IONICE(1)                                           User Commands                                           IONICE(1)

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

ionice [-c class] [-n level] [-t] -p PID…
ionice [-c class] [-n level] [-t] command [argument…]

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 permitted for an ordinary user (since kernel 2.6.25).

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 0-7, 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 “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 scheduling class.  The I/O priority is derived from the CPU nice level of the  process
(same as before kernel 2.6.26).

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.

-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.

-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 insufficient privileges or  an
old kernel version.

-h, –help
Display help and exit.

-V, –version
Display version information and exit.

# 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.

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

Jens Axboe <[email protected]k>
Karel Zak <[email protected]>

The   ionice   command   is   part   of   the   util-linux   package  and  is  available  from  ftp://ftp.ker‐

util-linux                                            July 2011                                             IONICE(1)


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