1IOPRIO_GET(2) Linux Programmer's Manual IOPRIO_GET(2)
2
6 ioprio_get, ioprio_set - get/set I/O scheduling class and priority
7
9 #include <errno.h>
10 #include <linux/unistd.h>
11 _syscall2(int, ioprio_get, int, which, int, who);
13 _syscall3(int, ioprio_set, int, which, int, who, int, ioprio);
14 /* Using syscall(2) might be preferable; see intro(2) */
15 int ioprio_get(int which, int who);
17 int ioprio_set(int which, int who, int ioprio);
18
20 The ioprio_get() and ioprio_set() system calls respectively get and set
21 the I/O scheduling class and priority of one or more processes.
22 The which and who arguments identify the process(es) on which the sys‐
24 tem calls operate. The which argument determines how who is inter‐
25 preted, and has one of the following values:
26 IOPRIO_WHO_PROCESS
28 who is a process ID identifying a single process.
29 IOPRIO_WHO_PGRP
31 who is a process group ID identifying all the members of a
32 process group.
33 IOPRIO_WHO_USER
35 who is a user ID identifying all of the processes that have a
36 matching real UID.
37 If which is specified as IOPRIO_WHO_PGRP or IOPRIO_WHO_USER when call‐
39 ing ioprio_get(), and more than one process matches who, then the
40 returned priority will be the highest one found among all of the match‐
41 ing processes. One priority is said to be higher than another one if
42 it belongs to a higher priority class (IOPRIO_CLASS_RT is the highest
43 priority class; IOPRIO_CLASS_IDLE is the lowest) or if it belongs to
44 the same priority class as the other process but has a higher priority
45 level (a lower priority number means a higher priority level).
46 The ioprio argument given to ioprio_set() is a bit mask that specifies
48 both the scheduling class and the priority to be assigned to the target
49 process(es). The following macros are used for assembling and dissect‐
50 ing ioprio values:
51 IOPRIO_PRIO_VALUE(class, data)
53 Given a scheduling class and priority (data), this macro com‐
54 bines the two values to produce an ioprio value, which is
55 returned as the result of the macro.
56 IOPRIO_PRIO_CLASS(mask)
58 Given mask (an ioprio value), this macro returns its I/O class
59 component, that is, one of the values IOPRIO_CLASS_RT,
60 IOPRIO_CLASS_BE, or IOPRIO_CLASS_IDLE.
61 IOPRIO_PRIO_DATA(mask)
63 Given mask (an ioprio value), this macro returns its priority
64 (data) component.
65 See the NOTES section for more information on scheduling classes and
67 priorities.
68 I/O priorities are supported for reads and for synchronous (O_DIRECT,
70 O_SYNC) writes. I/O priorities are not supported for asynchronous
71 writes because they are issued outside the context of the program
72 dirtying the memory, and thus program-specific priorities do not apply.
73
76 On success, ioprio_get() returns the ioprio value of the process with
77 highest I/O priority of any of the processes that match the criteria
78 specified in which and who. On error, -1 is returned, and errno is set
79 to indicate the error.
80 On success, ioprio_set() returns 0. On error, -1 is returned, and
82 errno is set to indicate the error.
83
85 EPERM The calling process does not have the privilege needed to assign
86 this ioprio to the specified process(es). See the NOTES section
87 for more information on required privileges for ioprio_set().
88 ESRCH No process(es) could be found that matched the specification in
90 which and who.
91 EINVAL Invalid value for which or ioprio. Refer to the NOTES section
93 for available scheduler classes and priority levels for ioprio.
94
96 These system calls only have an effect when used in conjunction with an
97 I/O scheduler that supports I/O priorities. As at kernel 2.6.17 the
98 only such scheduler is the Completely Fair Queuing (CFQ) I/O scheduler.
99 Selecting an I/O Scheduler
101 I/O Schedulers are selected on a per-device basis via the special file
102 /sys/block/<device>/queue/scheduler.
103 One can view the current I/O scheduler via the /sys file system. For
105 example, the following command displays a list of all schedulers cur‐
106 rently loaded in the kernel:
107 $ cat /sys/block/hda/queue/scheduler
109 noop anticipatory deadline [cfq]
110 The scheduler surrounded by brackets is the one actually in use for the
112 device (hda in the example). Setting another scheduler is done by
113 writing the name of the new scheduler to this file. For example, the
114 following command will set the scheduler for the hda device to cfq:
115 $ su
117 Password:
118 # echo cfq > /sys/block/hda/queue/scheduler
119 The Completely Fair Queuing (CFQ) I/O Scheduler
121 Since v3 (aka CFQ Time Sliced) CFQ implements I/O nice levels similar
122 to those of CPU scheduling. These nice levels are grouped in three
123 scheduling classes each one containing one or more priority levels:
124 IOPRIO_CLASS_RT (1)
126 This is the real-time I/O class. This scheduling class is given
127 higher priority than any other class: processes from this class
128 are given first access to the disk every time. Thus this I/O
129 class needs to be used with some care: one I/O real-time process
130 can starve the entire system. Within the real-time class, there
131 are 8 levels of class data (priority) that determine exactly how
132 much time this process needs the disk for on each service. The
133 highest real-time priority level is 0; the lowest is 7. In the
134 future this might change to be more directly mappable to perfor‐
135 mance, by passing in a desired data rate instead.
136 IOPRIO_CLASS_BE (2)
138 This is the best-effort scheduling class, which is the default
139 for any process that hasn't set a specific I/O priority. The
140 class data (priority) determines how much I/O bandwidth the
141 process will get. Best-effort priority levels are analogous to
142 CPU nice values (see getpriority(2)). The priority level deter‐
143 mines a priority relative to other processes in the best-effort
144 scheduling class. Priority levels range from 0 (highest) to 7
145 (lowest).
146 IOPRIO_CLASS_IDLE (3)
148 This is the idle scheduling class. Processes running at this
149 level only get I/O time when no one else needs the disk. The
150 idle class has no class data. Attention is required when
151 assigning this priority class to a process, since it may become
152 starved if higher priority processes are constantly accessing
153 the disk.
154 Refer to Documentation/block/ioprio.txt for more information on the CFQ
156 I/O Scheduler and an example program.
157 Required permissions to set I/O priorities
159 Permission to change a process's priority is granted or denied based on
160 two assertions:
161 Process ownership
163 An unprivileged process may only set the I/O priority of a
164 process whose real UID matches the real or effective UID of the
165 calling process. A process which has the CAP_SYS_NICE capabil‐
166 ity can change the priority of any process.
167 What is the desired priority
169 Attempts to set very high priorities (IOPRIO_CLASS_RT) or very
170 low ones (IOPRIO_CLASS_IDLE) require the CAP_SYS_ADMIN capabil‐
171 ity.
172 A call to ioprio_set() must follow both rules, or the call will fail
174 with the error EPERM.
175
177 Glibc does not yet provide a suitable header file defining the function
178 prototypes and macros described on this page. Suitable definitions can
179 be found in linux/ioprio.h.
180
182 These system calls have been available on Linux since kernel 2.6.13.
183
185 These system calls are Linux specific.
186
188 getpriority(2), open(2), capabilities(7)
189 Documentation/block/ioprio.txt in the kernel source tree.
1912.6.13 2006-04-27 IOPRIO_GET(2)