linux/Documentation/block/ioprio.txt
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   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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