[okular] [Bug 396137] Okular hangs when scrolling fast on both pdf and djvu documents

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https://bugs.kde.org/show_bug.cgi?id=396137

antonini44354 at gmail.com changed:

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                 CC|                            |antonini44354 at gmail.com

--- Comment #12 from antonini44354 at gmail.com ---
I think I discovered a cause for this, whuch is related to cgroups and limiting
Okular RAM. 

My version of Okular is 1.7.2. My PC has 4 GB of RAM. I was often running into
slowdowns because of thrashing very often. So I set up a few cgroups in order
to actually take control of RAM. I put in cgroups preety much every program
that used more than 100 MB. Okular often went over that much RAM because I
viewed several PDF tabs. I set up cgroups according to
https://askubuntu.com/questions/836469/install-cgconfig-in-ubuntu-16-04. 

I was tinkering with the cgroups and I settled for this cgroup config on Okular
like this in /etc/cgconfig:

group main/documents {
    memory {
        memory.limit_in_bytes = 72000000;
        memory.soft_limit_in_bytes = 4000000;
    }
}

How did I find it out? Yesterday, I set up a Kubuntu VM. I was also testing
Okular (Kubuntu came with Okular 1.7.3, I'm aware this bug wasn't patched on
it) to try to make the bug happen. I copied to the VM a large PDF with hundreds
of pages. I scrolled it for several minutes, page view set to fit width. I did
it franticallu almost, but nothing happened. Surprisingly, Okular didn't slow
down,  nor did this bug triggered.

Today I disabled the cgroup on Okular. I opened the same PDF, and I could not
possibly trigger the bug either. I also noticed Okular beheves preety smoothly.
When the cgroup was enabled, PDFs rendered noticeably slowly. Obviously!

I just found Okular memory  settings. I set it to run using as little as
needed. 

Thanks Tobias Deiminger for patching out the bug in newer versions. 

I hope that this helps with anything.

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