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NAME
ioprio_get, ioprio_set - get/set I/O scheduling class and priority
SYNOPSIS
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.
DESCRIPTION
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:
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.
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 errno 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.
VERSIONS
These system calls have been available on Linux since kernel 2.6.13.
CONFORMING TO
These system calls are Linux-specific.
NOTES
Glibc does not provide a wrapper for these system calls; call them using syscall(2).
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.
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/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
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 assertions:
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 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)
Documentation/block/ioprio.txt in the Linux kernel source tree
COLOPHON
This page is part of release 3.54 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 2013-02-12 IOPRIO_SET(2)