ioprio_set() - Unix, Linux System Call
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);
ioprio_set() system calls respectively get and set the I/O scheduling class and
priority of one or more processes.
who arguments identify the process(es) on which the system
which argument determines how
who is interpreted, and has one of the following values:
See the NOTES section for more
information on scheduling classes and priorities.
who is a process ID identifying a single process.
who is a process group ID identifying all the members of a process group.
who is a user ID identifying all of the processes that
have a matching real UID.
which is specified as
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
(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).
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_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 value), this macro returns its I/O class component, that is,
one of the values
IOPRIO_CLASS_RT, IOPRIO_CLASS_BE, or
ioprio value), this macro returns its priority
I/O priorities are supported for reads and for synchronous (O_DIRECT,
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.
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
who. On error, -1 is returned, and
errno is set to indicate the error.
ioprio_set() returns 0.
On error, -1 is returned, and
errno is set to indicate the error.
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
No process(es) could be found that matched the specification in
Invalid value for
ioprio. Refer to the NOTES section for available scheduler
classes and priority levels for
These system calls have been available on Linux since
These system calls are Linux specific.
Glibc does not provide wrapper for these system calls; call them using
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
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
# 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:
Documentation/block/ioprio.txt for more information on the CFQ I/O Scheduler and an example program.
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 hasnt 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
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
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.
Required permissions to set I/O priorities
Permission to change a processs priority is granted or denied based
on two assertions:
A call to
ioprio_set() must follow both rules, or the call will fail with the error
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) or very low ones
(IOPRIO_CLASS_IDLE) require the
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
Documentation/block/ioprio.txt in the kernel source tree.