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

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

SYNOPSIS
       #include <errno.h>
       #include <linux/unistd.h>

       _syscall2(int, ioprio_get, int, which, int, who);
       _syscall3(int, ioprio_set, int, which, int, who, int, ioprio);
	       /* Using syscall(2) might be preferable; see intro(2) */

       int ioprio_get(int which, int who);
       int ioprio_set(int which, int who, int ioprio);

DESCRIPTION
       The ioprio_get() and ioprio_set() system calls respectively get and set
       the I/O scheduling class and priority of one or more processes.

       The which and who arguments identify the process(es) on which the  sys‐
       tem  calls  operate.   The  which argument determines how who is inter‐
       preted, and has one of the following values:

       IOPRIO_WHO_PROCESS
	      who is a process ID identifying a single process.

       IOPRIO_WHO_PGRP
	      who is a process group ID	 identifying  all  the	members	 of  a
	      process group.

       IOPRIO_WHO_USER
	      who  is  a  user ID identifying all of the processes that have a
	      matching real UID.

       If who 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  pro‐
       cesses.	 One  priority	is  said  to  be higher than another one if it
       belongs to a higher priority class (IOPRIO_CLASS_RT is the highest pri‐
       ority  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.

       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
       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.

       EINVAL Invalid value for which or ioprio.  Refer to the	NOTES  section
	      for available scheduler classes and priority levels for ioprio.

NOTES
       These system calls only have an effect 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 file	 system.   For
       example,	 the  following command displays a list of all schedulers cur‐
       rently 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 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 perfor‐
	      mance, 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
	      (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 assign‐
	      ing  this priority class 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 only	set  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  capabil‐
	      ity can change the priority of any process.

       What is the desired priority
	      Attempts	to  set very high priorities (IOPRIO_CLASS_RT) or very
	      low ones (IOPRIO_CLASS_IDLE) require the CAP_SYS_ADMIN  capabil‐
	      ity.

       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.

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

CONFORMING TO
       These system calls are Linux specific.

SEE ALSO
       getpriority(2), open(2), capabilities(7)

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

2.6.13				  2006-04-27			 IOPRIO_GET(2)
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