IONICE(1) manual page
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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:
- 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).
- 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 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).
- 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.
- -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 insufficient 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.
- # 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 <jens@axboe.dk>
Karel Zak <kzak@redhat.com>
The ionice command is part of the util-linux package and is
available from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.
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