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

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

SYNOPSIS
       #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
       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 (IO‐
       PRIO_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  IO‐
              PRIO_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.

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
       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 dis‐
       tinct 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, specify‐
       ing 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
           Password:
           # 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)  deter‐
              mines 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 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 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.

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

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

Linux                                                                          2021-06-20                                                                  IOPRIO_SET(2)