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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);
     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  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
     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  cur-
     rently loaded in the kernel:
         $  cat  /sys/block/sda/queue/scheduler  noop  anticipatory deadline
     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 Password: # 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:
            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  per-
            formance, by passing in a desired data rate instead.
            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  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 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.


     ionice(1), getpriority(2), open(2), capabilities(7), cgroups(7)
     Documentation/block/ioprio.txt in the Linux kernel source tree


     This  page  is  part of release 4.16 of the Linux man-pages project.  A
     description of the project, information about reporting bugs,  and  the
     latest     version     of     this    page,    can    be    found    at

Linux 2017-09-15 IOPRIO_SET(2)

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