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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);

       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 pro-
       cess 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 prior-
       ity 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

              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  mem-
       ory, and thus program-specific priorities do not apply.

       On  success,  ioprio_get() returns the ioprio value of the process with highest I/O priority of any of the pro-
       cesses that match the criteria specified in which and who.  On error, -1 is returned, and errno is set to indi-
       cate 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 pri-
              ority levels for ioprio.

       EPERM  The calling process does not have the privilege needed to assign  this  ioprio  to  the  specified  pro-
              cess(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 priori-
       ties.  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 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

       IOPRIO_CLASS_BE (2)
              This is the best-effort scheduling class, which is the default for any process that hasn't  set  a  spe-
              cific  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 required.

       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.

       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at

Linux                             2008-07-09                     IOPRIO_SET(2)