ionice – get/set program io scheduling class and priority


ionice – get/set program io scheduling class and priority

ionice [[-c class] [-n classdata] [-t]] -p PID [PID]…
ionice [-c class] [-n classdata] [-t] COMMAND [ARG]…

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

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

Idle A program running with idle io priority will only get disk time when no other program has asked for disk io
for a defined grace period. The impact of idle io processes 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).

Best effort
This is the effective scheduling class for any process that has not asked for a specific io priority. This
class takes a priority argument from 0-7, with 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 io priority formally uses “none" as
scheduling class, but the io 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 CFQ io scheduler a process that has not asked for an io priority inherits CPU
scheduling class. The io priority is derived from the cpu nice level of the process (same as before kernel

Real time
The RT scheduling class is given first access to the disk, regardless of what else is going on in the sys‐
tem. 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
The scheduling class. 0 for none, 1 for real time, 2 for best-effort, 3 for idle.

-n classdata
The scheduling class data. This defines the class data, if the class accepts an argument. For real time and
best-effort, 0-7 is valid data.

-p pid Pass in process PID(s) to view or change already running processes. If this argument is not given, ionice
will run the listed program with the given parameters.

-t Ignore failure to set requested priority. If COMMAND or PID(s) is specified, run it even in case it was not
possible to set desired scheduling priority, what can happen due to insufficient privilegies or old kernel

# ionice -c 3 -p 89

Sets process with PID 89 as an idle io 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 io scheduling priorities and classes since 2.6.13 with the CFQ io scheduler.

Jens Axboe

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

ionice August 2005 ionice(1)



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