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Bad desktop performance with Linux 5.8

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Hello,

I am running Debian Buster (XFCE) on a Dell Laptop[1] with some packages
upgraded from backports[2], notably linux-image-amd64.  After upgrading
to version 5.8, I started noticing some UI "choppiness".  To give a few
examples:

- when typing (in any application, e.g. Emacs, Terminator, Firefox): at
  its worst, I can type a bunch of words, sit back, wait a few seconds,
  then watch the words appear letter by letter.

- when launching programs: startup times are very inconsistent; e.g. if
  I loop on
    while time emacs -Q --eval '(kill-emacs)' ; do continue ; done
  or
    while time terminator -x "bash -c exit" ; do continue ; done
  times vary between 1× and 3× what I observe on kernel 5.7 (where times
  barely vary at all, ±5% at most).

- when scrolling a PDF with Evince with the touchpad: the display
  "freezes" (the pages do not seem to be scrolling) and jumps to the new
  position after a few seconds; nonetheless, the pages *do* scroll
  continuously, as the mouse pointer changes from an arrow to an I-beam,
  presumably as text and whitespace move past it.

Neither htop, journalctl nor dmesg report anything suspicious to my
untrained eye; the only difference I spotted was the CPU governor: on
5.7 it is "powersave"; on 5.8 it is "ondemand".  Naively cpufreq-setting
all cores to powersave on 5.8 does not make the problem go away, though.

One curious thing is that looking at
    watch -n0.5 grep MHz /proc/cpuinfo

- on 5.7, the system is very responsive, and the idle frequency is
  around 800 MHz;
- on 5.8, the system lags despite the idle frequency never going below
  1700 MHz.

This might all be a red herring though, for all I know.  I did not
immediately realize that the performance drops came from 5.8; I only
understood that after studying a bunch of other hypotheses (uninstalling
xserver-xorg-video-intel, disabling services such as speech-dispatcher
and tor), coming up short, rebooting, opening Grub's "advanced options"
and realizing that 5.7 was still on my system.

I've since rebooted multiple times on 5.7 and 5.8, and I'm positive that
the problem only occurs with 5.8.


I'm not sure what to look at next.  I've searched the Web for people
having performance issues with 5.8, but nothing caught my eye.

I realize that "performance bad ☹" testimonies like this are probably
not very actionable; let me know if there's anything I can do to make
this "diagnosis" more precise.  Also, I hope this is the right place for
this kind of support request; I assume it is too vague as-is for a
proper bug report…  Let me know if I should post this elsewhere.


Thank you for your time, and sorry for the poor report.


[1] Dell Latitude 3190.  Attached outputs from lscpu and lspci:

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Thread

Bad desktop performance with Linux 5.8 Kévin Le Gouguec <kevin.legouguec@gmail.com> - 2020-11-14 23:30 +0100
  Re: Bad desktop performance with Linux 5.8 Georgi Naplatanov <gosho@oles.biz> - 2020-11-15 00:50 +0100
    Re: Bad desktop performance with Linux 5.8 Kévin Le Gouguec <kevin.legouguec@gmail.com> - 2020-11-15 15:30 +0100
    Re: Bad desktop performance with Linux 5.8 Kévin Le Gouguec <kevin.legouguec@gmail.com> - 2020-12-06 16:10 +0100

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