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

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 inter-
       preted, 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.   How-
       ever,  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 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 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  val-
	      ues (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 3.53 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/.

Linux								    2013-02-12							     IOPRIO_SET(2)