Document Linux's circular buffering capabilities

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			       CIRCULAR BUFFERS

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By: David Howells <dhowells@redhat.com>

    Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Linux provides a number of features that can be used to implement circular

buffering.  There are two sets of such features:

 (1) Convenience functions for determining information about power-of-2 sized

     buffers.

 (2) Memory barriers for when the producer and the consumer of objects in the

     buffer don't want to share a lock.

To use these facilities, as discussed below, there needs to be just one

producer and just one consumer.  It is possible to handle multiple producers by

serialising them, and to handle multiple consumers by serialising them.

Contents:

 (*) What is a circular buffer?

 (*) Measuring power-of-2 buffers.

 (*) Using memory barriers with circular buffers.

     - The producer.

     - The consumer.

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WHAT IS A CIRCULAR BUFFER?

==========================

First of all, what is a circular buffer?  A circular buffer is a buffer of

fixed, finite size into which there are two indices:

 (1) A 'head' index - the point at which the producer inserts items into the

     buffer.

 (2) A 'tail' index - the point at which the consumer finds the next item in

     the buffer.

Typically when the tail pointer is equal to the head pointer, the buffer is

empty; and the buffer is full when the head pointer is one less than the tail

pointer.

The head index is incremented when items are added, and the tail index when

items are removed.  The tail index should never jump the head index, and both

indices should be wrapped to 0 when they reach the end of the buffer, thus

allowing an infinite amount of data to flow through the buffer.

Typically, items will all be of the same unit size, but this isn't strictly

required to use the techniques below.  The indices can be increased by more

than 1 if multiple items or variable-sized items are to be included in the

buffer, provided that neither index overtakes the other.  The implementer must

be careful, however, as a region more than one unit in size may wrap the end of

the buffer and be broken into two segments.

============================

MEASURING POWER-OF-2 BUFFERS

============================

Calculation of the occupancy or the remaining capacity of an arbitrarily sized

circular buffer would normally be a slow operation, requiring the use of a

modulus (divide) instruction.  However, if the buffer is of a power-of-2 size,

then a much quicker bitwise-AND instruction can be used instead.

Linux provides a set of macros for handling power-of-2 circular buffers.  These

can be made use of by:

	#include <linux/circ_buf.h>

The macros are:

 (*) Measure the remaining capacity of a buffer:

	CIRC_SPACE(head_index, tail_index, buffer_size);

     This returns the amount of space left in the buffer[1] into which items

     can be inserted.

 (*) Measure the maximum consecutive immediate space in a buffer:

	CIRC_SPACE_TO_END(head_index, tail_index, buffer_size);

     This returns the amount of consecutive space left in the buffer[1] into

     which items can be immediately inserted without having to wrap back to the

     beginning of the buffer.

 (*) Measure the occupancy of a buffer:

	CIRC_CNT(head_index, tail_index, buffer_size);

     This returns the number of items currently occupying a buffer[2].

 (*) Measure the non-wrapping occupancy of a buffer:

	CIRC_CNT_TO_END(head_index, tail_index, buffer_size);

     This returns the number of consecutive items[2] that can be extracted from

     the buffer without having to wrap back to the beginning of the buffer.

Each of these macros will nominally return a value between 0 and buffer_size-1,

however:

 [1] CIRC_SPACE*() are intended to be used in the producer.  To the producer

     they will return a lower bound as the producer controls the head index,

     but the consumer may still be depleting the buffer on another CPU and

     moving the tail index.

     To the consumer it will show an upper bound as the producer may be busy

     depleting the space.

 [2] CIRC_CNT*() are intended to be used in the consumer.  To the consumer they

     will return a lower bound as the consumer controls the tail index, but the

     producer may still be filling the buffer on another CPU and moving the

     head index.

     To the producer it will show an upper bound as the consumer may be busy

     emptying the buffer.

 [3] To a third party, the order in which the writes to the indices by the

     producer and consumer become visible cannot be guaranteed as they are

     independent and may be made on different CPUs - so the result in such a

     situation will merely be a guess, and may even be negative.

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USING MEMORY BARRIERS WITH CIRCULAR BUFFERS

===========================================

By using memory barriers in conjunction with circular buffers, you can avoid

the need to:

 (1) use a single lock to govern access to both ends of the buffer, thus

     allowing the buffer to be filled and emptied at the same time; and

 (2) use atomic counter operations.

There are two sides to this: the producer that fills the buffer, and the

consumer that empties it.  Only one thing should be filling a buffer at any one

time, and only one thing should be emptying a buffer at any one time, but the

two sides can operate simultaneously.

THE PRODUCER

------------

The producer will look something like this:

	spin_lock(&producer_lock);

	unsigned long head = buffer->head;

	unsigned long tail = ACCESS_ONCE(buffer->tail);

	if (CIRC_SPACE(head, tail, buffer->size) >= 1) {

		/* insert one item into the buffer */

		struct item *item = buffer[head];

		produce_item(item);

		smp_wmb(); /* commit the item before incrementing the head */

		buffer->head = (head + 1) & (buffer->size - 1);

		/* wake_up() will make sure that the head is committed before

		 * waking anyone up */

		wake_up(consumer);

	}

	spin_unlock(&producer_lock);

This will instruct the CPU that the contents of the new item must be written

before the head index makes it available to the consumer and then instructs the

CPU that the revised head index must be written before the consumer is woken.

Note that wake_up() doesn't have to be the exact mechanism used, but whatever

is used must guarantee a (write) memory barrier between the update of the head

index and the change of state of the consumer, if a change of state occurs.

THE CONSUMER

------------

The consumer will look something like this:

	spin_lock(&consumer_lock);

	unsigned long head = ACCESS_ONCE(buffer->head);

	unsigned long tail = buffer->tail;

	if (CIRC_CNT(head, tail, buffer->size) >= 1) {

		/* read index before reading contents at that index */

		smp_read_barrier_depends();

		/* extract one item from the buffer */

		struct item *item = buffer[tail];

		consume_item(item);

		smp_mb(); /* finish reading descriptor before incrementing tail */

		buffer->tail = (tail + 1) & (buffer->size - 1);

	}

	spin_unlock(&consumer_lock);

This will instruct the CPU to make sure the index is up to date before reading

the new item, and then it shall make sure the CPU has finished reading the item

before it writes the new tail pointer, which will erase the item.

Note the use of ACCESS_ONCE() in both algorithms to read the opposition index.

This prevents the compiler from discarding and reloading its cached value -

which some compilers will do across smp_read_barrier_depends().  This isn't

strictly needed if you can be sure that the opposition index will _only_ be

used the once.

===============

FURTHER READING

===============

See also Documentation/memory-barriers.txt for a description of Linux's memory

barrier facilities.

			 ============================

By: David Howells <dhowells@redhat.com>

    Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Contents:

     - And then there's the Alpha.

 (*) Example uses.

     - Circular buffers.

 (*) References.

See the subsection on "Cache Coherency" above.

============

EXAMPLE USES

============

CIRCULAR BUFFERS

----------------

Memory barriers can be used to implement circular buffering without the need

of a lock to serialise the producer with the consumer.  See:

	Documentation/circular-buffers.txt

for details.

==========

REFERENCES

==========

/*

 * See Documentation/circular-buffers.txt for more information.

 */

#ifndef _LINUX_CIRC_BUF_H

#define _LINUX_CIRC_BUF_H 1