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After a writer has written to its reserved write location, it can only make the location available for reading if all of the writers with lower locations have finished.
433 lines
11 KiB
C
433 lines
11 KiB
C
/* GStreamer
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* Copyright (C) 2009 Edward Hervey <bilboed@bilboed.com>
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* 2011 Wim Taymans <wim.taymans@gmail.com>
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*
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* gstatomicqueue.c:
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*
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* This library is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Library General Public
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* License as published by the Free Software Foundation; either
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* version 2 of the License, or (at your option) any later version.
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*
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* This library is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* Library General Public License for more details.
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*
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* You should have received a copy of the GNU Library General Public
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* License along with this library; if not, write to the
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* Free Software Foundation, Inc., 59 Temple Place - Suite 330,
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* Boston, MA 02111-1307, USA.
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*/
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#include "gst_private.h"
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#include <string.h>
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#include <gst/gst.h>
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#include "gstatomicqueue.h"
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#include "glib-compat-private.h"
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/**
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* SECTION:gstatomicqueue
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* @title: GstAtomicQueue
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* @short_description: An atomic queue implementation
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*
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* The #GstAtomicQueue object implements a queue that can be used from multiple
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* threads without performing any blocking operations.
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*
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* Since: 0.10.33
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*/
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/* By default the queue uses 2 * sizeof(gpointer) * clp2 (max_items) of
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* memory. clp2(x) is the next power of two >= than x.
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*
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* The queue can operate in low memory mode, in which it consumes almost
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* half the memory at the expense of extra overhead in the readers. This
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* is disabled by default because even without LOW_MEM mode, the memory
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* consumption is still lower than a plain GList.
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*/
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#undef LOW_MEM
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typedef struct _GstAQueueMem GstAQueueMem;
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struct _GstAQueueMem
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{
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gint size;
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gpointer *array;
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volatile gint head;
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volatile gint tail_write;
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volatile gint tail_read;
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GstAQueueMem *next;
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GstAQueueMem *free;
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};
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static guint
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clp2 (guint n)
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{
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guint res = 1;
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while (res < n)
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res <<= 1;
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return res;
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}
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static GstAQueueMem *
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new_queue_mem (guint size, gint pos)
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{
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GstAQueueMem *mem;
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mem = g_new (GstAQueueMem, 1);
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/* we keep the size as a mask for performance */
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mem->size = clp2 (MAX (size, 16)) - 1;
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mem->array = g_new0 (gpointer, mem->size + 1);
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mem->head = pos;
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mem->tail_write = pos;
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mem->tail_read = pos;
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mem->next = NULL;
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mem->free = NULL;
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return mem;
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}
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static void
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free_queue_mem (GstAQueueMem * mem)
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{
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g_free (mem->array);
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g_free (mem);
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}
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struct _GstAtomicQueue
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{
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volatile gint refcount;
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#ifdef LOW_MEM
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gint num_readers;
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#endif
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GstAQueueMem *head_mem;
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GstAQueueMem *tail_mem;
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GstAQueueMem *free_list;
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};
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static void
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add_to_free_list (GstAtomicQueue * queue, GstAQueueMem * mem)
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{
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do {
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mem->free = g_atomic_pointer_get (&queue->free_list);
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} while (!g_atomic_pointer_compare_and_exchange (&queue->free_list,
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mem->free, mem));
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}
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static void
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clear_free_list (GstAtomicQueue * queue)
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{
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GstAQueueMem *free_list;
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/* take the free list and replace with NULL */
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do {
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free_list = g_atomic_pointer_get (&queue->free_list);
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if (free_list == NULL)
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return;
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} while (!g_atomic_pointer_compare_and_exchange (&queue->free_list, free_list,
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NULL));
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while (free_list) {
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GstAQueueMem *next = free_list->free;
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free_queue_mem (free_list);
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free_list = next;
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}
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}
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/**
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* gst_atomic_queue_new:
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* @initial_size: initial queue size
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*
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* Create a new atomic queue instance. @initial_size will be rounded up to the
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* nearest power of 2 and used as the initial size of the queue.
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*
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* Returns: a new #GstAtomicQueue
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*
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* Since: 0.10.33
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*/
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GstAtomicQueue *
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gst_atomic_queue_new (guint initial_size)
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{
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GstAtomicQueue *queue;
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queue = g_new (GstAtomicQueue, 1);
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queue->refcount = 1;
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#ifdef LOW_MEM
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queue->num_readers = 0;
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#endif
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queue->head_mem = queue->tail_mem = new_queue_mem (initial_size, 0);
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queue->free_list = NULL;
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return queue;
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}
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/**
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* gst_atomic_queue_ref:
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* @queue: a #GstAtomicQueue
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*
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* Increase the refcount of @queue.
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*
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* Since: 0.10.33
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*/
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void
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gst_atomic_queue_ref (GstAtomicQueue * queue)
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{
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g_return_if_fail (queue != NULL);
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g_atomic_int_inc (&queue->refcount);
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}
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static void
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gst_atomic_queue_free (GstAtomicQueue * queue)
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{
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free_queue_mem (queue->head_mem);
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if (queue->head_mem != queue->tail_mem)
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free_queue_mem (queue->tail_mem);
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clear_free_list (queue);
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g_free (queue);
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}
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/**
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* gst_atomic_queue_unref:
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* @queue: a #GstAtomicQueue
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*
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* Unref @queue and free the memory when the refcount reaches 0.
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*
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* Since: 0.10.33
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*/
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void
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gst_atomic_queue_unref (GstAtomicQueue * queue)
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{
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g_return_if_fail (queue != NULL);
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if (g_atomic_int_dec_and_test (&queue->refcount))
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gst_atomic_queue_free (queue);
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}
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/**
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* gst_atomic_queue_peek:
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* @queue: a #GstAtomicQueue
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*
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* Peek the head element of the queue without removing it from the queue.
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*
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* Returns: the head element of @queue or NULL when the queue is empty.
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*
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* Since: 0.10.33
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*/
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gpointer
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gst_atomic_queue_peek (GstAtomicQueue * queue)
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{
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GstAQueueMem *head_mem;
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gint head, tail, size;
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g_return_val_if_fail (queue != NULL, NULL);
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while (TRUE) {
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GstAQueueMem *next;
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head_mem = g_atomic_pointer_get (&queue->head_mem);
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head = g_atomic_int_get (&head_mem->head);
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tail = g_atomic_int_get (&head_mem->tail_read);
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size = head_mem->size;
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/* when we are not empty, we can continue */
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if (G_LIKELY (head != tail))
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break;
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/* else array empty, try to take next */
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next = g_atomic_pointer_get (&head_mem->next);
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if (next == NULL)
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return NULL;
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/* now we try to move the next array as the head memory. If we fail to do that,
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* some other reader managed to do it first and we retry */
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if (!g_atomic_pointer_compare_and_exchange (&queue->head_mem, head_mem,
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next))
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continue;
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/* when we managed to swing the head pointer the old head is now
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* useless and we add it to the freelist. We can't free the memory yet
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* because we first need to make sure no reader is accessing it anymore. */
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add_to_free_list (queue, head_mem);
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}
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return head_mem->array[head & size];
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}
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/**
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* gst_atomic_queue_pop:
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* @queue: a #GstAtomicQueue
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*
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* Get the head element of the queue.
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*
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* Returns: the head element of @queue or NULL when the queue is empty.
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*
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* Since: 0.10.33
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*/
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gpointer
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gst_atomic_queue_pop (GstAtomicQueue * queue)
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{
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gpointer ret;
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GstAQueueMem *head_mem;
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gint head, tail, size;
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g_return_val_if_fail (queue != NULL, NULL);
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#ifdef LOW_MEM
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g_atomic_int_inc (&queue->num_readers);
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#endif
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do {
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while (TRUE) {
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GstAQueueMem *next;
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head_mem = g_atomic_pointer_get (&queue->head_mem);
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head = g_atomic_int_get (&head_mem->head);
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tail = g_atomic_int_get (&head_mem->tail_read);
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size = head_mem->size;
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/* when we are not empty, we can continue */
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if G_LIKELY
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(head != tail)
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break;
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/* else array empty, try to take next */
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next = g_atomic_pointer_get (&head_mem->next);
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if (next == NULL)
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return NULL;
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/* now we try to move the next array as the head memory. If we fail to do that,
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* some other reader managed to do it first and we retry */
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if G_UNLIKELY
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(!g_atomic_pointer_compare_and_exchange (&queue->head_mem, head_mem,
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next))
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continue;
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/* when we managed to swing the head pointer the old head is now
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* useless and we add it to the freelist. We can't free the memory yet
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* because we first need to make sure no reader is accessing it anymore. */
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add_to_free_list (queue, head_mem);
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}
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ret = head_mem->array[head & size];
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} while G_UNLIKELY
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(!g_atomic_int_compare_and_exchange (&head_mem->head, head, head + 1));
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#ifdef LOW_MEM
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/* decrement number of readers, when we reach 0 readers we can be sure that
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* none is accessing the memory in the free list and we can try to clean up */
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if (g_atomic_int_dec_and_test (&queue->num_readers))
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clear_free_list (queue);
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#endif
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return ret;
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}
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/**
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* gst_atomic_queue_push:
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* @queue: a #GstAtomicQueue
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* @data: the data
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*
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* Append @data to the tail of the queue.
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*
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* Since: 0.10.33
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*/
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void
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gst_atomic_queue_push (GstAtomicQueue * queue, gpointer data)
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{
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GstAQueueMem *tail_mem;
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gint head, tail, size;
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g_return_if_fail (queue != NULL);
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do {
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while (TRUE) {
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GstAQueueMem *mem;
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tail_mem = g_atomic_pointer_get (&queue->tail_mem);
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head = g_atomic_int_get (&tail_mem->head);
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tail = g_atomic_int_get (&tail_mem->tail_write);
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size = tail_mem->size;
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/* we're not full, continue */
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if G_LIKELY
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(tail - head <= size)
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break;
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/* else we need to grow the array, we store a mask so we have to add 1 */
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mem = new_queue_mem ((size << 1) + 1, tail);
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/* try to make our new array visible to other writers */
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if G_UNLIKELY
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(!g_atomic_pointer_compare_and_exchange (&queue->tail_mem, tail_mem,
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mem)) {
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/* we tried to swap the new writer array but something changed. This is
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* because some other writer beat us to it, we free our memory and try
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* again */
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free_queue_mem (mem);
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continue;
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}
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/* make sure that readers can find our new array as well. The one who
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* manages to swap the pointer is the only one who can set the next
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* pointer to the new array */
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g_atomic_pointer_set (&tail_mem->next, mem);
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}
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} while G_UNLIKELY
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(!g_atomic_int_compare_and_exchange (&tail_mem->tail_write, tail, tail + 1));
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tail_mem->array[tail & size] = data;
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/* now wait until all writers have completed their write before we move the
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* tail_read to this new item. It is possible that other writers are still
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* updating the previous array slots and we don't want to reveal their changes
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* before they are done. FIXME, it would be nice if we didn't have to busy
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* wait here. */
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while G_UNLIKELY
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(!g_atomic_int_compare_and_exchange (&tail_mem->tail_read, tail, tail + 1));
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}
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/**
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* gst_atomic_queue_length:
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* @queue: a #GstAtomicQueue
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*
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* Get the amount of items in the queue.
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*
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* Returns: the number of elements in the queue.
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*
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* Since: 0.10.33
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*/
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guint
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gst_atomic_queue_length (GstAtomicQueue * queue)
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{
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GstAQueueMem *head_mem, *tail_mem;
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gint head, tail;
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g_return_val_if_fail (queue != NULL, 0);
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#ifdef LOW_MEM
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g_atomic_int_inc (&queue->num_readers);
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#endif
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head_mem = g_atomic_pointer_get (&queue->head_mem);
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head = g_atomic_int_get (&head_mem->head);
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tail_mem = g_atomic_pointer_get (&queue->tail_mem);
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tail = g_atomic_int_get (&tail_mem->tail_read);
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#ifdef LOW_MEM
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if (g_atomic_int_dec_and_test (&queue->num_readers))
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clear_free_list (queue);
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#endif
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return tail - head;
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}
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