ioprio_set(2) — Linux manual page


ioprio_set(2)              System Calls Manual             ioprio_set(2)

NAME         top

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

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #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).

DESCRIPTION         top

       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:

              who is a process ID or thread ID identifying a single
              process or thread.  If who is 0, then operate on the
              calling thread.

              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.

              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.

              Given mask (an ioprio value), this macro returns its I/O
              class component, that is, one of the values

              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.

RETURN VALUE         top

       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         top

       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.

STANDARDS         top


HISTORY         top

       Linux 2.6.13.

NOTES         top

       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)).
       Before Linux 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.

   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 filesystem.
       For example, the following command displays a list of all
       schedulers currently loaded in the kernel:

           $ cat /sys/block/sda/queue/scheduler
           noop anticipatory deadline [cfq]

       The scheduler surrounded by brackets is the one actually in use
       for the device (sda 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
       sda device to cfq:

           $ su
           # echo cfq > /sys/block/sda/queue/scheduler

   The Completely Fair Queuing (CFQ) I/O scheduler
       Since version 3 (also known as CFQ Time Sliced), CFQ implements
       I/O nice levels similar to those of CPU scheduling.  These nice
       levels are grouped into 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

              This is the idle scheduling class.  Processes running at
              this level get I/O time only 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 the kernel source file Documentation/block/ioprio.txt
       for more information on the CFQ I/O Scheduler and an example

   Required permissions to set I/O priorities
       Permission to change a process's priority is granted or denied
       based on two criteria:

       Process ownership
              An unprivileged process may set the I/O priority only for
              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

       What is the desired priority
              Attempts to set very high priorities (IOPRIO_CLASS_RT)
              require the CAP_SYS_ADMIN capability.  Up to Linux 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         top

       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.

SEE ALSO         top

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

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

COLOPHON         top

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Linux man-pages 6.9.1          2024-05-02                  ioprio_set(2)

Pages that refer to this page: ionice(1)gettid(2)io_uring_enter2(2)io_uring_enter(2)syscalls(2)systemd.exec(5)capabilities(7)cgroups(7)credentials(7)