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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  sys-
       tem  calls  operate.   The  which argument determines how who is inter-
       preted, 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  call-
       ing  ioprio_get(),  and  more  than  one  process matches who, then the
       returned priority will be the highest one found among all of the match-
       ing  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 dissect-
       ing ioprio values:

       IOPRIO_PRIO_VALUE(class, data)
              Given a scheduling class and priority (data),  this  macro  com-
              bines  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,

              Given mask (an ioprio value), this macro  returns  its  priority
              (data) component.

       See  the  NOTES  section for more information on scheduling classes and

       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.

       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

       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  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
              # 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 perfor-
              mance, 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 deter-
              mines a priority relative to other processes in the  best-effort
              scheduling  class.   Priority levels range from 0 (highest) to 7

              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 capabil-
              ity 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.

       This  page  is  part of release 3.05 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                             2008-07-09                     IOPRIO_SET(2)