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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);

       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 priority

       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,

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

       See the NOTES section for more information on  scheduling  classes  and
       priorities, as well as the meaning of specifying ioprio as 0.

       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 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.

       If  no I/O scheduler has been set for a thread, then by default the I/O
       priority will follow the CPU nice  value  (setpriority(2)).   In  Linux
       kernels  before version 2.6.24, once an I/O priority had been set using
       ioprio_set(), there was no way to reset the I/O scheduling behavior  to
       the default.  Since Linux 2.6.24, specifying ioprio as 0 can be used to
       reset to the default I/O scheduling behavior.

   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 following  command  displays  a  list  of  all  schedulers
       currently loaded in the kernel:

              $ cat /sys/block/sda/queue/scheduler
              noop anticipatory deadline [cfq]

       The scheduler surrounded by brackets is the one actually in use for the
       device (sda 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 sda device to cfq:

              $ su
              # echo cfq > /sys/block/sda/queue/scheduler

   The Completely Fair Queuing (CFQ) I/O scheduler
       Since version 3 (also known as CFQ Time  Sliced),  CFQ  implements  I/O
       nice  levels similar to those of CPU scheduling.  These nice levels are
       grouped into 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 get I/O time only 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 the kernel source file 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 criteria:

       Process ownership
              An unprivileged process may set the  I/O  priority  only  for  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


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