1ioprio_set(2)                 System Calls Manual                ioprio_set(2)
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NAME

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

LIBRARY

9       Standard C library (libc, -lc)
10

SYNOPSIS

12       #include <linux/ioprio.h>    /* Definition of IOPRIO_* constants */
13       #include <sys/syscall.h>     /* Definition of SYS_* constants */
14       #include <unistd.h>
15
16       int syscall(SYS_ioprio_get, int which, int who);
17       int syscall(SYS_ioprio_set, int which, int who, int ioprio);
18
19       Note:  glibc provides no wrappers for these system calls, necessitating
20       the use of syscall(2).
21

DESCRIPTION

23       The ioprio_get() and ioprio_set() system calls  get  and  set  the  I/O
24       scheduling class and priority of one or more threads.
25
26       The  which and who arguments identify the thread(s) on which the system
27       calls operate.  The which argument determines how who  is  interpreted,
28       and has one of the following values:
29
30       IOPRIO_WHO_PROCESS
31              who is a process ID or thread ID identifying a single process or
32              thread.  If who is 0, then operate on the calling thread.
33
34       IOPRIO_WHO_PGRP
35              who is a process group ID  identifying  all  the  members  of  a
36              process  group.   If who is 0, then operate on the process group
37              of which the caller is a member.
38
39       IOPRIO_WHO_USER
40              who is a user ID identifying all of the processes  that  have  a
41              matching real UID.
42
43       If  which is specified as IOPRIO_WHO_PGRP or IOPRIO_WHO_USER when call‐
44       ing ioprio_get(), and more than one process matches who, then  the  re‐
45       turned priority will be the highest one found among all of the matching
46       processes.  One priority is said to be higher than another  one  if  it
47       belongs to a higher priority class (IOPRIO_CLASS_RT is the highest pri‐
48       ority class; IOPRIO_CLASS_IDLE is the lowest) or if it belongs  to  the
49       same  priority  class  as  the  other process but has a higher priority
50       level (a lower priority number means a higher priority level).
51
52       The ioprio argument given to ioprio_set() is a bit mask that  specifies
53       both the scheduling class and the priority to be assigned to the target
54       process(es).  The following macros are used for assembling and dissect‐
55       ing ioprio values:
56
57       IOPRIO_PRIO_VALUE(class, data)
58              Given  a  scheduling  class and priority (data), this macro com‐
59              bines the two values to produce an ioprio value,  which  is  re‐
60              turned as the result of the macro.
61
62       IOPRIO_PRIO_CLASS(mask)
63              Given  mask  (an ioprio value), this macro returns its I/O class
64              component, that is,  one  of  the  values  IOPRIO_CLASS_RT,  IO‐
65              PRIO_CLASS_BE, or IOPRIO_CLASS_IDLE.
66
67       IOPRIO_PRIO_DATA(mask)
68              Given  mask  (an  ioprio value), this macro returns its priority
69              (data) component.
70
71       See the NOTES section for more information on  scheduling  classes  and
72       priorities, as well as the meaning of specifying ioprio as 0.
73
74       I/O  priorities  are supported for reads and for synchronous (O_DIRECT,
75       O_SYNC) writes.  I/O priorities  are  not  supported  for  asynchronous
76       writes  because  they  are  issued  outside  the context of the program
77       dirtying the memory, and thus program-specific priorities do not apply.
78

RETURN VALUE

80       On success, ioprio_get() returns the ioprio value of the  process  with
81       highest  I/O  priority  of any of the processes that match the criteria
82       specified in which and who.  On error, -1 is returned, and errno is set
83       to indicate the error.
84
85       On  success, ioprio_set() returns 0.  On error, -1 is returned, and er‐
86       rno is set to indicate the error.
87

ERRORS

89       EINVAL Invalid value for which or ioprio.  Refer to the  NOTES  section
90              for available scheduler classes and priority levels for ioprio.
91
92       EPERM  The calling process does not have the privilege needed to assign
93              this ioprio to the specified process(es).  See the NOTES section
94              for more information on required privileges for ioprio_set().
95
96       ESRCH  No  process(es) could be found that matched the specification in
97              which and who.
98

STANDARDS

100       Linux.
101

HISTORY

103       Linux 2.6.13.
104

NOTES

106       Two or more processes or threads can share an I/O context.   This  will
107       be  the case when clone(2) was called with the CLONE_IO flag.  However,
108       by default, the distinct threads of a process will not share  the  same
109       I/O context.  This means that if you want to change the I/O priority of
110       all threads in a process, you may need to call ioprio_set() on each  of
111       the  threads.   The thread ID that you would need for this operation is
112       the one that is returned by gettid(2) or clone(2).
113
114       These system calls have an effect only when used in conjunction with an
115       I/O  scheduler  that  supports I/O priorities.  As at kernel 2.6.17 the
116       only such scheduler is the Completely Fair Queuing (CFQ) I/O scheduler.
117
118       If no I/O scheduler has been set for a thread, then by default the  I/O
119       priority will follow the CPU nice value (setpriority(2)).  Before Linux
120       2.6.24, once an I/O priority had been set using ioprio_set(), there was
121       no  way  to  reset  the  I/O scheduling behavior to the default.  Since
122       Linux 2.6.24, specifying ioprio as 0 can be used to reset  to  the  de‐
123       fault I/O scheduling behavior.
124
125   Selecting an I/O scheduler
126       I/O  schedulers are selected on a per-device basis via the special file
127       /sys/block/device/queue/scheduler.
128
129       One can view the current I/O scheduler via the  /sys  filesystem.   For
130       example,  the  following command displays a list of all schedulers cur‐
131       rently loaded in the kernel:
132
133           $ cat /sys/block/sda/queue/scheduler
134           noop anticipatory deadline [cfq]
135
136       The scheduler surrounded by brackets is the one actually in use for the
137       device  (sda  in  the  example).   Setting another scheduler is done by
138       writing the name of the new scheduler to this file.  For  example,  the
139       following command will set the scheduler for the sda device to cfq:
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141           $ su
142           Password:
143           # echo cfq > /sys/block/sda/queue/scheduler
144
145   The Completely Fair Queuing (CFQ) I/O scheduler
146       Since  version  3  (also  known as CFQ Time Sliced), CFQ implements I/O
147       nice levels similar to those of CPU scheduling.  These nice levels  are
148       grouped  into three scheduling classes, each one containing one or more
149       priority levels:
150
151       IOPRIO_CLASS_RT (1)
152              This is the real-time I/O class.  This scheduling class is given
153              higher  priority than any other class: processes from this class
154              are given first access to the disk every time.  Thus,  this  I/O
155              class needs to be used with some care: one I/O real-time process
156              can starve the entire system.  Within the real-time class, there
157              are 8 levels of class data (priority) that determine exactly how
158              much time this process needs the disk for on each service.   The
159              highest  real-time priority level is 0; the lowest is 7.  In the
160              future, this might change to be more directly mappable  to  per‐
161              formance, by passing in a desired data rate instead.
162
163       IOPRIO_CLASS_BE (2)
164              This  is  the best-effort scheduling class, which is the default
165              for any process that hasn't set a specific  I/O  priority.   The
166              class  data  (priority)  determines  how  much I/O bandwidth the
167              process will get.  Best-effort priority levels are analogous  to
168              CPU nice values (see getpriority(2)).  The priority level deter‐
169              mines a priority relative to other processes in the  best-effort
170              scheduling  class.   Priority levels range from 0 (highest) to 7
171              (lowest).
172
173       IOPRIO_CLASS_IDLE (3)
174              This is the idle scheduling class.  Processes  running  at  this
175              level  get  I/O  time only when no one else needs the disk.  The
176              idle class has no class data.  Attention is  required  when  as‐
177              signing  this  priority  class to a process, since it may become
178              starved if higher priority processes  are  constantly  accessing
179              the disk.
180
181       Refer to the kernel source file Documentation/block/ioprio.txt for more
182       information on the CFQ I/O Scheduler and an example program.
183
184   Required permissions to set I/O priorities
185       Permission to change a process's priority is granted or denied based on
186       two criteria:
187
188       Process ownership
189              An  unprivileged  process  may  set  the I/O priority only for a
190              process whose real UID matches the real or effective UID of  the
191              calling  process.  A process which has the CAP_SYS_NICE capabil‐
192              ity can change the priority of any process.
193
194       What is the desired priority
195              Attempts to set very high priorities  (IOPRIO_CLASS_RT)  require
196              the  CAP_SYS_ADMIN capability.  Up to Linux 2.6.24 also required
197              CAP_SYS_ADMIN to set a very  low  priority  (IOPRIO_CLASS_IDLE),
198              but since Linux 2.6.25, this is no longer required.
199
200       A  call  to  ioprio_set() must follow both rules, or the call will fail
201       with the error EPERM.
202

BUGS

204       glibc does not yet provide a suitable header file defining the function
205       prototypes and macros described on this page.  Suitable definitions can
206       be found in linux/ioprio.h.
207

SEE ALSO

209       ionice(1), getpriority(2), open(2), capabilities(7), cgroups(7)
210
211       Documentation/block/ioprio.txt in the Linux kernel source tree
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215Linux man-pages 6.05              2023-04-03                     ioprio_set(2)
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