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       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);


       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  system  calls  operate.
       The which argument determines how who is interpreted, and has one of the following values:

              who is a process ID identifying a single process.

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

              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.

              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
       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 wrapper for these system calls; call them using syscall(2).

       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

       One  can  view  the  current  I/O  scheduler  via  the /sys file system.  For example, the
       following 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
              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  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 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 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.


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

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


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