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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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ioprio_set(2) System Calls Manual ioprio_set(2)
ioprio_get, ioprio_set - get/set I/O scheduling class and priority
Standard C library (libc, -lc)
#include <linux/ioprio.h> /* Definition of IOPRIO_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
int syscall(SYS_ioprio_get, int which, int who);
int syscall(SYS_ioprio_set, int which, int who, int ioprio);
Note: glibc provides no wrappers for these system calls,
necessitating the use of syscall(2).
The ioprio_get() and ioprio_set() system calls 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:
IOPRIO_WHO_PROCESS
who is a process ID or thread ID identifying a single
process or thread. If who is 0, then operate on the
calling thread.
IOPRIO_WHO_PGRP
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.
IOPRIO_WHO_USER
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 priority level).
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:
IOPRIO_PRIO_VALUE(class, data)
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.
IOPRIO_PRIO_CLASS(mask)
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.
IOPRIO_PRIO_DATA(mask)
Given mask (an ioprio value), this macro returns its
priority (data) component.
See the NOTES section for more information on scheduling classes
and priorities, as well as the meaning of specifying ioprio as 0.
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 apply.
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 ioprio.
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 ioprio_set().
ESRCH No process(es) could be found that matched the
specification in which and who.
Linux.
Linux 2.6.13.
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 clone(2).
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 scheduler.
If no I/O scheduler has been set for a thread, then by default the
I/O priority will follow the CPU nice value (setpriority(2)).
Before Linux 2.6.24, once an I/O priority had been set using
ioprio_set(), there was no way to reset the I/O scheduling
behavior to the default. Since Linux 2.6.24, specifying ioprio as
0 can be used to reset to the default I/O scheduling behavior.
Selecting an I/O scheduler
I/O schedulers are selected on a per-device basis via the special
file /sys/block/device/queue/scheduler.
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/sda/queue/scheduler
noop anticipatory deadline [cfq]
The scheduler surrounded by brackets is the one actually in use
for the device (sda 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 sda
device to cfq:
$ su
Password:
# echo cfq > /sys/block/sda/queue/scheduler
The Completely Fair Queuing (CFQ) I/O scheduler
Since version 3 (also known as CFQ Time Sliced), CFQ implements
I/O nice levels similar to those of CPU scheduling. These nice
levels are grouped into three scheduling classes, each one
containing one or more priority levels:
IOPRIO_CLASS_RT (1)
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.
IOPRIO_CLASS_BE (2)
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).
IOPRIO_CLASS_IDLE (3)
This is the idle scheduling class. Processes running at
this level get I/O time only 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 the kernel source file Documentation/block/ioprio.txt for
more information on the CFQ I/O Scheduler and an example program.
Required permissions to set I/O priorities
Permission to change a process's priority is granted or denied
based on two criteria:
Process ownership
An unprivileged process may set the I/O priority only for 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. Up to Linux 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 required.
A call to ioprio_set() must follow both rules, or the call will
fail with the error EPERM.
glibc does not yet provide a suitable header file defining the
function prototypes and macros described on this page. Suitable
definitions can be found in linux/ioprio.h.
ionice(1), getpriority(2), open(2), capabilities(7), cgroups(7)
Documentation/block/ioprio.txt in the Linux kernel source tree
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.15 2025-05-17 ioprio_set(2)
Pages that refer to this page: ionice(1), gettid(2), io_uring_enter2(2), io_uring_enter(2), syscalls(2), systemd.exec(5), capabilities(7), cgroups(7), credentials(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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