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

          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 get and set
          the I/O scheduling class and priority of one or more

          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 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 dissecting ioprio values:

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          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

               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

          On success, ioprio_set() returns 0.  On error, -1 is
          returned, and errno is set to indicate the error.

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

               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().

               No process(es) could be found that matched the specifi-
               cation in which and who.

          These system calls have been available on Linux since kernel

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          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

          These system calls have an effect only when used in conjunc-
          tion 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 /sys/block/<device>/queue/scheduler.

          One can view the current I/O scheduler via the /sys filesys-
          tem.  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

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        The Completely Fair Queuing (CFQ) I/O
          Since version 3 (also known as CFQ Time Sliced), CFQ imple-
          ments 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: pro-
               cesses 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 pri-
               ority relative to other processes in the best-effort
               scheduling class.  Priority levels range from 0 (high-
               est) to 7 (lowest).

          IOPRIO_CLASS_IDLE (3)
               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 effec-
               tive UID of the calling process.  A process which has
               the CAP_SYS_NICE capability can change the priority of

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               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 required.

          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

          This page is part of release 5.10 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

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