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]](../stylesheet/note.png) |
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-
whois a process ID or thread ID identifying a single process or thread. Ifwhois 0, then operate on the calling thread. IOPRIO_WHO_PGRP-
whois a process group ID identifying all the members of a process group. Ifwhois 0, then operate on the process group of which the caller is a member. IOPRIO_WHO_USER-
whois 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
classand priority (data), this macro combines the two values to produce aniopriovalue, which is returned as the result of the macro. IOPRIO_PRIO_CLASS(mask)-
Given
mask(aniopriovalue), this macro returns its I/O class component, that is, one of the valuesIOPRIO_CLASS_RT,IOPRIO_CLASS_BE, orIOPRIO_CLASS_IDLE. IOPRIO_PRIO_DATA(mask)-
Given
mask(aniopriovalue), 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 errno is set to indicate the error.
ERRORS
- EINVAL
-
Invalid value for
whichorioprio. Refer to the NOTES section for available scheduler classes and priority levels forioprio. - EPERM
-
The calling process does not have the privilege needed to assign this
ioprioto the specified process(es). See the NOTES section for more information on required privileges forioprio_set(). - ESRCH
-
No process(es) could be found that matched the specification in
whichandwho.
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.
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)). In Linux
kernels before version 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)). 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_NICEcapability can change the priority of any process. - What is the desired priority
-
Attempts to set very high priorities (
IOPRIO_CLASS_RT) require theCAP_SYS_ADMINcapability. Kernel versions up to 2.6.24 also requiredCAP_SYS_ADMINto 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
COLOPHON
This page is part of release 4.07 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
https://www.kernel.org/doc/man−pages/.
|
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