Linux – The `PRI` column of `ps` is inconsistent with man pages

linuxmanniceps

I've been trying to understand how scheduling priorities work in linux.
Here's what I've got:

  • The kernel uses a priority value for each process which ranges from 0 to 139. The lower the number, the more priority the process has.
  • Priority values from 0 to 99 are reserved, all user space processes have a priority from 100 to 139.
  • Linux provides the nice interface which is the priority exposed to and modifiable by the user. The nice value ranges from -20 to 19, and maps to priorities 100 to 139. As with the priority value, the lower the nice value, the more priority the process has.

My question now is: What does the PRI column of ps indicate?

The man page of ps says:

   pri         PRI       priority of the process.  Higher number means
                         lower priority.

But the value ranges from 0 to 39, and I empirically determined it is equal to 19 - nice.

It is impossible, given the relationship, that both nice and the priority shown in the PRI column satisfy "higher number means lower priority".

What am I missing?

Example showing this behaviour:

root@kali:~# ps -ao pid,comm,pri,nice
  PID COMMAND         PRI  NI
 6153 cat              19   0
 (···)
root@kali:~# renice -n -10 -p 6153
6153 (process ID) old priority 0, new priority -10
root@kali:~# ps -ao pid,comm,pri,nice
  PID COMMAND         PRI  NI
 6153 cat              29 -10
 (···)

Best Answer

The PRI from ps -o pri is 39 - priority, where priority is the 18th field from /proc/PID/stat.

If you want the unmangled field from proc/PID/stat, you can get it with ps -o priority.

If you want the real priority, you can obtain it with ps -o pri_baz.

Other interesting manglings of that value could be obtained with ps -o pri_foo, ps -o pri_bar and ps -o opri.

I kid you not. You can look at the source here.

Note about /proc/PID/stat:

The priority field (18th) in /proc/PID/stat is set by the kernel in fs/proc/array.c to task_struct->prio - 100 (via task_prio(); MAX_RT_PRIO is defined as 100).

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