# ioprio_set

## NAME



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


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <linux/ioprio.h>    **/* Definition of **IOPRIO_*** constants */
           **#include <sys/syscall.h>     **/* Definition of **SYS_*** constants */
           **#include <unistd.h>**
    
           **int syscall(SYS_ioprio_get, int** _which_**, int** _who_**);**
           **int syscall(SYS_ioprio_set, int** _which_**, int** _who_**, int** _ioprio_**);**
    
           _Note_ : glibc provides no wrappers for these system calls,
           necessitating the use of syscall(2).
    


## DESCRIPTION



           The **ioprio_get**() and **ioprio_set**() system calls 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:
    
           **IOPRIO_WHO_PROCESS**
                  _who_ is a process ID or thread ID identifying a single
                  process or thread.  If _who_ is 0, then operate on the
                  calling thread.
    
           **IOPRIO_WHO_PGRP**
                  _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.
    
           **IOPRIO_WHO_USER**
                  _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:
    
           **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.
    
           **IOPRIO_PRIO_CLASS(**_mask_**)**
                  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**.
    
           **IOPRIO_PRIO_DATA(**_mask_**)**
                  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.
    


## RETURN VALUE



           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.
    


## ERRORS



           **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_.
    


## STANDARDS



           Linux.
    


## NOTES



           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)).
           Before Linux 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_ 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**
               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:
    
           **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).
    
           **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 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.  Up to Linux 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**.
    


## BUGS



           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_.
    


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