1=================== 2Block io priorities 3=================== 4 5 6Intro 7----- 8 9The io priority feature enables users to io nice processes or process groups, 10similar to what has been possible with cpu scheduling for ages. Support for io 11priorities is io scheduler dependent and currently supported by bfq and 12mq-deadline. 13 14Scheduling classes 15------------------ 16 17Three generic scheduling classes are implemented for io priorities that 18determine how io is served 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 60ionice.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 70 extern int sys_ioprio_set(int, int, int); 71 extern 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 #else 83 #error "Unsupported arch" 84 #endif 85 86 static inline int ioprio_set(int which, int who, int ioprio) 87 { 88 return syscall(__NR_ioprio_set, which, who, ioprio); 89 } 90 91 static inline int ioprio_get(int which, int who) 92 { 93 return syscall(__NR_ioprio_get, which, who); 94 } 95 96 enum { 97 IOPRIO_CLASS_NONE, 98 IOPRIO_CLASS_RT, 99 IOPRIO_CLASS_BE, 100 IOPRIO_CLASS_IDLE, 101 }; 102 103 enum { 104 IOPRIO_WHO_PROCESS = 1, 105 IOPRIO_WHO_PGRP, 106 IOPRIO_WHO_USER, 107 }; 108 109 #define IOPRIO_CLASS_SHIFT 13 110 111 const char *to_prio[] = { "none", "realtime", "best-effort", "idle", }; 112 113 int main(int argc, char *argv[]) 114 { 115 int ioprio = 4, set = 0, ioprio_class = IOPRIO_CLASS_BE; 116 int c, pid = 0; 117 118 while ((c = getopt(argc, argv, "+n:c:p:")) != EOF) { 119 switch (c) { 120 case 'n': 121 ioprio = strtol(optarg, NULL, 10); 122 set = 1; 123 break; 124 case 'c': 125 ioprio_class = strtol(optarg, NULL, 10); 126 set = 1; 127 break; 128 case 'p': 129 pid = strtol(optarg, NULL, 10); 130 break; 131 } 132 } 133 134 switch (ioprio_class) { 135 case IOPRIO_CLASS_NONE: 136 ioprio_class = IOPRIO_CLASS_BE; 137 break; 138 case IOPRIO_CLASS_RT: 139 case IOPRIO_CLASS_BE: 140 break; 141 case IOPRIO_CLASS_IDLE: 142 ioprio = 7; 143 break; 144 default: 145 printf("bad prio class %d\n", ioprio_class); 146 return 1; 147 } 148 149 if (!set) { 150 if (!pid && argv[optind]) 151 pid = strtol(argv[optind], NULL, 10); 152 153 ioprio = ioprio_get(IOPRIO_WHO_PROCESS, pid); 154 155 printf("pid=%d, %d\n", pid, ioprio); 156 157 if (ioprio == -1) 158 perror("ioprio_get"); 159 else { 160 ioprio_class = ioprio >> IOPRIO_CLASS_SHIFT; 161 ioprio = ioprio & 0xff; 162 printf("%s: prio %d\n", to_prio[ioprio_class], ioprio); 163 } 164 } else { 165 if (ioprio_set(IOPRIO_WHO_PROCESS, pid, ioprio | ioprio_class << IOPRIO_CLASS_SHIFT) == -1) { 166 perror("ioprio_set"); 167 return 1; 168 } 169 170 if (argv[optind]) 171 execvp(argv[optind], &argv[optind]); 172 } 173 174 return 0; 175 } 176 177 178March 11 2005, Jens Axboe <jens.axboe@oracle.com> 179