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);
DESCRIPTION
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_PROCESS
who
is a process ID identifying a single process.
IOPRIO_WHO_PGRP
who
is a process group ID identifying all the members of a process group.
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.
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
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.
VERSIONS
These system calls have been available on Linux since
kernel 2.6.13.
CONFORMING TO
These system calls are Linux-specific.
NOTES
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.
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
file system.
For example, the following command
displays a list of all schedulers currently loaded in the kernel:
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
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:
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.
Required permissions to set I/O priorities
Permission to change a process's priority is granted or denied based
on two assertions:
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
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.
Kernel versions up to 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
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.
Documentation/block/ioprio.txt in the kernel source tree.
COLOPHON
This page is part of release 3.14 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/.
