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] −P PGID...
ionice [ −c class ] [ −n level ] [−t] −u UID...
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:
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).
Best-effortThis 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 classSpecify 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 levelSpecify 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,
−−ignoreIgnore 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,
−−helpDisplay help text and exit.
−u,
−−uid UID...Specify the user IDs of running processes for which to get or set the scheduling parameters.
−V,
−−versionDisplay version information and exit.