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