Why top and ps show diffrent priority?

There is some discrepancy in ps output caused by the fact that each system may use different values to represent the process priority and that the values have changed with the introduction of RT priorities.


The kernel stores the priority value in /proc//stat (let's call it p->prio) and ps reads the value and displays it in various ways to the user:

$ ps -A -o pri,opri,intpri,priority,pri_foo,pri_bar,pri_baz,pri_api,pid,commPRI PRI PRI PRI FOO BAR BAZ API PID COMMAND 19 80 80 20 0 21 120 -21 1 init 24 75 75 15 -5 16 115 -16 2 kthreadd139 -40 -40 -100 -120 -99 0 99 3 migration/0 24 75 75 15 -5 16 115 -16 4 ksoftirqd/0139 -40 -40 -100 -120 -99 0 99 5 watchdog/0139 -40 -40 -100 -120 -99 0 99 6 migration/1 24 75 75 15 -5 16 115 -16 7 ksoftirqd/1139 -40 -40 -100 -120 -99 0 99 8 watchdog/1 24 75 75 15 -5 16 115 -16 9 events/0

Yes, there are 8 undocumented values for the process priority that can be passed to -o option:

Option Computed as

prioirity p->priointpri 60 + p->prioopri 60 + p->priopri_foo p->prio - 20pri_bar p->prio + 1pri_baz p->prio + 100pri 39 - p->prioritypri_api -1 - p->priority

They were introduced to fit the values in certain intervals and compatibility with POSIX and other systems.