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

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