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
89_syscall3(int, ioprio_set, int, which, int, who, int, ioprio);
90_syscall2(int, ioprio_get, int, which, int, who);
91
92enum {
93	IOPRIO_CLASS_NONE,
94	IOPRIO_CLASS_RT,
95	IOPRIO_CLASS_BE,
96	IOPRIO_CLASS_IDLE,
97};
98
99enum {
100	IOPRIO_WHO_PROCESS = 1,
101	IOPRIO_WHO_PGRP,
102	IOPRIO_WHO_USER,
103};
104
105#define IOPRIO_CLASS_SHIFT	13
106
107const char *to_prio[] = { "none", "realtime", "best-effort", "idle", };
108
109int main(int argc, char *argv[])
110{
111	int ioprio = 4, set = 0, ioprio_class = IOPRIO_CLASS_BE;
112	int c, pid = 0;
113
114	while ((c = getopt(argc, argv, "+n:c:p:")) != EOF) {
115		switch (c) {
116		case 'n':
117			ioprio = strtol(optarg, NULL, 10);
118			set = 1;
119			break;
120		case 'c':
121			ioprio_class = strtol(optarg, NULL, 10);
122			set = 1;
123			break;
124		case 'p':
125			pid = strtol(optarg, NULL, 10);
126			break;
127		}
128	}
129
130	switch (ioprio_class) {
131		case IOPRIO_CLASS_NONE:
132			ioprio_class = IOPRIO_CLASS_BE;
133			break;
134		case IOPRIO_CLASS_RT:
135		case IOPRIO_CLASS_BE:
136			break;
137		case IOPRIO_CLASS_IDLE:
138			ioprio = 7;
139			break;
140		default:
141			printf("bad prio class %d\n", ioprio_class);
142			return 1;
143	}
144
145	if (!set) {
146		if (!pid && argv[optind])
147			pid = strtol(argv[optind], NULL, 10);
148
149		ioprio = ioprio_get(IOPRIO_WHO_PROCESS, pid);
150
151		printf("pid=%d, %d\n", pid, ioprio);
152
153		if (ioprio == -1)
154			perror("ioprio_get");
155		else {
156			ioprio_class = ioprio >> IOPRIO_CLASS_SHIFT;
157			ioprio = ioprio & 0xff;
158			printf("%s: prio %d\n", to_prio[ioprio_class], ioprio);
159		}
160	} else {
161		if (ioprio_set(IOPRIO_WHO_PROCESS, pid, ioprio | ioprio_class << IOPRIO_CLASS_SHIFT) == -1) {
162			perror("ioprio_set");
163			return 1;
164		}
165
166		if (argv[optind])
167			execvp(argv[optind], &argv[optind]);
168	}
169
170	return 0;
171}
172
173---> snip ionice.c tool <---
174
175
176March 11 2005, Jens Axboe <axboe@suse.de>
177