1IOPRIO_SET(2)              Linux Programmer's Manual             IOPRIO_SET(2)
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NAME

6       ioprio_get, ioprio_set - get/set I/O scheduling class and priority
7

SYNOPSIS

9       #include <linux/ioprio.h>    /* Definition of IOPRIO_* constants */
10       #include <sys/syscall.h>     /* Definition of SYS_* constants */
11       #include <unistd.h>
12
13       int syscall(SYS_ioprio_get, int which, int who);
14       int syscall(SYS_ioprio_set, int which, int who, int ioprio);
15
16       Note:  glibc provides no wrappers for these system calls, necessitating
17       the use of syscall(2).
18

DESCRIPTION

20       The ioprio_get() and ioprio_set() system calls  get  and  set  the  I/O
21       scheduling class and priority of one or more threads.
22
23       The  which and who arguments identify the thread(s) on which the system
24       calls operate.  The which argument determines how who  is  interpreted,
25       and has one of the following values:
26
27       IOPRIO_WHO_PROCESS
28              who is a process ID or thread ID identifying a single process or
29              thread.  If who is 0, then operate on the calling thread.
30
31       IOPRIO_WHO_PGRP
32              who is a process group ID  identifying  all  the  members  of  a
33              process  group.   If who is 0, then operate on the process group
34              of which the caller is a member.
35
36       IOPRIO_WHO_USER
37              who is a user ID identifying all of the processes  that  have  a
38              matching real UID.
39
40       If  which is specified as IOPRIO_WHO_PGRP or IOPRIO_WHO_USER when call‐
41       ing ioprio_get(), and more than one process matches who, then  the  re‐
42       turned priority will be the highest one found among all of the matching
43       processes.  One priority is said to be higher than another  one  if  it
44       belongs to a higher priority class (IOPRIO_CLASS_RT is the highest pri‐
45       ority class; IOPRIO_CLASS_IDLE is the lowest) or if it belongs  to  the
46       same  priority  class  as  the  other process but has a higher priority
47       level (a lower priority number means a higher priority level).
48
49       The ioprio argument given to ioprio_set() is a bit mask that  specifies
50       both the scheduling class and the priority to be assigned to the target
51       process(es).  The following macros are used for assembling and dissect‐
52       ing ioprio values:
53
54       IOPRIO_PRIO_VALUE(class, data)
55              Given  a  scheduling  class and priority (data), this macro com‐
56              bines the two values to produce an ioprio value,  which  is  re‐
57              turned as the result of the macro.
58
59       IOPRIO_PRIO_CLASS(mask)
60              Given  mask  (an ioprio value), this macro returns its I/O class
61              component, that is,  one  of  the  values  IOPRIO_CLASS_RT,  IO‐
62              PRIO_CLASS_BE, or IOPRIO_CLASS_IDLE.
63
64       IOPRIO_PRIO_DATA(mask)
65              Given  mask  (an  ioprio value), this macro returns its priority
66              (data) component.
67
68       See the NOTES section for more information on  scheduling  classes  and
69       priorities, as well as the meaning of specifying ioprio as 0.
70
71       I/O  priorities  are supported for reads and for synchronous (O_DIRECT,
72       O_SYNC) writes.  I/O priorities  are  not  supported  for  asynchronous
73       writes  because  they  are  issued  outside  the context of the program
74       dirtying the memory, and thus program-specific priorities do not apply.
75

RETURN VALUE

77       On success, ioprio_get() returns the ioprio value of the  process  with
78       highest  I/O  priority  of any of the processes that match the criteria
79       specified in which and who.  On error, -1 is returned, and errno is set
80       to indicate the error.
81
82       On  success, ioprio_set() returns 0.  On error, -1 is returned, and er‐
83       rno is set to indicate the error.
84

ERRORS

86       EINVAL Invalid value for which or ioprio.  Refer to the  NOTES  section
87              for available scheduler classes and priority levels for ioprio.
88
89       EPERM  The calling process does not have the privilege needed to assign
90              this ioprio to the specified process(es).  See the NOTES section
91              for more information on required privileges for ioprio_set().
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93       ESRCH  No  process(es) could be found that matched the specification in
94              which and who.
95

VERSIONS

97       These system calls have been available on Linux since kernel 2.6.13.
98

CONFORMING TO

100       These system calls are Linux-specific.
101

NOTES

103       Two or more processes or threads can share an I/O context.   This  will
104       be  the case when clone(2) was called with the CLONE_IO flag.  However,
105       by default, the distinct threads of a process will not share  the  same
106       I/O context.  This means that if you want to change the I/O priority of
107       all threads in a process, you may need to call ioprio_set() on each  of
108       the  threads.   The thread ID that you would need for this operation is
109       the one that is returned by gettid(2) or clone(2).
110
111       These system calls have an effect only when used in conjunction with an
112       I/O  scheduler  that  supports I/O priorities.  As at kernel 2.6.17 the
113       only such scheduler is the Completely Fair Queuing (CFQ) I/O scheduler.
114
115       If no I/O scheduler has been set for a thread, then by default the  I/O
116       priority  will  follow  the  CPU nice value (setpriority(2)).  In Linux
117       kernels before version 2.6.24, once an I/O priority had been set  using
118       ioprio_set(),  there was no way to reset the I/O scheduling behavior to
119       the default.  Since Linux 2.6.24, specifying ioprio as 0 can be used to
120       reset to the default I/O scheduling behavior.
121
122   Selecting an I/O scheduler
123       I/O  schedulers are selected on a per-device basis via the special file
124       /sys/block/<device>/queue/scheduler.
125
126       One can view the current I/O scheduler via the  /sys  filesystem.   For
127       example,  the  following command displays a list of all schedulers cur‐
128       rently loaded in the kernel:
129
130           $ cat /sys/block/sda/queue/scheduler
131           noop anticipatory deadline [cfq]
132
133       The scheduler surrounded by brackets is the one actually in use for the
134       device  (sda  in  the  example).   Setting another scheduler is done by
135       writing the name of the new scheduler to this file.  For  example,  the
136       following command will set the scheduler for the sda device to cfq:
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138           $ su
139           Password:
140           # echo cfq > /sys/block/sda/queue/scheduler
141
142   The Completely Fair Queuing (CFQ) I/O scheduler
143       Since  version  3  (also  known as CFQ Time Sliced), CFQ implements I/O
144       nice levels similar to those of CPU scheduling.  These nice levels  are
145       grouped  into three scheduling classes, each one containing one or more
146       priority levels:
147
148       IOPRIO_CLASS_RT (1)
149              This is the real-time I/O class.  This scheduling class is given
150              higher  priority than any other class: processes from this class
151              are given first access to the disk every time.  Thus,  this  I/O
152              class needs to be used with some care: one I/O real-time process
153              can starve the entire system.  Within the real-time class, there
154              are 8 levels of class data (priority) that determine exactly how
155              much time this process needs the disk for on each service.   The
156              highest  real-time priority level is 0; the lowest is 7.  In the
157              future, this might change to be more directly mappable  to  per‐
158              formance, by passing in a desired data rate instead.
159
160       IOPRIO_CLASS_BE (2)
161              This  is  the best-effort scheduling class, which is the default
162              for any process that hasn't set a specific  I/O  priority.   The
163              class  data  (priority)  determines  how  much I/O bandwidth the
164              process will get.  Best-effort priority levels are analogous  to
165              CPU nice values (see getpriority(2)).  The priority level deter‐
166              mines a priority relative to other processes in the  best-effort
167              scheduling  class.   Priority levels range from 0 (highest) to 7
168              (lowest).
169
170       IOPRIO_CLASS_IDLE (3)
171              This is the idle scheduling class.  Processes  running  at  this
172              level  get  I/O  time only when no one else needs the disk.  The
173              idle class has no class data.  Attention is  required  when  as‐
174              signing  this  priority  class to a process, since it may become
175              starved if higher priority processes  are  constantly  accessing
176              the disk.
177
178       Refer to the kernel source file Documentation/block/ioprio.txt for more
179       information on the CFQ I/O Scheduler and an example program.
180
181   Required permissions to set I/O priorities
182       Permission to change a process's priority is granted or denied based on
183       two criteria:
184
185       Process ownership
186              An  unprivileged  process  may  set  the I/O priority only for a
187              process whose real UID matches the real or effective UID of  the
188              calling  process.  A process which has the CAP_SYS_NICE capabil‐
189              ity can change the priority of any process.
190
191       What is the desired priority
192              Attempts to set very high priorities  (IOPRIO_CLASS_RT)  require
193              the CAP_SYS_ADMIN capability.  Kernel versions up to 2.6.24 also
194              required  CAP_SYS_ADMIN  to  set  a  very  low   priority   (IO‐
195              PRIO_CLASS_IDLE),  but since Linux 2.6.25, this is no longer re‐
196              quired.
197
198       A call to ioprio_set() must follow both rules, or the  call  will  fail
199       with the error EPERM.
200

BUGS

202       Glibc does not yet provide a suitable header file defining the function
203       prototypes and macros described on this page.  Suitable definitions can
204       be found in linux/ioprio.h.
205

SEE ALSO

207       ionice(1), getpriority(2), open(2), capabilities(7), cgroups(7)
208
209       Documentation/block/ioprio.txt in the Linux kernel source tree
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COLOPHON

212       This  page  is  part of release 5.13 of the Linux man-pages project.  A
213       description of the project, information about reporting bugs,  and  the
214       latest     version     of     this    page,    can    be    found    at
215       https://www.kernel.org/doc/man-pages/.
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219Linux                             2021-06-20                     IOPRIO_SET(2)
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