NAME

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

SYNOPSIS

       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.

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  (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.

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

       Glibc does not provide a wrapper for these system calls; call them using syscall(2).

       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
       /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) 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).

       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

COLOPHON

       This  page  is  part of release 5.05 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 https://www.kernel.org/doc/man-pages/.