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-.\" 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 <efleury@br.ibm.com>
-.\" with various additions by Michael Kerrisk <mtk.manpages@gmail.com>
-.\"
-.\"
-.TH IOPRIO_GET 2 2007-06-01 "Linux" "Linux Programmer's Manual"
-.SH NAME
-ioprio_get, ioprio_set \- get/set I/O scheduling class and priority
-.SH SYNOPSIS
-.nf
-.BI "int ioprio_get(int " which ", int " who );
-.BI "int ioprio_set(int " which ", int " who ", int " ioprio );
-.fi
-.SH DESCRIPTION
-The
-.BR ioprio_get ()
-and
-.BR ioprio_set ()
-system calls respectively get and set the I/O scheduling class and
-priority of one or more processes.
-
-The
-.I which
-and
-.I who
-arguments identify the process(es) on which the system
-calls operate.
-The
-.I which
-argument determines how
-.I who
-is interpreted, and has one of the following values:
-.TP
-.B IOPRIO_WHO_PROCESS
-.I who
-is a process ID identifying a single process.
-.TP
-.B IOPRIO_WHO_PGRP
-.I who
-is a process group ID identifying all the members of a process group.
-.TP
-.B IOPRIO_WHO_USER
-.I who
-is a user ID identifying all of the processes that
-have a matching real UID.
-.PP
-If
-.I which
-is specified as
-.B IOPRIO_WHO_PGRP
-or
-.B IOPRIO_WHO_USER
-when calling
-.BR ioprio_get (),
-and more than one process matches
-.IR 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
-.RB ( IOPRIO_CLASS_RT
-is the highest priority class;
-.B 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
-.I ioprio
-argument given to
-.BR 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
-.I ioprio
-values:
-.TP
-.BI IOPRIO_PRIO_VALUE( class ", " data )
-Given a scheduling
-.I class
-and priority
-.RI ( data ),
-this macro combines the two values to produce an
-.I ioprio
-value, which is returned as the result of the macro.
-.TP
-.BI IOPRIO_PRIO_CLASS( mask )
-Given
-.I mask
-(an
-.I ioprio
-value), this macro returns its I/O class component, that is,
-one of the values
-.BR IOPRIO_CLASS_RT ,
-.BR IOPRIO_CLASS_BE ,
-or
-.BR IOPRIO_CLASS_IDLE .
-.TP
-.BI IOPRIO_PRIO_DATA( mask )
-Given
-.I mask
-(an
-.I ioprio
-value), this macro returns its priority
-.RI ( data )
-component.
-.PP
-See the NOTES section for more
-information on scheduling classes and priorities.
-
-I/O priorities are supported for reads and for synchronous
-.RB ( O_DIRECT ,
-.BR 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.
-.SH "RETURN VALUE"
-On success,
-.BR ioprio_get ()
-returns the
-.I ioprio
-value of the process with highest I/O priority of any of the processes
-that match the criteria specified in
-.I which
-and
-.IR who .
-On error, \-1 is returned, and
-.I errno
-is set to indicate the error.
-.PP
-On success,
-.BR ioprio_set ()
-returns 0.
-On error, \-1 is returned, and
-.I errno
-is set to indicate the error.
-.SH ERRORS
-.TP
-.B EINVAL
-Invalid value for
-.I which
-or
-.IR ioprio .
-Refer to the NOTES section for available scheduler
-classes and priority levels for
-.IR ioprio .
-.TP
-.B EPERM
-The calling process does not have the privilege needed to assign this
-.I ioprio
-to the specified process(es).
-See the NOTES section for more information on required
-privileges for
-.BR ioprio_set ().
-.TP
-.B ESRCH
-No process(es) could be found that matched the specification in
-.I which
-and
-.IR who .
-.SH VERSIONS
-These system calls have been available on Linux since
-kernel 2.6.13.
-.SH "CONFORMING TO"
-These system calls are Linux specific.
-.SH NOTES
-Glibc does not provide wrapper for these system calls; call them using
-.BR 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.
-.SS "Selecting an I/O Scheduler"
-I/O Schedulers are selected on a per-device basis via the special
-file
-.IR /sys/block/<device>/queue/scheduler .
-
-One can view the current I/O scheduler via the
-.I /sys
-file system.
-For example, the following command
-displays a list of all schedulers currently loaded in the kernel:
-.sp
-.RS
-.nf
-$ cat /sys/block/hda/queue/scheduler
-noop anticipatory deadline [cfq]
-.fi
-.RE
-.sp
-The scheduler surrounded by brackets is the one actually
-in use for the device
-.RI ( 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
-.I hda
-device to
-.IR cfq :
-.sp
-.RS
-.nf
-$ su
-Password:
-# echo cfq > /sys/block/hda/queue/scheduler
-.fi
-.RE
-.SS "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:
-.TP
-.BR 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.
-.TP
-.BR 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
-.BR 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).
-.TP
-.BR 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.
-.PP
-Refer to
-.I Documentation/block/ioprio.txt
-for more information on the CFQ I/O Scheduler and an example program.
-.SS "Required permissions to set I/O priorities"
-Permission to change a process's priority is granted or denied based
-on two assertions:
-.TP
-.B "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
-.B CAP_SYS_NICE
-capability can change the priority of any process.
-.TP
-.B "What is the desired priority"
-Attempts to set very high priorities
-.RB ( IOPRIO_CLASS_RT )
-or very low ones
-.RB ( IOPRIO_CLASS_IDLE )
-require the
-.B CAP_SYS_ADMIN
-capability.
-.PP
-A call to
-.BR ioprio_set ()
-must follow both rules, or the call will fail with the error
-.BR EPERM .
-.SH BUGS
-.\" 6 May 07: Bug report raised:
-.\" http://sources.redhat.com/bugzilla/show_bug.cgi?id=4464
-.\" Ulriich Drepper replied that he wasn't going to add these
-.\" to glibc.
-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
-.IR linux/ioprio.h .
-.SH "SEE ALSO"
-.BR getpriority "(2), " open "(2), " capabilities (7)
-.sp
-Documentation/block/ioprio.txt in the kernel source tree.
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