[PATCH] CFQ io scheduler, add ioprio documentation

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>