atomicqueue: add an atomic queue

Add an atomic queue. The queue can be used from multiple threads simultaneously
and without taking any locks or doing any blocking operations. This makes it
highly scalable for things like the bus, bufferpools and object recycling.

gst/Makefile.am

@@ -75,6 +75,7 @@ libgstreamer_@GST_MAJORMINOR@_la_SOURCES = \
 	gstinfo.c		\
 	gstinterface.c		\
 	gstiterator.c		\
+	gstatomicqueue.c	\
 	gstmessage.c		\
 	gstminiobject.c		\
 	gstpad.c		\
@@ -167,6 +168,7 @@ gst_headers =			\
 	gstinfo.h		\
 	gstinterface.h		\
 	gstiterator.h		\
+	gstatomicqueue.h	\
 	gstmacros.h		\
 	gstmessage.h		\
 	gstminiobject.h		\

gst/gstatomicqueue.c

@@ -0,0 +1,311 @@
+/* GStreamer
+ * Copyright (C) 2009 Edward Hervey <bilboed@bilboed.com>
+ *               2011 Wim Taymans <wim.taymans@gmail.com>
+ *
+ * gstatomicqueue.c:
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#include <string.h>
+#include <gst/gst.h>
+#include "gstatomicqueue.h"
+
+/* By default the queue uses 2 * sizeof(gpointer) * clp2 (max_items) of
+ * memory. clp2(x) is the next power of two >= than x.
+ *
+ * The queue can operate in low memory mode, in which it consumes almost
+ * half the memory at the expense of extra overhead in the readers. This
+ * is disabled by default because even without LOW_MEM mode, the memory
+ * consumption is still lower than a plain GList.
+ */
+#undef LOW_MEM
+
+typedef struct _GstAQueueMem GstAQueueMem;
+
+struct _GstAQueueMem
+{
+  gint size;
+  gpointer *array;
+  volatile gint head;
+  volatile gint tail;
+  GstAQueueMem *next;
+  GstAQueueMem *free;
+};
+
+static guint
+clp2 (guint n)
+{
+  guint res = 1;
+
+  while (res < n)
+    res <<= 1;
+
+  return res;
+}
+
+static GstAQueueMem *
+new_queue_mem (guint size, gint pos)
+{
+  GstAQueueMem *mem;
+
+  mem = g_new (GstAQueueMem, 1);
+
+  /* we keep the size as a mask for performance */
+  mem->size = clp2 (size) - 1;
+  mem->array = g_new0 (gpointer, mem->size);
+  mem->head = pos;
+  mem->tail = pos;
+  mem->next = NULL;
+  mem->free = NULL;
+
+  return mem;
+}
+
+static void
+free_queue_mem (GstAQueueMem * mem)
+{
+  g_free (mem->array);
+  g_free (mem);
+}
+
+struct _GstAtomicQueue
+{
+#ifdef LOW_MEM
+  gint num_readers;
+#endif
+  GstAQueueMem *head_mem;
+  GstAQueueMem *tail_mem;
+  GstAQueueMem *free_list;
+};
+
+static void
+add_to_free_list (GstAtomicQueue * queue, GstAQueueMem * mem)
+{
+  do {
+    mem->free = g_atomic_pointer_get (&queue->free_list);
+  } while (!g_atomic_pointer_compare_and_exchange ((gpointer *) &
+          queue->free_list, mem->free, mem));
+}
+
+static void
+clear_free_list (GstAtomicQueue * queue)
+{
+  GstAQueueMem *free_list;
+
+  /* take the free list and replace with NULL */
+  do {
+    free_list = g_atomic_pointer_get (&queue->free_list);
+    if (free_list == NULL)
+      return;
+  } while (!g_atomic_pointer_compare_and_exchange ((gpointer *) &
+          queue->free_list, free_list, NULL));
+
+  while (free_list) {
+    GstAQueueMem *next = free_list->free;
+
+    free_queue_mem (free_list);
+
+    free_list = next;
+  }
+}
+
+GstAtomicQueue *
+gst_atomic_queue_new (guint initial_size)
+{
+  GstAtomicQueue *queue;
+
+  queue = g_new (GstAtomicQueue, 1);
+
+#ifdef LOW_MEM
+  queue->num_readers = 0;
+#endif
+  queue->head_mem = queue->tail_mem = new_queue_mem (initial_size, 0);
+  queue->free_list = NULL;
+
+  return queue;
+}
+
+void
+gst_atomic_queue_free (GstAtomicQueue * queue)
+{
+  free_queue_mem (queue->head_mem);
+  if (queue->head_mem != queue->tail_mem)
+    free_queue_mem (queue->tail_mem);
+  clear_free_list (queue);
+  g_free (queue);
+}
+
+gpointer
+gst_atomic_queue_peek (GstAtomicQueue * queue)
+{
+  GstAQueueMem *head_mem;
+  gint head, tail, size;
+
+  while (TRUE) {
+    GstAQueueMem *next;
+
+    head_mem = g_atomic_pointer_get (&queue->head_mem);
+
+    head = g_atomic_int_get (&head_mem->head);
+    tail = g_atomic_int_get (&head_mem->tail);
+    size = head_mem->size;
+
+    /* when we are not empty, we can continue */
+    if (G_LIKELY (head != tail))
+      break;
+
+    /* else array empty, try to take next */
+    next = g_atomic_pointer_get (&head_mem->next);
+    if (next == NULL)
+      return NULL;
+
+    /* now we try to move the next array as the head memory. If we fail to do that,
+     * some other reader managed to do it first and we retry */
+    if (!g_atomic_pointer_compare_and_exchange ((gpointer *) &
+            queue->head_mem, head_mem, next))
+      continue;
+
+    /* when we managed to swing the head pointer the old head is now
+     * useless and we add it to the freelist. We can't free the memory yet
+     * because we first need to make sure no reader is accessing it anymore. */
+    add_to_free_list (queue, head_mem);
+  }
+
+  return head_mem->array[head & size];
+}
+
+gpointer
+gst_atomic_queue_pop (GstAtomicQueue * queue)
+{
+  gpointer ret;
+  GstAQueueMem *head_mem;
+  gint head, tail, size;
+
+#ifdef LOW_MEM
+  g_atomic_int_inc (&queue->num_readers);
+#endif
+
+  do {
+    while (TRUE) {
+      GstAQueueMem *next;
+
+      head_mem = g_atomic_pointer_get (&queue->head_mem);
+
+      head = g_atomic_int_get (&head_mem->head);
+      tail = g_atomic_int_get (&head_mem->tail);
+      size = head_mem->size;
+
+      /* when we are not empty, we can continue */
+      if (G_LIKELY (head != tail))
+        break;
+
+      /* else array empty, try to take next */
+      next = g_atomic_pointer_get (&head_mem->next);
+      if (next == NULL)
+        return NULL;
+
+      /* now we try to move the next array as the head memory. If we fail to do that,
+       * some other reader managed to do it first and we retry */
+      if (!g_atomic_pointer_compare_and_exchange ((gpointer *) &
+              queue->head_mem, head_mem, next))
+        continue;
+
+      /* when we managed to swing the head pointer the old head is now
+       * useless and we add it to the freelist. We can't free the memory yet
+       * because we first need to make sure no reader is accessing it anymore. */
+      add_to_free_list (queue, head_mem);
+    }
+
+    ret = head_mem->array[head & size];
+  } while (!g_atomic_int_compare_and_exchange (&head_mem->head, head,
+          head + 1));
+
+#ifdef LOW_MEM
+  /* decrement number of readers, when we reach 0 readers we can be sure that
+   * none is accessing the memory in the free list and we can try to clean up */
+  if (g_atomic_int_dec_and_test (&queue->num_readers))
+    clear_free_list (queue);
+#endif
+
+  return ret;
+}
+
+void
+gst_atomic_queue_push (GstAtomicQueue * queue, gpointer data)
+{
+  GstAQueueMem *tail_mem;
+  gint head, tail, size;
+
+  do {
+    while (TRUE) {
+      GstAQueueMem *mem;
+
+      tail_mem = g_atomic_pointer_get (&queue->tail_mem);
+      head = g_atomic_int_get (&tail_mem->head);
+      tail = g_atomic_int_get (&tail_mem->tail);
+      size = tail_mem->size;
+
+      /* we're not full, continue */
+      if (tail - head < size)
+        break;
+
+      /* else we need to grow the array, we store a mask so we have to add 1 */
+      mem = new_queue_mem ((size << 1) + 1, tail);
+
+      /* try to make our new array visible to other writers */
+      if (!g_atomic_pointer_compare_and_exchange ((gpointer *) &
+              queue->tail_mem, tail_mem, mem)) {
+        /* we tried to swap the new writer array but something changed. This is
+         * because some other writer beat us to it, we free our memory and try
+         * again */
+        free_queue_mem (mem);
+        continue;
+      }
+      /* make sure that readers can find our new array as well. The one who
+       * manages to swap the pointer is the only one who can set the next
+       * pointer to the new array */
+      g_atomic_pointer_set (&tail_mem->next, mem);
+    }
+  } while (!g_atomic_int_compare_and_exchange (&tail_mem->tail, tail,
+          tail + 1));
+
+  tail_mem->array[tail & size] = data;
+}
+
+guint
+gst_atomic_queue_length (GstAtomicQueue * queue)
+{
+  GstAQueueMem *head_mem, *tail_mem;
+  gint head, tail;
+
+#ifdef LOW_MEM
+  g_atomic_int_inc (&queue->num_readers);
+#endif
+
+  head_mem = g_atomic_pointer_get (&queue->head_mem);
+  head = g_atomic_int_get (&head_mem->head);
+
+  tail_mem = g_atomic_pointer_get (&queue->tail_mem);
+  tail = g_atomic_int_get (&tail_mem->tail);
+
+#ifdef LOW_MEM
+  if (g_atomic_int_dec_and_test (&queue->num_readers))
+    clear_free_list (queue);
+#endif
+
+  return tail - head;
+}

gst/gstatomicqueue.h

@@ -0,0 +1,43 @@
+/* GStreamer
+ * Copyright (C) 2009-2010 Edward Hervey <bilboed@bilboed.com>
+ *           (C) 2011 Wim Taymans <wim.taymans@gmail.com>
+ *
+ * gstatomicqueue.h:
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#include <glib.h>
+
+#ifndef __GST_ATOMIC_QUEUE_H__
+#define __GST_ATOMIC_QUEUE_H__
+
+typedef struct _GstAtomicQueue GstAtomicQueue;
+
+G_BEGIN_DECLS
+
+GstAtomicQueue *   gst_atomic_queue_new         (guint initial_size);
+void               gst_atomic_queue_free        (GstAtomicQueue * queue);
+
+void               gst_atomic_queue_push        (GstAtomicQueue* queue, gpointer data);
+gpointer           gst_atomic_queue_pop         (GstAtomicQueue* queue);
+gpointer           gst_atomic_queue_peek        (GstAtomicQueue* queue);
+
+guint              gst_atomic_queue_length      (GstAtomicQueue * queue);
+
+G_END_DECLS
+
+#endif /* __GST_ATOMIC_QUEUE_H__ */