Root/Documentation/block/ioprio.txt

1Block io priorities
2===================
3
4
5Intro
6-----
7
8With the introduction of cfq v3 (aka cfq-ts or time sliced cfq), basic io
9priorities are supported for reads on files. This enables users to io nice
10processes or process groups, similar to what has been possible with cpu
11scheduling for ages. This document mainly details the current possibilities
12with cfq; other io schedulers do not support io priorities thus far.
13
14Scheduling classes
15------------------
16
17CFQ implements three generic scheduling classes that determine how io is
18served for a process.
19
20IOPRIO_CLASS_RT: This is the realtime io class. This scheduling class is given
21higher priority than any other in the system, processes from this class are
22given first access to the disk every time. Thus it needs to be used with some
23care, one io RT process can starve the entire system. Within the RT class,
24there are 8 levels of class data that determine exactly how much time this
25process needs the disk for on each service. In the future this might change
26to be more directly mappable to performance, by passing in a wanted data
27rate instead.
28
29IOPRIO_CLASS_BE: This is the best-effort scheduling class, which is the default
30for any process that hasn't set a specific io priority. The class data
31determines how much io bandwidth the process will get, it's directly mappable
32to the cpu nice levels just more coarsely implemented. 0 is the highest
33BE prio level, 7 is the lowest. The mapping between cpu nice level and io
34nice level is determined as: io_nice = (cpu_nice + 20) / 5.
35
36IOPRIO_CLASS_IDLE: This is the idle scheduling class, processes running at this
37level only get io time when no one else needs the disk. The idle class has no
38class data, since it doesn't really apply here.
39
40Tools
41-----
42
43See below for a sample ionice tool. Usage:
44
45# ionice -c<class> -n<level> -p<pid>
46
47If pid isn't given, the current process is assumed. IO priority settings
48are inherited on fork, so you can use ionice to start the process at a given
49level:
50
51# ionice -c2 -n0 /bin/ls
52
53will run ls at the best-effort scheduling class at the highest priority.
54For a running process, you can give the pid instead:
55
56# ionice -c1 -n2 -p100
57
58will change pid 100 to run at the realtime scheduling class, at priority 2.
59
60---> snip ionice.c tool <---
61
62#include <stdio.h>
63#include <stdlib.h>
64#include <errno.h>
65#include <getopt.h>
66#include <unistd.h>
67#include <sys/ptrace.h>
68#include <asm/unistd.h>
69
70extern int sys_ioprio_set(int, int, int);
71extern int sys_ioprio_get(int, int);
72
73#if defined(__i386__)
74#define __NR_ioprio_set 289
75#define __NR_ioprio_get 290
76#elif defined(__ppc__)
77#define __NR_ioprio_set 273
78#define __NR_ioprio_get 274
79#elif defined(__x86_64__)
80#define __NR_ioprio_set 251
81#define __NR_ioprio_get 252
82#elif defined(__ia64__)
83#define __NR_ioprio_set 1274
84#define __NR_ioprio_get 1275
85#else
86#error "Unsupported arch"
87#endif
88
89static inline int ioprio_set(int which, int who, int ioprio)
90{
91    return syscall(__NR_ioprio_set, which, who, ioprio);
92}
93
94static inline int ioprio_get(int which, int who)
95{
96    return syscall(__NR_ioprio_get, which, who);
97}
98
99enum {
100    IOPRIO_CLASS_NONE,
101    IOPRIO_CLASS_RT,
102    IOPRIO_CLASS_BE,
103    IOPRIO_CLASS_IDLE,
104};
105
106enum {
107    IOPRIO_WHO_PROCESS = 1,
108    IOPRIO_WHO_PGRP,
109    IOPRIO_WHO_USER,
110};
111
112#define IOPRIO_CLASS_SHIFT 13
113
114const char *to_prio[] = { "none", "realtime", "best-effort", "idle", };
115
116int main(int argc, char *argv[])
117{
118    int ioprio = 4, set = 0, ioprio_class = IOPRIO_CLASS_BE;
119    int c, pid = 0;
120
121    while ((c = getopt(argc, argv, "+n:c:p:")) != EOF) {
122        switch (c) {
123        case 'n':
124            ioprio = strtol(optarg, NULL, 10);
125            set = 1;
126            break;
127        case 'c':
128            ioprio_class = strtol(optarg, NULL, 10);
129            set = 1;
130            break;
131        case 'p':
132            pid = strtol(optarg, NULL, 10);
133            break;
134        }
135    }
136
137    switch (ioprio_class) {
138        case IOPRIO_CLASS_NONE:
139            ioprio_class = IOPRIO_CLASS_BE;
140            break;
141        case IOPRIO_CLASS_RT:
142        case IOPRIO_CLASS_BE:
143            break;
144        case IOPRIO_CLASS_IDLE:
145            ioprio = 7;
146            break;
147        default:
148            printf("bad prio class %d\n", ioprio_class);
149            return 1;
150    }
151
152    if (!set) {
153        if (!pid && argv[optind])
154            pid = strtol(argv[optind], NULL, 10);
155
156        ioprio = ioprio_get(IOPRIO_WHO_PROCESS, pid);
157
158        printf("pid=%d, %d\n", pid, ioprio);
159
160        if (ioprio == -1)
161            perror("ioprio_get");
162        else {
163            ioprio_class = ioprio >> IOPRIO_CLASS_SHIFT;
164            ioprio = ioprio & 0xff;
165            printf("%s: prio %d\n", to_prio[ioprio_class], ioprio);
166        }
167    } else {
168        if (ioprio_set(IOPRIO_WHO_PROCESS, pid, ioprio | ioprio_class << IOPRIO_CLASS_SHIFT) == -1) {
169            perror("ioprio_set");
170            return 1;
171        }
172
173        if (argv[optind])
174            execvp(argv[optind], &argv[optind]);
175    }
176
177    return 0;
178}
179
180---> snip ionice.c tool <---
181
182
183March 11 2005, Jens Axboe <jens.axboe@oracle.com>
184

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