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           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 pro-
                  cess 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  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,
                  IOPRIO_CLASS_BE, or IOPRIO_CLASS_IDLE.
           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.
       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 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
                  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
           IOPRIO_CLASS_IDLE (3)
                  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 set only 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) 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.



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