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
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).
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 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.
The scheduling class. 0 for none, 1 for real time, 2 for best-
effort, 3 for idle.
The scheduling class data. This defines the class data, if the
class accepts an argument. For real time and best-effort, 0-7 is
-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 version.
# 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 <email@example.com>
The ionice command is part of the util-linux-ng package and is
available from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.