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CentOS 7.0 - man page for ioprio_get (centos section 2)

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 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 pri-
       ority 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  indi-
       cate the error.

ERRORS
       EINVAL Invalid value for which or ioprio.  Refer to the NOTES section for available sched-
	      uler 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 file system.  For example, the follow-
       ing 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 lev-
	      els 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  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)


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