RTPRIO(1)

NAME
rtprio, idprio - execute, examine or modify a utility's or process's realtime or idletime scheduling priority

SYNOPSIS
[id|rt]prio
[id|rt]prio [-]pid
[id|rt]prio priority command [args]
[id|rt]prio priority - pid
[id|rt]prio -t command [args]
[id|rt]prio -t - pid

DESCRIPTION
The rtprio utility is used for controlling realtime process scheduling.

The idprio utility is used for controlling idletime process scheduling, and can be called with the same options as rtprio.

A process with a realtime priority is not subject to priority degradation, and will only be preempted by another process of equal or higher realtime priority.

A process with an idle priority will run only when no other process is runnable and then only if its idle priority is equal or greater than all other runnable idle priority processes.

Both rtprio or idprio when called without arguments will return the realtime priority of the current process.

If rtprio is called with 1 argument, it will return the realtime priority of the process with the specified pid.

If priority is specified, the process or program is run at that realtime priority. If -t is specified, the process or program is run as a normal (non-realtime) process.

If -pid is specified, the process with the process identifier pid will be modified, else if command is specified, that program is run with its arguments.

Priority is an integer between 0 and RTP_PRIO_MAX (usually 31). 0 is the highest priority

Pid of 0 means "the current process".

Only root is allowed to set realtime or idle priority for a process. Exceptional privileges can be granted through the mac_priority(4) policy and the realtime and idletime user groups. The sysctl(8) variable security.bsd.unprivileged_idprio is deprecated. If set to non-zero, it lets any user modify the idle priority of processes they own.

Note that idle priority increases the chance that a deadlock can occur if a process locks a required resource and then does not get to run.

EXIT STATUS
If rtprio execute a command, the exit value is that of the command executed. In all other cases, rtprio exits 0 on success, and 1 for all other errors.

EXAMPLES
To see which realtime priority the current process is at:
    rtprio

To see which realtime priority of process 1423:
    rtprio 1423

To run cron(8) at the lowest realtime priority:
    rtprio 31 cron

To change the realtime priority of process 1423 to 16:
    rtprio 16 -1423

To run tcpdump(1) without realtime priority:
    rtprio -t tcpdump

To change the realtime priority of process 1423 to RTP_PRIO_NORMAL (non-realtime/normal priority):
    rtprio -t -1423

To make depend while not disturbing other machine usage:
    idprio 31 make depend

SEE ALSO
nice(1), ps(1), rtprio(2), setpriority(2), nice(3), mac_priority(4), renice(8)

HISTORY
The rtprio utility appeared in FreeBSD 2.0, but is similar to the HP-UX version.

AUTHORS
-nosplit Henrik Vestergaard Draboel <hvd@terry.ping.dk> is the original author. This implementation in FreeBSD was substantially rewritten by David Greenman.

CAVEATS
You can lock yourself out of the system by placing a cpu-heavy process in a realtime priority.

BUGS
There is no way to set/view the realtime priority of process 0 (swapper) (see ps(1)).

There is in FreeBSD no way to ensure that a process page is present in memory therefore the process may be stopped for pagein (see mprotect(2), madvise(2)).

Under FreeBSD system calls are currently never preempted, therefore non-realtime processes can starve realtime processes, or idletime processes can starve normal priority processes.