-.\" This is _*_ nroff _*_ source. Emacs, gimme all those colors :)
-.\"
.\" Copyright (c) International Business Machines orp., 2006
.\"
+.\" %%%LICENSE_START(GPLv2+_SW_3_PARA)
.\" This program is free software; you can redistribute it and/or
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.\" published by the Free Software Foundation; either version 2 of
.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See
.\" the GNU General Public License for more details.
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-.\" Foundation, Inc., 59 Temple Place, Suite 330, Boston,
-.\" MA 02111-1307 USA
+.\" You should have received a copy of the GNU General Public
+.\" License along with this manual; if not, see
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.\"
.\" 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_SET 2 2008-07-09 "Linux" "Linux Programmer's Manual"
+.TH IOPRIO_SET 2 2013-02-12 "Linux" "Linux Programmer's Manual"
.SH NAME
ioprio_get, ioprio_set \- get/set I/O scheduling class and priority
.SH SYNOPSIS
.BI "int ioprio_get(int " which ", int " who );
.BI "int ioprio_set(int " which ", int " who ", int " ioprio );
.fi
+
+.IR Note :
+There are no glibc wrappers for these system calls; see NOTES.
.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.
+priority of one or more threads.
The
.I which
and
.I who
-arguments identify the process(es) on which the system
+arguments identify the thread(s) on which the system
calls operate.
The
.I which
.TP
.B IOPRIO_WHO_PROCESS
.I who
-is a process ID identifying a single process.
+is a process ID or thread ID identifying a single process or thread.
+If
+.I who
+is 0, then operate on the calling thread.
.TP
.B IOPRIO_WHO_PGRP
.I who
is a process group ID identifying all the members of a process group.
+If
+.I who
+is 0, then operate on the process group of which the caller is a member.
.TP
.B IOPRIO_WHO_USER
.I who
is a user ID identifying all of the processes that
have a matching real UID.
+.\" FIXME who==0 needs to be documented,
+.\" See http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=652443
.PP
If
.I which
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"
+.SH RETURN VALUE
On success,
.BR ioprio_get ()
returns the
.SH VERSIONS
These system calls have been available on Linux since
kernel 2.6.13.
-.SH "CONFORMING TO"
+.SH CONFORMING TO
These system calls are Linux-specific.
.SH NOTES
-Glibc does not provide wrapper for these system calls; call them using
+Glibc does not provide a wrapper for these system calls; call them using
.BR syscall (2).
+Two or more processes or threads can share an I/O context.
+This will be the case when
+.BR clone (2)
+was called with the
+.B CLONE_IO
+flag.
+However, by default, the distinct threads of a process will
+.B 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
+.BR ioprio_set ()
+on each of the threads.
+The thread ID that you would need for this operation
+is the one that is returned by
+.BR gettid (2)
+or
+.BR clone (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"
+.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 .
.RB "#" " echo cfq > /sys/block/hda/queue/scheduler"
.fi
.RE
-.SS "The Completely Fair Queuing (CFQ) I/O Scheduler"
+.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.
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"
+.SS Required permissions to set I/O priorities
Permission to change a process's priority is granted or denied based
on two assertions:
.TP
the function prototypes and macros described on this page.
Suitable definitions can be found in
.IR linux/ioprio.h .
-.SH "SEE ALSO"
+.SH SEE ALSO
+.BR ionice (1),
.BR getpriority (2),
.BR open (2),
.BR capabilities (7)
-.sp
-Documentation/block/ioprio.txt in the kernel source tree.
+
+.I Documentation/block/ioprio.txt
+in the Linux kernel source tree
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