Manpages - ioprio_set.2

ioprio_get, ioprio_set - get/set I/O scheduling class and priority

  #include <linux/ioprio.h> /* Definition of IOPRIO_* constants */
  #include <sys/syscall.h> /* Definition of SYS_* constants */
  #include <unistd.h>

  int syscall(SYS_ioprio_get, int which, int who);
  int syscall(SYS_ioprio_set, int which, int who, int ioprio);

Note: glibc provides no wrappers for these system calls, necessitating the use of *syscall*(2).

The *ioprio_get*() and *ioprio_set*() system calls get and set the I/O scheduling class and priority of one or more threads.

The which and who arguments identify the thread(s) 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 or thread ID identifying a single process or thread. If who is 0, then operate on the calling thread.

IOPRIO_WHO_PGRP

who is a process group ID identifying all the members of a process group. If who is 0, then operate on the process group of which the caller is a member.

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, as well as the meaning of specifying ioprio as 0.

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.

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.

These system calls have been available on Linux since kernel 2.6.13.

These system calls are Linux-specific.

Two or more processes or threads can share an I/O context. This will be the case when clone*(2) was called with the *CLONE_IO flag. However, by default, the distinct threads of a process will 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 *ioprio_set*() on each of the threads. The thread ID that you would need for this operation is the one that is returned by *gettid*(2) or *clone*(2).

These system calls have an effect only 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.

If no I/O scheduler has been set for a thread, then by default the I/O priority will follow the CPU nice value (*setpriority*(2)). In Linux kernels before version 2.6.24, once an I/O priority had been set using *ioprio_set*(), there was no way to reset the I/O scheduling behavior to the default. Since Linux 2.6.24, specifying ioprio as 0 can be used to reset to the default I/O scheduling behavior.

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.

*ionice*(1), *getpriority*(2), *open*(2), *capabilities*(7), *cgroups*(7)

Documentation/block/ioprio.txt in the Linux kernel source tree

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