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