NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON
IOPRIO_SET(2) Linux Programmer's Manual IOPRIO_SET(2)
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);
The ioprio_get() and ioprio_set() system calls respectively get and set the
I/O scheduling class and priority of one or more processes.
The which and who arguments identify the process(es) 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 identifying a single process.
IOPRIO_WHO_PGRP
who is a process group ID identifying all the members of a process
group.
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.
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.
These system calls have been available on Linux since kernel 2.6.13.
These system calls are Linux-specific.
Glibc does not provide wrapper for these system calls; call them using
syscall(2).
These system calls only have an effect 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.
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 file system. 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 cfq:
$ su
Password:
# 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:
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.
Permission to change a process's priority is granted or denied based on two
assertions:
Process ownership
An unprivileged process may only set 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 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.
getpriority(2), open(2), capabilities(7)
Documentation/block/ioprio.txt in the kernel source tree.
This page is part of release 3.23 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2008-07-09 IOPRIO_SET(2)