IOPRIO_SET(2)               Linux Programmer's Manual              IOPRIO_SET(2)

       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);

       Note: There are no glibc wrappers for these system calls; see NOTES.

       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 IOPRIO_CLASS_RT,

              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.

       Glibc does not provide a wrapper for these system calls; call them using

       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.

   Selecting an I/O scheduler
       I/O schedulers are selected on a per-device basis via the special file

       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

       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).

              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 program.

   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 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

       A call to ioprio_set() must follow both rules, or the call will fail with
       the error EPERM.

       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

       This page is part of release 5.11 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                              2019-03-06                      IOPRIO_SET(2)