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

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

       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.

       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:

	      who is a process ID or thread ID identifying a single process or
	      thread.  If who is 0, then operate on the calling thread.

	      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.

	      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	 call-
       ing  ioprio_get(),  and	more  than  one	 process matches who, then the
       returned priority will be the highest one found among all of the match-
       ing  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 dissect-
       ing ioprio values:

       IOPRIO_PRIO_VALUE(class, data)
	      Given a scheduling class and priority (data),  this  macro  com-
	      bines  the  two  values  to  produce  an	ioprio value, which is
	      returned as the result of the macro.

	      Given mask (an ioprio value), this macro returns its  I/O	 class
	      component,   that	  is,	one  of	 the  values  IOPRIO_CLASS_RT,

	      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.

       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.

       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.

       These system calls have been available on Linux since kernel 2.6.13.

       These system calls are Linux-specific.

       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

       One can view the current I/O scheduler via the  /sys  filesystem.   For
       example,	 the  following command displays a list of all schedulers cur-
       rently 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
	      # 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  per-
	      formance, 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 deter-
	      mines a priority relative to other processes in the  best-effort
	      scheduling  class.   Priority levels range from 0 (highest) to 7

	      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 capabil-
	      ity 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

       A call to ioprio_set() must follow both rules, or the  call  will  fail
       with the error EPERM.

       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.

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

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

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Linux				  2016-05-09			 IOPRIO_SET(2)