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Jiskya
Asked: 2012-05-06 14:37:52 +0800 CST

Do all apps start with a normal priority on the system monitor?

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I noticed that on my system monitor, one of my processes, titled pulseaudio, had a very high priority, while all of my other running apps had a normal priority. I don't recall ever changing it, so it must have been its default priority, right? Are there any other apps like this?

pulseaudio process process-priority

2 Answers

  1. Best Answer

    Technically, yes. In Linux — Ubuntu's kernel — all programs start with a priority or "niceness" value of 0. They may, while executing, request a higher or lower priority by making the system call nice(int niceness) from unistd.h.

    Normally, programs the user runs only have the permission to lower their priority to positive niceness. This is useful for non-critical janitorial tasks your computer performs from time to time.

    However, Ubuntu also uses a framework called AppArmor, which grants specific user programs administrative ("root") level rights. Ubuntu's AppArmor is configured to allow certain important programs to request a high priority (negative niceness), most notably PulseAudio. This prevents breaks in playing audio while your system is under heavy load.

    In summary, yes all applications start with a normal priority. However, in practice, many applications request a different priority immediately after launching.

    And as a little geeky addendum

    You can start (or to be technically accurate, fork) your own programs with low/high priority in a commandline like this:

    nice -n 10 my_app                 # this application is low priority, very nice
sudo nice -n -10 my_other_app     # this application is high priority, very mean

    No guarantees they'll keep it, of course. ;)

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  2. Usually there's a pretty good reason why some process have an elevated priority – PulseAudio handles audio mixing, high priority reduces audio drop outs.

    However PulseAudio is mostly an exception. You need root permissions to boost process' priority, so you probably won't see any other such processes owned by a regular user.

    There are many system processes and threads running with maximum priority (you can see them in System Monitor by switching to View > All Processes), but there's no need to be concerned about those.

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