ioprio_get, ioprio_set - get/set I/O scheduling class and priority
int ioprio_get(int which, int who);
int ioprio_set(int which, int who, int ioprio);
Note: There are no glibc wrappers for these system calls; see NOTES.
The ioprio_get() and ioprio_set() system calls respectively get and
set the I/O scheduling class and priority of one or more threads.
The which and who arguments identify the thread(s) on which the
system calls operate. The which argument determines how who is
interpreted, and has one of the following values:
who is a process ID or thread ID identifying a single process
or thread. If who is 0, then operate on the calling thread.
who is a process group ID identifying all the members of a
process group. If who is 0, then operate on the process group
of which the caller is a member.
who is a user ID identifying all of the processes that have a
matching real UID.
If which is specified as IOPRIO_WHO_PGRP or IOPRIO_WHO_USER when
calling ioprio_get(), and more than one process matches who, then the
returned priority will be the highest one found among all of the
matching processes. One priority is said to be higher than another
one if it belongs to a higher priority class (IOPRIO_CLASS_RT is the
highest priority class; IOPRIO_CLASS_IDLE is the lowest) or if it
belongs to the same priority class as the other process but has a
higher priority level (a lower priority number means a higher
The ioprio argument given to ioprio_set() is a bit mask that
specifies both the scheduling class and the priority to be assigned
to the target process(es). The following macros are used for
assembling and dissecting ioprio values:
Given a scheduling class and priority (data), this macro
combines the two values to produce an ioprio value, which is
returned as the result of the macro.
Given mask (an ioprio value), this macro returns its I/O class
component, that is, one of the values IOPRIO_CLASS_RT,
IOPRIO_CLASS_BE, or IOPRIO_CLASS_IDLE.
Given mask (an ioprio value), this macro returns its priority
See the NOTES section for more information on scheduling classes and
I/O priorities are supported for reads and for synchronous (O_DIRECT,
O_SYNC) writes. I/O priorities are not supported for asynchronous
writes because they are issued outside the context of the program
dirtying the memory, and thus program-specific priorities do not
On success, ioprio_get() returns the ioprio value of the process with
highest I/O priority of any of the processes that match the criteria
specified in which and who. On error, -1 is returned, and errno is
set to indicate the error.
On success, ioprio_set() returns 0. On error, -1 is returned, and
errno is set to indicate the error.
EINVAL Invalid value for which or ioprio. Refer to the NOTES section
for available scheduler classes and priority levels for
EPERM The calling process does not have the privilege needed to
assign this ioprio to the specified process(es). See the
NOTES section for more information on required privileges for
ESRCH No process(es) could be found that matched the specification
in which and who.
These system calls have been available on Linux since kernel 2.6.13.
These system calls are Linux-specific.
Glibc does not provide a wrapper for these system calls; call them
Two or more processes or threads can share an I/O context. This will
be the case when clone(2) was called with the CLONE_IO flag.
However, by default, the distinct threads of a process will not share
the same I/O context. This means that if you want to change the I/O
priority of all threads in a process, you may need to call
ioprio_set() on each of the threads. The thread ID that you would
need for this operation is the one that is returned by gettid(2) or
These system calls have an effect only when used in conjunction with
an I/O scheduler that supports I/O priorities. As at kernel 2.6.17
the only such scheduler is the Completely Fair Queuing (CFQ) I/O
I/O Schedulers are selected on a per-device basis via the special
One can view the current I/O scheduler via the /sys filesystem. For
example, the following command displays a list of all schedulers
currently loaded in the kernel:
$ cat /sys/block/hda/queue/scheduler
noop anticipatory deadline [cfq]
The scheduler surrounded by brackets is the one actually in use for
the device (hda in the example). Setting another scheduler is done
by writing the name of the new scheduler to this file. For example,
the following command will set the scheduler for the hda device to
# echo cfq > /sys/block/hda/queue/scheduler
Since v3 (aka CFQ Time Sliced) CFQ implements I/O nice levels similar
to those of CPU scheduling. These nice levels are grouped in three
scheduling classes each one containing one or more priority levels:
This is the real-time I/O class. This scheduling class is
given higher priority than any other class: processes from
this class are given first access to the disk every time.
Thus this I/O class needs to be used with some care: one I/O
real-time process can starve the entire system. Within the
real-time class, there are 8 levels of class data (priority)
that determine exactly how much time this process needs the
disk for on each service. The highest real-time priority
level is 0; the lowest is 7. In the future this might change
to be more directly mappable to performance, by passing in a
desired data rate instead.
This is the best-effort scheduling class, which is the default
for any process that hasn't set a specific I/O priority. The
class data (priority) determines how much I/O bandwidth the
process will get. Best-effort priority levels are analogous
to CPU nice values (see getpriority(2)). The priority level
determines a priority relative to other processes in the best-
effort scheduling class. Priority levels range from 0
(highest) to 7 (lowest).
This is the idle scheduling class. Processes running at this
level only get I/O time when no-one else needs the disk. The
idle class has no class data. Attention is required when
assigning this priority class to a process, since it may
become starved if higher priority processes are constantly
accessing the disk.
Refer to Documentation/block/ioprio.txt for more information on the
CFQ I/O Scheduler and an example program.
Permission to change a process's priority is granted or denied based
on two assertions:
An unprivileged process may set only the I/O priority of a
process whose real UID matches the real or effective UID of
the calling process. A process which has the CAP_SYS_NICE
capability can change the priority of any process.
What is the desired priority
Attempts to set very high priorities (IOPRIO_CLASS_RT) require
the CAP_SYS_ADMIN capability. Kernel versions up to 2.6.24
also required CAP_SYS_ADMIN to set a very low priority
(IOPRIO_CLASS_IDLE), but since Linux 2.6.25, this is no longer
A call to ioprio_set() must follow both rules, or the call will fail
with the error EPERM.