NAME

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

LIBRARY

       Standard C library (libc, -lc)

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 (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.

STANDARDS

       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 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)).  Before Linux 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.   Up  to  Linux  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