ioprio_set(2) - Linux manual page (original) (raw)
ioprioset(2) System Calls Manual ioprioset(2)
NAME top
ioprio_get, ioprio_set - get/set I/O scheduling class and priorityLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#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)](../man2/syscall.2.html).DESCRIPTION top
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.RETURN VALUE top
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](../man3/errno.3.html)_ is set to indicate the error.
On success, **ioprio_set**() returns 0. On error, -1 is returned, and
_[errno](../man3/errno.3.html)_ is set to indicate the error.ERRORS top
**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 top
Linux.HISTORY top
Linux 2.6.13.NOTES top
Two or more processes or threads can share an I/O context. This
will be the case when [clone(2)](../man2/clone.2.html) 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)](../man2/gettid.2.html)
or [clone(2)](../man2/clone.2.html).
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)](../man2/setpriority.2.html)).
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)](../man2/getpriority.2.html)). 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**.BUGS top
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 top
[ionice(1)](../man1/ionice.1.html), [getpriority(2)](../man2/getpriority.2.html), [open(2)](../man2/open.2.html), [capabilities(7)](../man7/capabilities.7.html), [cgroups(7)](../man7/cgroups.7.html)
_Documentation/block/ioprio.txt_ in the Linux kernel source treeCOLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 ioprioset(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)