238 lines
8.2 KiB
ReStructuredText
238 lines
8.2 KiB
ReStructuredText
================
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Circular Buffers
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================
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:Author: David Howells <dhowells@redhat.com>
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:Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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Linux provides a number of features that can be used to implement circular
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buffering. There are two sets of such features:
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(1) Convenience functions for determining information about power-of-2 sized
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buffers.
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(2) Memory barriers for when the producer and the consumer of objects in the
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buffer don't want to share a lock.
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To use these facilities, as discussed below, there needs to be just one
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producer and just one consumer. It is possible to handle multiple producers by
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serialising them, and to handle multiple consumers by serialising them.
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.. Contents:
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(*) What is a circular buffer?
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(*) Measuring power-of-2 buffers.
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(*) Using memory barriers with circular buffers.
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- The producer.
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- The consumer.
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What is a circular buffer?
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==========================
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First of all, what is a circular buffer? A circular buffer is a buffer of
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fixed, finite size into which there are two indices:
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(1) A 'head' index - the point at which the producer inserts items into the
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buffer.
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(2) A 'tail' index - the point at which the consumer finds the next item in
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the buffer.
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Typically when the tail pointer is equal to the head pointer, the buffer is
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empty; and the buffer is full when the head pointer is one less than the tail
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pointer.
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The head index is incremented when items are added, and the tail index when
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items are removed. The tail index should never jump the head index, and both
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indices should be wrapped to 0 when they reach the end of the buffer, thus
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allowing an infinite amount of data to flow through the buffer.
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Typically, items will all be of the same unit size, but this isn't strictly
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required to use the techniques below. The indices can be increased by more
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than 1 if multiple items or variable-sized items are to be included in the
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buffer, provided that neither index overtakes the other. The implementer must
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be careful, however, as a region more than one unit in size may wrap the end of
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the buffer and be broken into two segments.
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Measuring power-of-2 buffers
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============================
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Calculation of the occupancy or the remaining capacity of an arbitrarily sized
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circular buffer would normally be a slow operation, requiring the use of a
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modulus (divide) instruction. However, if the buffer is of a power-of-2 size,
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then a much quicker bitwise-AND instruction can be used instead.
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Linux provides a set of macros for handling power-of-2 circular buffers. These
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can be made use of by::
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#include <linux/circ_buf.h>
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The macros are:
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(#) Measure the remaining capacity of a buffer::
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CIRC_SPACE(head_index, tail_index, buffer_size);
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This returns the amount of space left in the buffer[1] into which items
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can be inserted.
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(#) Measure the maximum consecutive immediate space in a buffer::
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CIRC_SPACE_TO_END(head_index, tail_index, buffer_size);
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This returns the amount of consecutive space left in the buffer[1] into
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which items can be immediately inserted without having to wrap back to the
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beginning of the buffer.
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(#) Measure the occupancy of a buffer::
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CIRC_CNT(head_index, tail_index, buffer_size);
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This returns the number of items currently occupying a buffer[2].
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(#) Measure the non-wrapping occupancy of a buffer::
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CIRC_CNT_TO_END(head_index, tail_index, buffer_size);
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This returns the number of consecutive items[2] that can be extracted from
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the buffer without having to wrap back to the beginning of the buffer.
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Each of these macros will nominally return a value between 0 and buffer_size-1,
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however:
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(1) CIRC_SPACE*() are intended to be used in the producer. To the producer
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they will return a lower bound as the producer controls the head index,
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but the consumer may still be depleting the buffer on another CPU and
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moving the tail index.
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To the consumer it will show an upper bound as the producer may be busy
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depleting the space.
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(2) CIRC_CNT*() are intended to be used in the consumer. To the consumer they
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will return a lower bound as the consumer controls the tail index, but the
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producer may still be filling the buffer on another CPU and moving the
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head index.
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To the producer it will show an upper bound as the consumer may be busy
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emptying the buffer.
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(3) To a third party, the order in which the writes to the indices by the
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producer and consumer become visible cannot be guaranteed as they are
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independent and may be made on different CPUs - so the result in such a
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situation will merely be a guess, and may even be negative.
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Using memory barriers with circular buffers
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===========================================
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By using memory barriers in conjunction with circular buffers, you can avoid
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the need to:
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(1) use a single lock to govern access to both ends of the buffer, thus
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allowing the buffer to be filled and emptied at the same time; and
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(2) use atomic counter operations.
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There are two sides to this: the producer that fills the buffer, and the
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consumer that empties it. Only one thing should be filling a buffer at any one
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time, and only one thing should be emptying a buffer at any one time, but the
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two sides can operate simultaneously.
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The producer
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------------
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The producer will look something like this::
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spin_lock(&producer_lock);
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unsigned long head = buffer->head;
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/* The spin_unlock() and next spin_lock() provide needed ordering. */
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unsigned long tail = READ_ONCE(buffer->tail);
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if (CIRC_SPACE(head, tail, buffer->size) >= 1) {
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/* insert one item into the buffer */
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struct item *item = buffer[head];
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produce_item(item);
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smp_store_release(buffer->head,
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(head + 1) & (buffer->size - 1));
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/* wake_up() will make sure that the head is committed before
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* waking anyone up */
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wake_up(consumer);
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}
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spin_unlock(&producer_lock);
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This will instruct the CPU that the contents of the new item must be written
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before the head index makes it available to the consumer and then instructs the
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CPU that the revised head index must be written before the consumer is woken.
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Note that wake_up() does not guarantee any sort of barrier unless something
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is actually awakened. We therefore cannot rely on it for ordering. However,
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there is always one element of the array left empty. Therefore, the
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producer must produce two elements before it could possibly corrupt the
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element currently being read by the consumer. Therefore, the unlock-lock
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pair between consecutive invocations of the consumer provides the necessary
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ordering between the read of the index indicating that the consumer has
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vacated a given element and the write by the producer to that same element.
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The Consumer
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------------
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The consumer will look something like this::
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spin_lock(&consumer_lock);
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/* Read index before reading contents at that index. */
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unsigned long head = smp_load_acquire(buffer->head);
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unsigned long tail = buffer->tail;
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if (CIRC_CNT(head, tail, buffer->size) >= 1) {
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/* extract one item from the buffer */
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struct item *item = buffer[tail];
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consume_item(item);
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/* Finish reading descriptor before incrementing tail. */
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smp_store_release(buffer->tail,
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(tail + 1) & (buffer->size - 1));
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}
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spin_unlock(&consumer_lock);
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This will instruct the CPU to make sure the index is up to date before reading
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the new item, and then it shall make sure the CPU has finished reading the item
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before it writes the new tail pointer, which will erase the item.
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Note the use of READ_ONCE() and smp_load_acquire() to read the
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opposition index. This prevents the compiler from discarding and
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reloading its cached value. This isn't strictly needed if you can
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be sure that the opposition index will _only_ be used the once.
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The smp_load_acquire() additionally forces the CPU to order against
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subsequent memory references. Similarly, smp_store_release() is used
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in both algorithms to write the thread's index. This documents the
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fact that we are writing to something that can be read concurrently,
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prevents the compiler from tearing the store, and enforces ordering
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against previous accesses.
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Further reading
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===============
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See also Documentation/memory-barriers.txt for a description of Linux's memory
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barrier facilities.
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