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898bd37a | 1 | =================== |
52a5e15f JA |
2 | Block io priorities |
3 | =================== | |
4 | ||
5 | ||
6 | Intro | |
7 | ----- | |
8 | ||
9 | With the introduction of cfq v3 (aka cfq-ts or time sliced cfq), basic io | |
a982ac06 ML |
10 | priorities are supported for reads on files. This enables users to io nice |
11 | processes or process groups, similar to what has been possible with cpu | |
12 | scheduling for ages. This document mainly details the current possibilities | |
13 | with cfq; other io schedulers do not support io priorities thus far. | |
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14 | |
15 | Scheduling classes | |
16 | ------------------ | |
17 | ||
18 | CFQ implements three generic scheduling classes that determine how io is | |
19 | served for a process. | |
20 | ||
21 | IOPRIO_CLASS_RT: This is the realtime io class. This scheduling class is given | |
22 | higher priority than any other in the system, processes from this class are | |
23 | given first access to the disk every time. Thus it needs to be used with some | |
24 | care, one io RT process can starve the entire system. Within the RT class, | |
25 | there are 8 levels of class data that determine exactly how much time this | |
26 | process needs the disk for on each service. In the future this might change | |
27 | to be more directly mappable to performance, by passing in a wanted data | |
28 | rate instead. | |
29 | ||
30 | IOPRIO_CLASS_BE: This is the best-effort scheduling class, which is the default | |
31 | for any process that hasn't set a specific io priority. The class data | |
32 | determines how much io bandwidth the process will get, it's directly mappable | |
33 | to the cpu nice levels just more coarsely implemented. 0 is the highest | |
34 | BE prio level, 7 is the lowest. The mapping between cpu nice level and io | |
35 | nice level is determined as: io_nice = (cpu_nice + 20) / 5. | |
36 | ||
37 | IOPRIO_CLASS_IDLE: This is the idle scheduling class, processes running at this | |
38 | level only get io time when no one else needs the disk. The idle class has no | |
39 | class data, since it doesn't really apply here. | |
40 | ||
41 | Tools | |
42 | ----- | |
43 | ||
898bd37a | 44 | See below for a sample ionice tool. Usage:: |
52a5e15f | 45 | |
898bd37a | 46 | # ionice -c<class> -n<level> -p<pid> |
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47 | |
48 | If pid isn't given, the current process is assumed. IO priority settings | |
49 | are inherited on fork, so you can use ionice to start the process at a given | |
898bd37a | 50 | level:: |
52a5e15f | 51 | |
898bd37a | 52 | # ionice -c2 -n0 /bin/ls |
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53 | |
54 | will run ls at the best-effort scheduling class at the highest priority. | |
898bd37a | 55 | For a running process, you can give the pid instead:: |
52a5e15f | 56 | |
898bd37a | 57 | # ionice -c1 -n2 -p100 |
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58 | |
59 | will change pid 100 to run at the realtime scheduling class, at priority 2. | |
60 | ||
898bd37a MCC |
61 | ionice.c tool:: |
62 | ||
63 | #include <stdio.h> | |
64 | #include <stdlib.h> | |
65 | #include <errno.h> | |
66 | #include <getopt.h> | |
67 | #include <unistd.h> | |
68 | #include <sys/ptrace.h> | |
69 | #include <asm/unistd.h> | |
70 | ||
71 | extern int sys_ioprio_set(int, int, int); | |
72 | extern int sys_ioprio_get(int, int); | |
73 | ||
74 | #if defined(__i386__) | |
75 | #define __NR_ioprio_set 289 | |
76 | #define __NR_ioprio_get 290 | |
77 | #elif defined(__ppc__) | |
78 | #define __NR_ioprio_set 273 | |
79 | #define __NR_ioprio_get 274 | |
80 | #elif defined(__x86_64__) | |
81 | #define __NR_ioprio_set 251 | |
82 | #define __NR_ioprio_get 252 | |
83 | #elif defined(__ia64__) | |
84 | #define __NR_ioprio_set 1274 | |
85 | #define __NR_ioprio_get 1275 | |
86 | #else | |
87 | #error "Unsupported arch" | |
88 | #endif | |
89 | ||
90 | static inline int ioprio_set(int which, int who, int ioprio) | |
91 | { | |
3317fedb | 92 | return syscall(__NR_ioprio_set, which, who, ioprio); |
898bd37a | 93 | } |
3317fedb | 94 | |
898bd37a MCC |
95 | static inline int ioprio_get(int which, int who) |
96 | { | |
3317fedb | 97 | return syscall(__NR_ioprio_get, which, who); |
898bd37a | 98 | } |
52a5e15f | 99 | |
898bd37a | 100 | enum { |
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101 | IOPRIO_CLASS_NONE, |
102 | IOPRIO_CLASS_RT, | |
103 | IOPRIO_CLASS_BE, | |
104 | IOPRIO_CLASS_IDLE, | |
898bd37a | 105 | }; |
52a5e15f | 106 | |
898bd37a | 107 | enum { |
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108 | IOPRIO_WHO_PROCESS = 1, |
109 | IOPRIO_WHO_PGRP, | |
110 | IOPRIO_WHO_USER, | |
898bd37a | 111 | }; |
52a5e15f | 112 | |
898bd37a | 113 | #define IOPRIO_CLASS_SHIFT 13 |
52a5e15f | 114 | |
898bd37a | 115 | const char *to_prio[] = { "none", "realtime", "best-effort", "idle", }; |
52a5e15f | 116 | |
898bd37a MCC |
117 | int main(int argc, char *argv[]) |
118 | { | |
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119 | int ioprio = 4, set = 0, ioprio_class = IOPRIO_CLASS_BE; |
120 | int c, pid = 0; | |
121 | ||
122 | while ((c = getopt(argc, argv, "+n:c:p:")) != EOF) { | |
123 | switch (c) { | |
124 | case 'n': | |
125 | ioprio = strtol(optarg, NULL, 10); | |
126 | set = 1; | |
127 | break; | |
128 | case 'c': | |
129 | ioprio_class = strtol(optarg, NULL, 10); | |
130 | set = 1; | |
131 | break; | |
132 | case 'p': | |
133 | pid = strtol(optarg, NULL, 10); | |
134 | break; | |
135 | } | |
136 | } | |
137 | ||
138 | switch (ioprio_class) { | |
139 | case IOPRIO_CLASS_NONE: | |
140 | ioprio_class = IOPRIO_CLASS_BE; | |
141 | break; | |
142 | case IOPRIO_CLASS_RT: | |
143 | case IOPRIO_CLASS_BE: | |
144 | break; | |
145 | case IOPRIO_CLASS_IDLE: | |
146 | ioprio = 7; | |
147 | break; | |
148 | default: | |
149 | printf("bad prio class %d\n", ioprio_class); | |
150 | return 1; | |
151 | } | |
152 | ||
153 | if (!set) { | |
154 | if (!pid && argv[optind]) | |
155 | pid = strtol(argv[optind], NULL, 10); | |
156 | ||
157 | ioprio = ioprio_get(IOPRIO_WHO_PROCESS, pid); | |
158 | ||
159 | printf("pid=%d, %d\n", pid, ioprio); | |
160 | ||
161 | if (ioprio == -1) | |
162 | perror("ioprio_get"); | |
163 | else { | |
164 | ioprio_class = ioprio >> IOPRIO_CLASS_SHIFT; | |
165 | ioprio = ioprio & 0xff; | |
166 | printf("%s: prio %d\n", to_prio[ioprio_class], ioprio); | |
167 | } | |
168 | } else { | |
169 | if (ioprio_set(IOPRIO_WHO_PROCESS, pid, ioprio | ioprio_class << IOPRIO_CLASS_SHIFT) == -1) { | |
170 | perror("ioprio_set"); | |
171 | return 1; | |
172 | } | |
173 | ||
174 | if (argv[optind]) | |
175 | execvp(argv[optind], &argv[optind]); | |
176 | } | |
177 | ||
178 | return 0; | |
898bd37a | 179 | } |
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180 | |
181 | ||
26bbb29a | 182 | March 11 2005, Jens Axboe <jens.axboe@oracle.com> |