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_PROCESSwho is a
process ID identifying a single process.
IOPRIO_WHO_PGRPwho is a
process group ID identifying all the members of a
process group.
IOPRIO_WHO_USERwho 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.
Invalid value for which or ioprio. Refer to the
NOTES section for available scheduler classes and
priority levels for ioprio.
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.
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:
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.
Attempts to set very high priorities (IOPRIO_CLASS_RT) or very low ones
(IOPRIO_CLASS_IDLE)
require the CAP_SYS_ADMIN capability.
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 2.79 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/.
|
This is _*_ nroff _*_ source. Emacs, gimme all those colors :) Copyright (c) International Business Machines orp., 2006 This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA HISTORY: 2006-04-27, created by Eduardo M. Fleury <efleurybr.ibm.com> with various additions by Michael Kerrisk <mtk.manpagesgmail.com> |