Ubuntu Noble:
gnome-shell process cpu% when camera is still: 20-55%
gnome-shell process cpu% when camera is moving: 30-55%
Memory Bandwidth (CPU and GPU shared): 50%
In both cases GPU utilization is observed to be near 100%.
So in summary, large performance regression regression which appears
to be due to gnome-shell. There is nearly no gnome-shell cpu% usage on
ubuntu jammy when there the camera is still (even with a fps graph
drawn onscreen) in stark contrast to ubuntu noble.
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