What does '-P' mean in the context of the 'ps' command?

Question

If is executed ps -p 3384 3395 (observe -p is lowercase) then the output is as follows:

  PID TTY      STAT   TIME COMMAND
 3384 tty6     S+     0:00 man ls
 3395 tty6     S+     0:00 pager

Until here all fine and is expected.

Just by mistake was executed ps -P 3384 3395 (observe -P is uppercase) then the output is as follows:

 PID PSR TTY      STAT   TIME COMMAND
 3384   2 tty6     S+     0:00 man ls
 3395   3 tty6     S+     0:00 pager

Observe in this output appears a new header - it is PSR

Question

• What does -P mean in the context of the ps command?

And yes, I already read both man ps and ps --help all where appears the documentation for the -p option/parameter as follows respectively:

  # Approach 1
   p pidlist
     Select by process ID. Identical to -p and --pid.

  -p pidlist
     Select by PID. This selects the processes whose process ID numbers appear in pidlist.  
     Identical to p and --pid.

  # Approach 2
  -p, p, --pid <PID> process id
        --ppid <PID> parent process id

But about -P does not appear nothing. To be honest when the ps command was executed with -P - theoretically I expected an error because -P does not exist, it because -P is not documented.

Extra Questions

• If -P theoretically does not exist because is not documented, Why an error was not thrown?
• What does PSR mean?

Answer 1

I don't have any knowledge of why -P is not documented. I wonder if the feature was not fully supported at some point in the past?

-P (or -o psr) sets the output to include PSR, which the manual states is:

       psr         PSR       processor that process is currently assigned to.

There is a comment in the help code for ps that suggests -P is "missing" from the help. Going back to the initial checkin that I can find, several other flags used to be so marked (including -c, -L, and -M). This is the only of the "dash" options I see still listed as missing from the help page.

Answer 2

It indicates which processor is currently being used to execute the process. Even though the -P option is not documented, PSR is described under STANDARD FORMAT SPECIFIERS in the manpage for ps.