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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 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.

              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.

              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

       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.

       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.

       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.

   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  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
              # 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).

              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

       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)

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

       This page is part of release 3.74 of the Linux man-pages project.  A  description  of  the
       project,  information  about  reporting  bugs, and the latest version of this page, can be
       found at http://www.kernel.org/doc/man-pages/.

Linux                                       2013-02-12                              IOPRIO_SET(2)

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