[okular] [Bug 451059] New: Okular freezes for up to 10 seconds when loading a document or switching windows

[Amaury Pouly]

https://bugs.kde.org/show_bug.cgi?id=451059

            Bug ID: 451059
           Summary: Okular freezes for up to 10 seconds when loading a
                    document or switching windows
           Product: okular
           Version: 21.12.2
          Platform: Debian unstable
                OS: Linux
            Status: REPORTED
          Severity: major
          Priority: NOR
         Component: general
          Assignee: okular-devel at kde.org
          Reporter: amaury.pouly at gmail.com
  Target Milestone: ---

Created attachment 147249
  --> https://bugs.kde.org/attachment.cgi?id=147249&action=edit
D-BUS log of a such an instance

I have a large number of okular instances running (typically 20 or more) and
very often either loading a new document or just switching between windows (or
switching between virtual desktops) will make okular completely freeze for a
long time. Quite regularly, the freeze is short enough to be an annoyance (a
couple of seconds), but every now and then, it can freeze for 10 seconds and
more.

I tried to debug the code myself but I am out of my depth. I got to suspect,
however, that it is related to okular asking other okular instances questions
over D-BUS, specifically the canOpenDocs call. I attach a D-BUS trace showing
an instance when opening a document will trigger several very slow canOpenDocs
calls (5 seconds).

Unfortunately, I haven't found yet a 100% recipe to reproduce it but clearly:
1) it only happens with many okular instances running
2) switching virtual desktops seems to make it worse

Finally, the bug is not new, I has been happening for some time but
progressively got worse as my number of open documents has increased recently.

I happy to provide more logs/recompile stuff if needed, I just don't really
know where to start at the moment.

STEPS TO REPRODUCE
1. open lots of okular documents (I use PDFs), the size of the PDFs does not
seem to matter
2. switch between windows/desktops
3. open a new document, hope to reproduce the bug, otherwise close it
4. return to step 2

OBSERVED RESULT
opening/switching to a windows will take anywhere between 2 to 10 seconds

EXPECTED RESULT
should be almost instantenous


SOFTWARE/OS VERSIONS
Linux:  5.16.0-3-amd64
KDE Plasma Version: 5.24.2
KDE Frameworks Version: 5.90.0
Qt Version: 5.15.2

ADDITIONAL INFORMATION

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