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 respectively 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.

       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.

   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/hda/queue/scheduler
              noop anticipatory deadline [cfq]

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

              $ su
              Password:
              # echo cfq > /sys/block/hda/queue/scheduler

   The Completely Fair Queuing (CFQ) I/O scheduler
       Since v3 (aka CFQ Time Sliced) CFQ implements I/O nice levels similar to those of CPU scheduling.   These
       nice levels are grouped in 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 only get I/O time 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 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 assertions:

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

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

COLOPHON

       This page is part of release 3.54 of the Linux man-pages project.  A  description  of  the  project,  and
       information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.