===================
Block io priorities
===================


Intro
-----

With the introduction of cfq v3 (aka cfq-ts or time sliced cfq), basic io
priorities are supported for reads on files.  This enables users to io nice
processes or process groups, similar to what has been possible with cpu
scheduling for ages.  This document mainly details the current possibilities
with cfq; other io schedulers do not support io priorities thus 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.

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
  #else
  #error "Unsupported arch"
  #endif

  static inline int ioprio_set(int which, int who, int ioprio)
  {
	return syscall(__NR_ioprio_set, which, who, ioprio);
  }

  static inline int ioprio_get(int which, int who)
  {
	return syscall(__NR_ioprio_get, which, 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;
  }


March 11 2005, Jens Axboe <jens.axboe@oracle.com>
