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/* Generic associative array implementation.
 *
 * See Documentation/core-api/assoc_array.rst for information.
 *
 * Copyright (C) 2013 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public Licence
 * as published by the Free Software Foundation; either version
 * 2 of the Licence, or (at your option) any later version.
 */

#ifndef _LINUX_ASSOC_ARRAY_H
#define _LINUX_ASSOC_ARRAY_H

#ifdef CONFIG_ASSOCIATIVE_ARRAY

#include <linux/types.h>

#define ASSOC_ARRAY_KEY_CHUNK_SIZE BITS_PER_LONG /* Key data retrieved in chunks of this size */

/*
 * Generic associative array.
 */
struct assoc_array {
	struct assoc_array_ptr	*root;		/* The node at the root of the tree */
	unsigned long		nr_leaves_on_tree;
};

/*
 * Operations on objects and index keys for use by array manipulation routines.
 */
struct assoc_array_ops {
	/* Method to get a chunk of an index key from caller-supplied data */
	unsigned long (*get_key_chunk)(const void *index_key, int level);

	/* Method to get a piece of an object's index key */
	unsigned long (*get_object_key_chunk)(const void *object, int level);

	/* Is this the object we're looking for? */
	bool (*compare_object)(const void *object, const void *index_key);

	/* How different is an object from an index key, to a bit position in
	 * their keys? (or -1 if they're the same)
	 */
	int (*diff_objects)(const void *object, const void *index_key);

	/* Method to free an object. */
	void (*free_object)(void *object);
};

/*
 * Access and manipulation functions.
 */
struct assoc_array_edit;

static inline void assoc_array_init(struct assoc_array *array)
{
	array->root = NULL;
	array->nr_leaves_on_tree = 0;
}

extern int assoc_array_iterate(const struct assoc_array *array,
			       int (*iterator)(const void *object,
					       void *iterator_data),
			       void *iterator_data);
extern void *assoc_array_find(const struct assoc_array *array,
			      const struct assoc_array_ops *ops,
			      const void *index_key);
extern void assoc_array_destroy(struct assoc_array *array,
				const struct assoc_array_ops *ops);
extern struct assoc_array_edit *assoc_array_insert(struct assoc_array *array,
						   const struct assoc_array_ops *ops,
						   const void *index_key,
						   void *object);
extern void assoc_array_insert_set_object(struct assoc_array_edit *edit,
					  void *object);
extern struct assoc_array_edit *assoc_array_delete(struct assoc_array *array,
						   const struct assoc_array_ops *ops,
						   const void *index_key);
extern struct assoc_array_edit *assoc_array_clear(struct assoc_array *array,
						  const struct assoc_array_ops *ops);
extern void assoc_array_apply_edit(struct assoc_array_edit *edit);
extern void assoc_array_cancel_edit(struct assoc_array_edit *edit);
extern int assoc_array_gc(struct assoc_array *array,
			  const struct assoc_array_ops *ops,
			  bool (*iterator)(void *object, void *iterator_data),
			  void *iterator_data);

#endif /* CONFIG_ASSOCIATIVE_ARRAY */
#endif /* _LINUX_ASSOC_ARRAY_H */
/proc/diskstats, and the other is within the sysfs file system, which must be mounted in order to obtain the information. Throughout this document we'll assume that sysfs is mounted on /sys, although of course it may be mounted anywhere. Both /proc/diskstats and sysfs use the same source for the information and so should not differ. Here are examples of these different formats: 2.4: 3 0 39082680 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160 3 1 9221278 hda1 35486 0 35496 38030 0 0 0 0 0 38030 38030 2.6 sysfs: 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160 35486 38030 38030 38030 2.6 diskstats: 3 0 hda 446216 784926 9550688 4382310 424847 312726 5922052 19310380 0 3376340 23705160 3 1 hda1 35486 38030 38030 38030 On 2.4 you might execute "grep 'hda ' /proc/partitions". On 2.6, you have a choice of "cat /sys/block/hda/stat" or "grep 'hda ' /proc/diskstats". The advantage of one over the other is that the sysfs choice works well if you are watching a known, small set of disks. /proc/diskstats may be a better choice if you are watching a large number of disks because you'll avoid the overhead of 50, 100, or 500 or more opens/closes with each snapshot of your disk statistics. In 2.4, the statistics fields are those after the device name. In the above example, the first field of statistics would be 446216. By contrast, in 2.6 if you look at /sys/block/hda/stat, you'll find just the eleven fields, beginning with 446216. If you look at /proc/diskstats, the eleven fields will be preceded by the major and minor device numbers, and device name. Each of these formats provide eleven fields of statistics, each meaning exactly the same things. All fields except field 9 are cumulative since boot. Field 9 should go to zero as I/Os complete; all others only increase. Yes, these are 32 bit unsigned numbers, and on a very busy or long-lived system they may wrap. Applications should be prepared to deal with that; unless your observations are measured in large numbers of minutes or hours, they should not wrap twice before you notice them. Each set of stats only applies to the indicated device; if you want system-wide stats you'll have to find all the devices and sum them all up. Field 1 -- # of reads issued This is the total number of reads completed successfully. Field 2 -- # of reads merged, field 6 -- # of writes merged Reads and writes which are adjacent to each other may be merged for efficiency. Thus two 4K reads may become one 8K read before it is ultimately handed to the disk, and so it will be counted (and queued) as only one I/O. This field lets you know how often this was done. Field 3 -- # of sectors read This is the total number of sectors read successfully. Field 4 -- # of milliseconds spent reading This is the total number of milliseconds spent by all reads (as measured from __make_request() to end_that_request_last()). Field 5 -- # of writes completed This is the total number of writes completed successfully. Field 7 -- # of sectors written This is the total number of sectors written successfully. Field 8 -- # of milliseconds spent writing This is the total number of milliseconds spent by all writes (as measured from __make_request() to end_that_request_last()). Field 9 -- # of I/Os currently in progress The only field that should go to zero. Incremented as requests are given to appropriate struct request_queue and decremented as they finish. Field 10 -- # of milliseconds spent doing I/Os This field is increases so long as field 9 is nonzero. Field 11 -- weighted # of milliseconds spent doing I/Os This field is incremented at each I/O start, I/O completion, I/O merge, or read of these stats by the number of I/Os in progress (field 9) times the number of milliseconds spent doing I/O since the last update of this field. This can provide an easy measure of both I/O completion time and the backlog that may be accumulating. To avoid introducing performance bottlenecks, no locks are held while modifying these counters. This implies that minor inaccuracies may be introduced when changes collide, so (for instance) adding up all the read I/Os issued per partition should equal those made to the disks ... but due to the lack of locking it may only be very close. In 2.6, there are counters for each cpu, which made the lack of locking almost a non-issue. When the statistics are read, the per-cpu counters are summed (possibly overflowing the unsigned 32-bit variable they are summed to) and the result given to the user. There is no convenient user interface for accessing the per-cpu counters themselves. Disks vs Partitions ------------------- There were significant changes between 2.4 and 2.6 in the I/O subsystem. As a result, some statistic information disappeared. The translation from a disk address relative to a partition to the disk address relative to the host disk happens much earlier. All merges and timings now happen at the disk level rather than at both the disk and partition level as in 2.4. Consequently, you'll see a different statistics output on 2.6 for partitions from that for disks. There are only *four* fields available for partitions on 2.6 machines. This is reflected in the examples above. Field 1 -- # of reads issued This is the total number of reads issued to this partition. Field 2 -- # of sectors read This is the total number of sectors requested to be read from this partition. Field 3 -- # of writes issued This is the total number of writes issued to this partition. Field 4 -- # of sectors written This is the total number of sectors requested to be written to this partition. Note that since the address is translated to a disk-relative one, and no record of the partition-relative address is kept, the subsequent success or failure of the read cannot be attributed to the partition. In other words, the number of reads for partitions is counted slightly before time of queuing for partitions, and at completion for whole disks. This is a subtle distinction that is probably uninteresting for most cases. Additional notes ---------------- In 2.6, sysfs is not mounted by default. If your distribution of Linux hasn't added it already, here's the line you'll want to add to your /etc/fstab: none /sys sysfs defaults 0 0 In 2.6, all disk statistics were removed from /proc/stat. In 2.4, they appear in both /proc/partitions and /proc/stat, although the ones in /proc/stat take a very different format from those in /proc/partitions (see proc(5), if your system has it.) -- ricklind@us.ibm.com