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Diffstat (limited to 'Documentation/block')
| -rw-r--r-- | Documentation/block/ioprio.txt | 176 |
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diff --git a/Documentation/block/ioprio.txt b/Documentation/block/ioprio.txt new file mode 100644 index 000000000000..96ccf681075e --- /dev/null +++ b/Documentation/block/ioprio.txt @@ -0,0 +1,176 @@ +Block io priorities +=================== + + +Intro +----- + +With the introduction of cfq v3 (aka cfq-ts or time sliced cfq), basic io +priorities is supported for reads on files. This enables users to io nice +processes or process groups, similar to what has been possible to cpu +scheduling for ages. This document mainly details the current possibilites +with cfq, other io schedulers do not support io priorities so far. + +Scheduling classes +------------------ + +CFQ implements three generic scheduling classes that determine how io is +served for a process. + +IOPRIO_CLASS_RT: This is the realtime io class. This scheduling class is given +higher priority than any other in the system, processes from this class are +given first access to the disk every time. Thus it needs to be used with some +care, one io RT process can starve the entire system. Within the RT class, +there are 8 levels of class data that determine exactly how much time this +process needs the disk for on each service. In the future this might change +to be more directly mappable to performance, by passing in a wanted data +rate instead. + +IOPRIO_CLASS_BE: This is the best-effort scheduling class, which is the default +for any process that hasn't set a specific io priority. The class data +determines how much io bandwidth the process will get, it's directly mappable +to the cpu nice levels just more coarsely implemented. 0 is the highest +BE prio level, 7 is the lowest. The mapping between cpu nice level and io +nice level is determined as: io_nice = (cpu_nice + 20) / 5. + +IOPRIO_CLASS_IDLE: This is the idle scheduling class, processes running at this +level only get io time when no one else needs the disk. The idle class has no +class data, since it doesn't really apply here. + +Tools +----- + +See below for a sample ionice tool. Usage: + +# ionice -c<class> -n<level> -p<pid> + +If pid isn't given, the current process is assumed. IO priority settings +are inherited on fork, so you can use ionice to start the process at a given +level: + +# ionice -c2 -n0 /bin/ls + +will run ls at the best-effort scheduling class at the highest priority. +For a running process, you can give the pid instead: + +# ionice -c1 -n2 -p100 + +will change pid 100 to run at the realtime scheduling class, at priority 2. + +---> snip ionice.c tool <--- + +#include <stdio.h> +#include <stdlib.h> +#include <errno.h> +#include <getopt.h> +#include <unistd.h> +#include <sys/ptrace.h> +#include <asm/unistd.h> + +extern int sys_ioprio_set(int, int, int); +extern int sys_ioprio_get(int, int); + +#if defined(__i386__) +#define __NR_ioprio_set 289 +#define __NR_ioprio_get 290 +#elif defined(__ppc__) +#define __NR_ioprio_set 273 +#define __NR_ioprio_get 274 +#elif defined(__x86_64__) +#define __NR_ioprio_set 251 +#define __NR_ioprio_get 252 +#elif defined(__ia64__) +#define __NR_ioprio_set 1274 +#define __NR_ioprio_get 1275 +#else +#error "Unsupported arch" +#endif + +_syscall3(int, ioprio_set, int, which, int, who, int, ioprio); +_syscall2(int, ioprio_get, int, which, int, who); + +enum { + IOPRIO_CLASS_NONE, + IOPRIO_CLASS_RT, + IOPRIO_CLASS_BE, + IOPRIO_CLASS_IDLE, +}; + +enum { + IOPRIO_WHO_PROCESS = 1, + IOPRIO_WHO_PGRP, + IOPRIO_WHO_USER, +}; + +#define IOPRIO_CLASS_SHIFT 13 + +const char *to_prio[] = { "none", "realtime", "best-effort", "idle", }; + +int main(int argc, char *argv[]) +{ + int ioprio = 4, set = 0, ioprio_class = IOPRIO_CLASS_BE; + int c, pid = 0; + + while ((c = getopt(argc, argv, "+n:c:p:")) != EOF) { + switch (c) { + case 'n': + ioprio = strtol(optarg, NULL, 10); + set = 1; + break; + case 'c': + ioprio_class = strtol(optarg, NULL, 10); + set = 1; + break; + case 'p': + pid = strtol(optarg, NULL, 10); + break; + } + } + + switch (ioprio_class) { + case IOPRIO_CLASS_NONE: + ioprio_class = IOPRIO_CLASS_BE; + break; + case IOPRIO_CLASS_RT: + case IOPRIO_CLASS_BE: + break; + case IOPRIO_CLASS_IDLE: + ioprio = 7; + break; + default: + printf("bad prio class %d\n", ioprio_class); + return 1; + } + + if (!set) { + if (!pid && argv[optind]) + pid = strtol(argv[optind], NULL, 10); + + ioprio = ioprio_get(IOPRIO_WHO_PROCESS, pid); + + printf("pid=%d, %d\n", pid, ioprio); + + if (ioprio == -1) + perror("ioprio_get"); + else { + ioprio_class = ioprio >> IOPRIO_CLASS_SHIFT; + ioprio = ioprio & 0xff; + printf("%s: prio %d\n", to_prio[ioprio_class], ioprio); + } + } else { + if (ioprio_set(IOPRIO_WHO_PROCESS, pid, ioprio | ioprio_class << IOPRIO_CLASS_SHIFT) == -1) { + perror("ioprio_set"); + return 1; + } + + if (argv[optind]) + execvp(argv[optind], &argv[optind]); + } + + return 0; +} + +---> snip ionice.c tool <--- + + +March 11 2005, Jens Axboe <axboe@suse.de> |