Add nice setting or command line option

[FrostBird347]

Is your feature request related to a problem? Please describe.
Creating a timeshift snapshot can at times use up a significant amount of resources. Since the process is run as root with a nice value of 0, it takes priority over all other processes running under the user which includes almost all parts of the UI that the user directly interacts with.
Setting the parent process's nice value does not appear to do anything, nor does launching timeshift with nice. Opening plasma-systemmonitor to check the process's nice value reveals that it's automatically resetting it's value back to 0 for some reason.

Describe the solution you'd like
Either an option to specify a custom nice value or a way to ensure timeshift keeps it's initial nice value would be greatly appreciated.

Describe alternatives you've considered
I could try to modify my personal backup script to launch timeshift in the background, wait a couple seconds and then modify the nice value of all rsync processes.
This isn't ideal as it wouldn't be guaranteed to always work and it may also change the nice value of other unrelated rsync processes that are running in that moment.
This solution also wouldn't work for anyone who intends to backup from the GUI nor would it work for anyone who intends to use the built-in automatic backup system.

Additional context
I haven't tested if ionice faces a similar issue or not, as I backup to an external disk.

[ygerlach]

A nice value of 0 is the default. See here: https://www.man7.org/linux/man-pages/man1/renice.1.html#:~:text=0%20(the%20%22base%22%20scheduling%20priority). All programs with that prio should have somewhat the same chance to be scheduled.

I was able to reproduce your issue with the timeshift version: 24.06.6, but not with the current master version (25.12.4).
This is because the logic to set the prio was removed by me in #428 with 1f138f7. At that point i thought that background_mode was ever the same value and therefore should never change the prio. The case that the user may start the program with a higher or lower prio value did not occur to me then.
But i think its rude for a program to raise its own prio over the value it was started with. So i think it is still correct to leave the setprio code removed.

What version of timeshift are you having the issue with (Check timeshift --version)?

As for the ionice: i have a PR open to set the ionice of background snapshot jobs a bit lower than average. #492

[FrostBird347]

I assumed it was taking priority due to the process being run as root, but it being given equal priority would explain why my system doesn't completely freeze whenever any root process does a cpu intensive operation and instead just stutters a ton.

I have version 24.06.6, which is the latest version for Questing Quokka in ubuntu's official apt source.