ioprio_set(2) System Calls Manual ioprio_set(2)
NAME
ioprio_get, ioprio_set - get/set I/O scheduling class and priority
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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).
DESCRIPTION
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 call-
ing ioprio_get(), and more than one process matches who, then the re-
turned 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 pri-
ority 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 dissect-
ing ioprio values:
IOPRIO_PRIO_VALUE(class, data)
Given a scheduling class and priority (data), this macro com-
bines the two values to produce an ioprio value, which is re-
turned 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, IO-
PRIO_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.
RETURN VALUE
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 er-
rno is set to indicate the error.
ERRORS
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.
STANDARDS
Linux.
HISTORY
Linux 2.6.13.
NOTES
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 de-
fault 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 cur-
rently 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 per-
formance, 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 deter-
mines 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 as-
signing 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 capabil-
ity 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.
BUGS
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.
SEE ALSO
ionice(1), getpriority(2), open(2), capabilities(7), cgroups(7)
Documentation/block/ioprio.txt in the Linux kernel source tree
Linux man-pages 6.7 2023-10-31 ioprio_set(2)
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