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);
: There are no glibc wrappers for these system calls; see NOTES.
() and ioprio_set
() system calls respectively get
and set the I/O scheduling class and priority of one or more threads.
arguments identify the thread(s) on which the
system calls operate. The which
argument determines how who
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.
is specified as IOPRIO_WHO_PGRP
when calling ioprio_get
(), and more than one process matches
, 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
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).
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
- 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.
- 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 (data) component.
See the NOTES section for more information on scheduling classes and priorities.
I/O priorities are supported for reads and for synchronous (O_DIRECT
) 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
. On error, -1 is returned, and
is set to indicate the error.
On success, ioprio_set
() returns 0. On error, -1 is returned, and
is set to indicate the error.
- Invalid value for which or ioprio. Refer to the NOTES
section for available scheduler classes and priority levels for
- 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().
- 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 using
Two or more processes or threads can share an I/O context. This will be the case
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)
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.
Selecting an I/O scheduler¶
I/O Schedulers are selected on a per-device basis via the special file
One can view the current I/O scheduler via the /sys
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
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 cfq
# echo cfq > /sys/block/hda/queue/scheduler
The Completely Fair Queuing (CFQ) I/O 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:
- 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 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.
Required permissions to set I/O priorities¶
Permission to change a process's priority is granted or denied based on two
- Process ownership
- 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
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
in the Linux kernel source tree
This page is part of release 3.74 of the Linux man-pages
description of the project, information about reporting bugs, and the latest
version of this page, can be found at