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.TH IONICE "1" "July 2011" "util-linux" "User Commands"
.SH NAME
ionice \- sets or gets process I/O scheduling class and priority
.SH SYNOPSIS
.B ionice
[[ \fB\-c\fR
.IR class ]
.RB [ \fB-n\fR
.IR level ]
.RB [ \fB-t\fR ]]
.BI \fB-p\fR
PID
.RI [ PID ]...
.br
.B ionice
.RB [ \fB-c\fR
.IR class ]
.RB [ \fB-n\fR
.IR level ]
.RB [ \fB-t\fR ]
.IR COMMAND
[ ARG ]...
.SH DESCRIPTION
This program sets or gets the I/O scheduling class and priority for a program.
If no arguments or just \fB\-p\fR is given, \fBionice\fR will query the current
I/O scheduling class and priority for that process.
If no class is given, then
.I COMMAND
will be executed with the "best-effort" scheduling class. The default
priority argument is 4.
As of this writing, a process can be in one of three scheduling classes:
.IP "\fBIdle\fP"
A program running with idle I/O priority will only get disk time when no other
program has asked for disk I/O for a defined grace period. The impact of an
idle I/O process on normal system activity should be zero. This scheduling
class does not take a priority argument. Presently, this scheduling class
is permitted for an ordinary user (since kernel 2.6.25).
.IP "\fBBest-effort\fP"
This is the effective scheduling class for any process that has not asked for
a specific I/O priority.
This class takes a priority argument from \fI0-7\fR, with a lower
number being higher priority. Programs running at the same best-effort
priority are served in a round-robin fashion.
Note that before kernel 2.6.26 a process that has not asked for an I/O priority
formally uses "\fBnone\fP" as scheduling class, but the I/O scheduler will treat
such processes as if it were in the best-effort class. The priority within the
best-effort class will be dynamically derived from the CPU nice level of the
process: io_priority = (cpu_nice + 20) / 5.
For kernels after 2.6.26 with the CFQ I/O scheduler, a process that has not asked
for an I/O priority inherits its CPU scheduling class. The I/O priority is derived
from the CPU nice level of the process (same as before kernel 2.6.26).
.IP "\fBRealtime\fP"
The RT scheduling class is given first access to the disk, regardless of
what else is going on in the system. Thus the RT class needs to be used with
some care, as it can starve other processes. As with the best-effort class,
8 priority levels are defined denoting how big a time slice a given process
will receive on each scheduling window. This scheduling class is not
permitted for an ordinary (i.e., non-root) user.
.SH OPTIONS
.TP
\fB\-c\fR, \fB\-\-class\fR \fICLASS\fR
The scheduling class name or number; \fI0\fR for none, \fI1\fR for realtime, \fI2\fR for
best-effort, \fI3\fR for idle.
.TP
\fB\-n\fR, \fB\-\-classdata\fR \fINUM\fR
The scheduling class data. This defines the class data, if the class
accepts an argument. For realtime and best-effort, \fI0-7\fR is valid data.
.TP
\fB\-p\fR, \fB\-\-pid\fR \fIPID\fR
Pass in process PID(s) to view or change already running processes. If this argument
is not given, \fBionice\fP will run the listed program with the given parameters.
.TP
\fB\-t\fR, \fB\-\-ignore\fR
Ignore failure to set the requested priority. If COMMAND or PID(s) is specified,
run it even in case it was not possible to set the desired scheduling priority,
which can happen due to insufficient privileges or an old kernel version.
.TP
\fB\-h\fR, \fB\-\-help\fR
Display help and exit.
.TP
\fB\-V\fR, \fB\-\-version\fR
Display version information and exit.
.SH EXAMPLES
.LP
.TP 7
# \fBionice\fP -c 3 -p 89
.TP 7
Sets process with PID 89 as an idle I/O process.
.TP 7
# \fBionice\fP -c 2 -n 0 bash
.TP 7
Runs 'bash' as a best-effort program with highest priority.
.TP 7
# \fBionice\fP -p 89 91
.TP 7
Prints the class and priority of the processes with PID 89 and 91.
.SH NOTES
Linux supports I/O scheduling priorities and classes since 2.6.13 with the CFQ
I/O scheduler.
.SH AUTHORS
.nf
Jens Axboe <jens@axboe.dk>
Karel Zak <kzak@redhat.com>
.fi
.SH AVAILABILITY
The ionice command is part of the util-linux package and is available from
ftp://ftp.kernel.org/pub/linux/utils/util-linux/.
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