[konsole] [Bug 439614] New: Running commands in Konsole becomes slow / laggy when it has been open a long time

[Adam Fontenot]

https://bugs.kde.org/show_bug.cgi?id=439614

            Bug ID: 439614
           Summary: Running commands in Konsole becomes slow / laggy when
                    it has been open a long time
           Product: konsole
           Version: 21.04.2
          Platform: Archlinux Packages
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: general
          Assignee: konsole-devel at kde.org
          Reporter: adam.m.fontenot+kde at gmail.com
  Target Milestone: ---

Created attachment 139927
  --> https://bugs.kde.org/attachment.cgi?id=139927&action=edit
desktop recording of konsole slowdown problem

SUMMARY

I have a Konsole instance that has been running for about a week. I've been
using it every day for various tasks (system updates, utilities that print
things, and so on). Now, new commands take far too long to execute. (The
processes themselves still run just as quickly, but Konsole is slow to print
the output.)

I've included a video of this happening. In the foreground you can see an XTerm
instance which shows the exact frame I press the enter key on. If you count the
frames from the instant the <enter> key is shown, Konsole consistently takes
around 37 frames to show the result. As this video was recorded at ~60 fps,
that's over 0.6 seconds and is easily noticeable to the user.

As you can see in the video, I run the command using `time`, and the program
runs in about ~0.05 seconds. In other words, over half a second of the time to
return to the shell prompt is due to Konsole's output lagging.

Typing in the terminal is *not* noticeably slow.

Ctrl-shift-n to open a new window *does not* solve this problem. The resulting
window is running under the same Konsole pid as the existing window, and is
just as slow. If I start a new Konsole instance (e.g. by entering `konsole` at
the prompt in Konsole), it is not slow. 

The reason I didn't show `echo test` in the video is that this command (which
uses the zsh echo built-in) is almost always very fast. Once out of every 10 or
20 times I enter it it'll lag slightly. I'm not sure why this behaves
differently than the command I show in the video.

STEPS TO REPRODUCE
1. Leave Konsole running for a long time and do normal tasks in it from time to
time
2. Try to run a simple command that should return ~instantly

OBSERVED RESULT

Konsole takes a lot of time to print the output from commands and return to the
shell prompt.

EXPECTED RESULT

Konsole should take the normal amount of time to do the above.

SOFTWARE/OS VERSIONS
Linux: Arch Linux x86_64 kernel version 5.12.13
KDE Plasma: 5.22.2
KDE Plasma Version: 5.22.2
KDE Frameworks Version: 5.83.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION

I have a fairly large scrollback of 100k lines, but my system is not even close
to being out of memory. I have a pretty decent number of terminals open under
this PID right now.

I have "reflow lines" enabled (I think it's a default). It's the only other
thing I've ever seen slow down Konsole - if I accidentally print an extremely
long line (millions of characters), Konsole can slow to a crawl. I don't have a
good reason for thinking it's related.

I will try to leave this Konsole instance running for at least another day or
two, in case anyone has debugging steps they would like me to try.

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