IOPRIO_GET(2)              Linux Programmer's Manual             IOPRIO_GET(2)

       ioprio_get, ioprio_set - get/set I/O scheduling class and priority

       #include <errno.h>
       #include <linux/unistd.h>

       _syscall2(int, ioprio_get, int, which, int, who);
       _syscall3(int, ioprio_set, int, which, int, who, int, ioprio);
               /* Using syscall(2) might be preferable; see intro(2) */

       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  pro-
              cess group.

              who  is  a  user ID identifying all of the processes that have a
              matching real UID.

       If who 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  pro-
       cesses.   One  priority  is  said  to  be higher than another one if it
       belongs to a higher priority class (IOPRIO_CLASS_RT is the highest pri-
       ority  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.

       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.

       EINVAL Invalid  value  for which or ioprio.  Refer to the NOTES section
              for available scheduler classes and priority levels for  ioprio.

       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 set the scheduler for the hda device  to

              $ 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 pro-
              cess 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

              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 assign-
              ing this priority class 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 pro-
              cess 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)  or  very
              low  ones (IOPRIO_CLASS_IDLE) require the CAP_SYS_ADMIN capabil-

       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.

       These system calls have been available on Linux since kernel 2.6.13.

       These system calls are Linux specific.

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

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

2.6.13                            2006-04-27                     IOPRIO_GET(2)

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