freebsd-dev/usr.sbin/rtprio/rtprio.1

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.Dd July 23, 1994
.Dt RTPRIO 1
.Os 
.Sh NAME
.Nm rtprio ,
.Nm idprio
.Nd execute, examine or modify a utilitys or process realtime 
or idletime scheduling priority
.Sh SYNOPSIS

.Nm [id|rt]prio
.Nm [id|rt]prio 
.Ar pid
.Nm [id|rt]prio 
.Ar priority
.Ar command
.Op args
.Nm [id|rt]prio 
.Ar priority
.Ar -pid
.Nm [id|rt]prio 
.Ar -t
.Ar command
.Op args
.Nm [id|rt]prio 
.Ar -t
.Ar -pid
.Sh DESCRIPTION
.Nm Rtprio
is used for controlling realtime process scheduling. 

.Nm Idprio
is used for controlling idletime process scheduling, and can be called
with the same options as 
.Nm 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 it's idle priority is equal or
greater than all other runnable idle priority processes.

.Nm Rtprio 
or
.Nm Idprio 
when called without arguments will return the realtime priority
of the current process.

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

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

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

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

.Ar Pid
of 0 means "the current process".

Only root is allowed to set realtime priorities. Non-root processes may
set idle priority levels for the current process only.
.Sh RETURN VALUE
If
.Nm rtprio
execute a command, the exit value is that of the command executed.
In all other cases,
.Nm
exits with 0 for success and 1 for all other errors.
.Sh EXAMPLES
.\LP

To see which realtime priority the current process is at:
.Bd -literal -offset indent -compact
\fBrtprio\fP
.Ed

.\.LP
To see which realtime priority of process \fI1423\fP:
.Bd -literal -offset indent -compact
\fBrtprio 1423\fP
.Ed

.\.LP
To run \fIcron\fP at the lowest realtime priority:
.Bd -literal -offset indent -compact
\fBrtprio 31 cron\fP
.Ed

.\.LP
To change the realtime priority of process \fI1423\fP to \fI16\fP:
.Bd -literal -offset indent -compact
\fBrtprio 16 -1423\fP
.Ed

.\.LP
To run \fItcpdump\fP without realtime priority:
.Bd -literal -offset indent -compact
\fBrtprio -t tcpdump\fP
.Ed

.\.LP
To change the realtime priority of process \fI1423\fP to RTP_PRIO_NORMAL
(non-realtime/"normal" priority):
.Bd -literal -offset indent -compact
\fBrtprio -t -1423\fP
.Ed

.\.LP
To make depend while not disturbing other machine usage:
.Bd -literal -offset indent -compact
\fBidprio 31 make depend\fP
.Ed
.Sh SEE ALSO
.Xr nice 1 ,
.Xr ps 1 ,
.Xr rtprio 2 ,
.Xr setpriority 2 ,
.Xr nice 3 ,
.Xr renice 8
.Sh HISTORY
The
.Nm rtprio
utility appeared in
.Fx 2.0 ,
but is similar to the HP-UX version.
.Sh CAVEATS
You can lock yourself out of the system by placing a cpu-heavy
process in a realtime priority.
.Sh BUGS
There is no way to set/view the realtime priority of process 0
(swapper) (see ps(1)).

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

Others ...
.Sh AUTHOR
Henrik Vestergaard Draboel - hvd@terry.ping.dk is the original author. This
implementation in
.Tn FreeBSD 
was substantially rewritten by David Greenman.