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![Christian Loehle](/assets/img/avatar_default.png)
This doc hasn't been touched in a while, in the meantime some new io schedulers were added (e.g. all of mq), some with ioprio support. Also reword the introduction to remove reference to CFQ and the limitation that io priorities only work on reads, which is no longer true. Signed-off-by: Christian Loehle <christian.loehle@arm.com> Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com> Link: https://lore.kernel.org/r/a86cfdc8-016f-40f1-8b58-0cb15d2a792c@arm.com Signed-off-by: Jens Axboe <axboe@kernel.dk>
179 lines
4.5 KiB
ReStructuredText
179 lines
4.5 KiB
ReStructuredText
===================
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Block io priorities
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===================
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Intro
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-----
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The io priority feature enables users to io nice processes or process groups,
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similar to what has been possible with cpu scheduling for ages. Support for io
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priorities is io scheduler dependent and currently supported by bfq and
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mq-deadline.
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Scheduling classes
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------------------
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Three generic scheduling classes are implemented for io priorities that
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determine how io is 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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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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#else
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#error "Unsupported arch"
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#endif
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static inline int ioprio_set(int which, int who, int ioprio)
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{
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return syscall(__NR_ioprio_set, which, who, ioprio);
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}
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static inline int ioprio_get(int which, int who)
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{
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return syscall(__NR_ioprio_get, which, who);
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}
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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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March 11 2005, Jens Axboe <jens.axboe@oracle.com>
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