ionice Man page

IONICE(1) User Commands IONICE(1)

NAME

ionice – set or get process I/O scheduling class and priority

SYNOPSIS

ionice [-c class] [-n level] [-t] -p PID…
ionice [-c class] [-n level] [-t] -P PGID…
ionice [-c class] [-n level] [-t] -u UID…
ionice [-c class] [-n level] [-t] command [argument…]

DESCRIPTION

This program sets or gets the I/O scheduling class and priority for a
program. If no arguments or just -p is given, ionice will query the
current I/O scheduling class and priority for that process.

When command is given, ionice will run this command with the given
arguments. If no class is specified, then command will be executed
with the “best-effort” scheduling class. The default priority level is
4.

As of this writing, a process can be in one of three scheduling
classes:

Idle 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 permit‐
ted for an ordinary user (since kernel 2.6.25).

Best-effort
This is the effective scheduling class for any process that has
not asked for a specific I/O priority. This class takes a pri‐
ority argument from 0-7, with a lower number being higher prior‐
ity. 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 “none” 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 schedul‐
ing class. The I/O priority is derived from the CPU nice level
of the process (same as before kernel 2.6.26).

Realtime
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.

OPTIONS

-c, –class class
Specify the name or number of the scheduling class to use; 0 for
none, 1 for realtime, 2 for best-effort, 3 for idle.

-n, –classdata level
Specify the scheduling class data. This only has an effect if
the class accepts an argument. For realtime and best-effort,
0-7 are valid data (priority levels).

-p, –pid PID…
Specify the process IDs of running processes for which to get or
set the scheduling parameters.

-P, –pgid PGID…
Specify the process group IDs of running processes for which to
get or set the scheduling parameters.

-t, –ignore
Ignore failure to set the requested priority. If command was
specified, run it even in case it was not possible to set the
desired scheduling priority, which can happen due to insuffi‐
cient privileges or an old kernel version.

-h, –help
Display help text and exit.

-u, –uid UID…
Specify the user IDs of running processes for which to get or
set the scheduling parameters.

-V, –version
Display version information and exit.

EXAMPLES
# ionice -c 3 -p 89

Sets process with PID 89 as an idle I/O process.

# ionice -c 2 -n 0 bash

Runs ‘bash’ as a best-effort program with highest priority.

# ionice -p 89 91

Prints the class and priority of the processes with PID 89 and 91.

NOTES
Linux supports I/O scheduling priorities and classes since 2.6.13 with
the CFQ I/O scheduler.

AUTHORS
Jens Axboe
Karel Zak

AVAILABILITY
The ionice command is part of the util-linux package and is available
from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.

util-linux July 2011 IONICE(1)