Improve performance of the ast_event cache functionality.

This code comes from svn/asterisk/team/russell/event_performance/.

Here is a summary of the changes that have been made, in order of both
invasiveness and performance impact, from smallest to largest.

1) Asterisk 1.6.1 introduces some additional logic to be able to handle
   distributed device state.  This functionality comes at a cost.
   One relatively minor change in this patch is that the extra processing
   required for distributed device state is now completely bypassed if
   it's not needed.

2) One of the things that I noticed when profiling this code was that a
   _lot_ of time was spent doing string comparisons.  I changed the way
   strings are represented in an event to include a hash value at the front.
   So, before doing a string comparison, we do an integer comparison on the
   hash.

3) Finally, the code that handles the event cache has been re-written.
   I tried to do this in a such a way that it had minimal impact on the API.
   I did have to change one API call, though - ast_event_queue_and_cache().
   However, the way it works now is nicer, IMO.  Each type of event that
   can be cached (MWI, device state) has its own hash table and rules for
   hashing and comparing objects.  This by far made the biggest impact on
   performance.

For additional details regarding this code and how it was tested, please see the
review request.

(closes issue #14738)
Reported by: russell

Review: http://reviewboard.digium.com/r/205/


git-svn-id: http://svn.digium.com/svn/asterisk/trunk@184339 f38db490-d61c-443f-a65b-d21fe96a405b

1 files changed, 350 insertions, 136 deletions

diff --git a/main/event.c b/main/event.c
index 956ce6f91..e29f057e3 100644
--- a/main/event.c
+++ b/main/event.c
@@ -28,6 +28,7 @@
 ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 
 #include "asterisk/_private.h"
+
 #include "asterisk/event.h"
 #include "asterisk/linkedlists.h"
 #include "asterisk/dlinkedlists.h"
@@ -36,6 +37,7 @@ ASTERISK_FILE_VERSION(__FILE__, "$Revision$")
 #include "asterisk/unaligned.h"
 #include "asterisk/utils.h"
 #include "asterisk/taskprocessor.h"
+#include "asterisk/astobj2.h"
 
 struct ast_taskprocessor *event_dispatcher;
 
@@ -55,6 +57,16 @@ struct ast_event_ie {
 } __attribute__((packed));
 
 /*!
+ * \brief The payload for a string information element
+ */
+struct ast_event_ie_str_payload {
+	/*! \brief A hash calculated with ast_str_hash(), to speed up comparisons */
+	uint32_t hash;
+	/*! \brief The actual string, null terminated */
+	char str[1];
+} __attribute__((packed));
+
+/*!
  * \brief An event
  *
  * An ast_event consists of an event header (this structure), and zero or
@@ -85,7 +97,10 @@ struct ast_event_ie_val {
 	enum ast_event_ie_pltype ie_pltype;
 	union {
 		uint32_t uint;
-		const char *str;
+		struct {
+			uint32_t hash;
+			const char *str;
+		};
 		void *raw;
 	} payload;
 	size_t raw_datalen;
@@ -107,11 +122,55 @@ static uint32_t sub_uniqueid;
  * The event subscribers are indexed by which event they are subscribed to */
 static AST_RWDLLIST_HEAD(ast_event_sub_list, ast_event_sub) ast_event_subs[AST_EVENT_TOTAL];
 
-/*! \brief Cached events
- * The event cache is indexed on the event type.  The purpose of this is 
- * for events that express some sort of state.  So, when someone first
- * needs to know this state, it can get the last known state from the cache. */
-static AST_RWLIST_HEAD(ast_event_ref_list, ast_event_ref) ast_event_cache[AST_EVENT_TOTAL];
+static int ast_event_cmp(void *obj, void *arg, int flags);
+static int ast_event_hash_mwi(const void *obj, const int flags);
+static int ast_event_hash_devstate(const void *obj, const int flags);
+static int ast_event_hash_devstate_change(const void *obj, const int flags);
+
+#ifdef LOW_MEMORY
+#define NUM_CACHE_BUCKETS 17
+#else
+#define NUM_CACHE_BUCKETS 563
+#endif
+
+#define MAX_CACHE_ARGS 8
+
+/*!
+ * \brief Event types that are kept in the cache.
+ */
+static struct {
+	/*! 
+	 * \brief Container of cached events
+	 *
+	 * \details This gets allocated in ast_event_init() when Asterisk starts
+	 * for the event types declared as using the cache.
+	 */
+	struct ao2_container *container;
+	/*! \brief Event type specific hash function */
+	ao2_hash_fn *hash_fn;
+	/*!
+	 * \brief Information Elements used for caching
+	 *
+	 * \details This array is the set of information elements that will be unique
+	 * among all events in the cache for this event type.  When a new event gets
+	 * cached, a previous event with the same values for these information elements
+	 * will be replaced.
+	 */
+	enum ast_event_ie_type cache_args[MAX_CACHE_ARGS];
+} ast_event_cache[AST_EVENT_TOTAL] = {
+	[AST_EVENT_MWI] = {
+		.hash_fn = ast_event_hash_mwi,
+		.cache_args = { AST_EVENT_IE_MAILBOX, AST_EVENT_IE_CONTEXT },
+	},
+	[AST_EVENT_DEVICE_STATE] = {
+		.hash_fn = ast_event_hash_devstate,
+		.cache_args = { AST_EVENT_IE_DEVICE, },
+	},
+	[AST_EVENT_DEVICE_STATE_CHANGE] = {
+		.hash_fn = ast_event_hash_devstate_change,
+		.cache_args = { AST_EVENT_IE_DEVICE, AST_EVENT_IE_EID, },
+	},
+};
 
 /*!
  * The index of each entry _must_ match the event type number!
@@ -237,6 +296,8 @@ static void ast_event_ie_val_destroy(struct ast_event_ie_val *ie_val)
 {
 	switch (ie_val->ie_pltype) {
 	case AST_EVENT_IE_PLTYPE_STR:
+		ast_free((char *) ie_val->payload.str);
+		break;
 	case AST_EVENT_IE_PLTYPE_RAW:
 		ast_free(ie_val->payload.raw);
 		break;
@@ -328,7 +389,8 @@ enum ast_event_subscriber_res ast_event_check_subscriber(enum ast_event_type typ
 	return res;
 }
 
-static int match_ie_val(struct ast_event *event, struct ast_event_ie_val *ie_val, struct ast_event *event2)
+static int match_ie_val(const struct ast_event *event,
+		const struct ast_event_ie_val *ie_val, const struct ast_event *event2)
 {
 	if (ie_val->ie_pltype == AST_EVENT_IE_PLTYPE_UINT) {
 		uint32_t val = event2 ? ast_event_get_ie_uint(event2, ie_val->ie_type) : ie_val->payload.uint;
@@ -338,9 +400,19 @@ static int match_ie_val(struct ast_event *event, struct ast_event_ie_val *ie_val
 	}
 
 	if (ie_val->ie_pltype == AST_EVENT_IE_PLTYPE_STR) {
-		const char *str = event2 ? ast_event_get_ie_str(event2, ie_val->ie_type) : ie_val->payload.str;
-		if (str && !strcmp(str, ast_event_get_ie_str(event, ie_val->ie_type)))
+		const char *str;
+		uint32_t hash;
+
+		hash = event2 ? ast_event_get_ie_str_hash(event2, ie_val->ie_type) : ie_val->payload.hash;
+		if (hash != ast_event_get_ie_str_hash(event, ie_val->ie_type)) {
+			return 0;
+		}
+
+		str = event2 ? ast_event_get_ie_str(event2, ie_val->ie_type) : ie_val->payload.str;
+		if (str && !strcmp(str, ast_event_get_ie_str(event, ie_val->ie_type))) {
 			return 1;
+		}
+
 		return 0;
 	}
 
@@ -360,26 +432,32 @@ static int match_ie_val(struct ast_event *event, struct ast_event_ie_val *ie_val
 	return 0;
 }
 
-/*! \brief Dump the event cache for the subscribed event type */
-void ast_event_dump_cache(const struct ast_event_sub *event_sub)
+static int dump_cache_cb(void *obj, void *arg, int flags)
 {
-	struct ast_event_ref *event_ref;
-	enum ast_event_type type = event_sub->type;
+	const struct ast_event_ref *event_ref = obj;
+	const struct ast_event *event = event_ref->event;
+	const struct ast_event_sub *event_sub = arg;
+	struct ast_event_ie_val *ie_val = NULL;
 
-	AST_RWLIST_RDLOCK(&ast_event_cache[type]);
-	AST_RWLIST_TRAVERSE_SAFE_BEGIN(&ast_event_cache[type], event_ref, entry) {
-		struct ast_event_ie_val *ie_val;
-		AST_LIST_TRAVERSE(&event_sub->ie_vals, ie_val, entry) {
-			if (!match_ie_val(event_ref->event, ie_val, NULL))
-				break;
-		}
-		if (!ie_val) {
-			/* All parameters were matched on this cache entry, so dump it */
-			event_sub->cb(event_ref->event, event_sub->userdata);
+	AST_LIST_TRAVERSE(&event_sub->ie_vals, ie_val, entry) {
+		if (!match_ie_val(event, ie_val, NULL)) {
+			break;
 		}
 	}
-	AST_RWLIST_TRAVERSE_SAFE_END
-	AST_RWLIST_UNLOCK(&ast_event_cache[type]);
+
+	if (!ie_val) {
+		/* All parameters were matched on this cache entry, so dump it */
+		event_sub->cb(event, event_sub->userdata);
+	}
+
+	return 0;
+}
+
+/*! \brief Dump the event cache for the subscribed event type */
+void ast_event_dump_cache(const struct ast_event_sub *event_sub)
+{
+	ao2_callback(ast_event_cache[event_sub->type].container, OBJ_NODATA,
+			dump_cache_cb, (void *) event_sub);
 }
 
 static struct ast_event *gen_sub_event(struct ast_event_sub *sub)
@@ -536,6 +614,8 @@ int ast_event_sub_append_ie_str(struct ast_event_sub *sub,
 		return -1;
 	}
 
+	ie_val->payload.hash = ast_str_hash(str);
+
 	AST_LIST_INSERT_TAIL(&sub->ie_vals, ie_val, entry);
 
 	return 0;
@@ -703,7 +783,11 @@ uint32_t ast_event_iterator_get_ie_uint(struct ast_event_iterator *iterator)
 
 const char *ast_event_iterator_get_ie_str(struct ast_event_iterator *iterator)
 {
-	return (const char*)iterator->ie->ie_payload;
+	const struct ast_event_ie_str_payload *str_payload;
+
+	str_payload = (struct ast_event_ie_str_payload *) iterator->ie->ie_payload;
+
+	return str_payload->str;
 }
 
 void *ast_event_iterator_get_ie_raw(struct ast_event_iterator *iterator)
@@ -725,9 +809,22 @@ uint32_t ast_event_get_ie_uint(const struct ast_event *event, enum ast_event_ie_
 	return ie_val ? ntohl(get_unaligned_uint32(ie_val)) : 0;
 }
 
+uint32_t ast_event_get_ie_str_hash(const struct ast_event *event, enum ast_event_ie_type ie_type)
+{
+	const struct ast_event_ie_str_payload *str_payload;
+
+	str_payload = ast_event_get_ie_raw(event, ie_type);
+
+	return str_payload->hash;
+}
+
 const char *ast_event_get_ie_str(const struct ast_event *event, enum ast_event_ie_type ie_type)
 {
-	return ast_event_get_ie_raw(event, ie_type);
+	const struct ast_event_ie_str_payload *str_payload;
+
+	str_payload = ast_event_get_ie_raw(event, ie_type);
+
+	return str_payload->str;
 }
 
 const void *ast_event_get_ie_raw(const struct ast_event *event, enum ast_event_ie_type ie_type)
@@ -746,7 +843,16 @@ const void *ast_event_get_ie_raw(const struct ast_event *event, enum ast_event_i
 int ast_event_append_ie_str(struct ast_event **event, enum ast_event_ie_type ie_type,
 	const char *str)
 {
-	return ast_event_append_ie_raw(event, ie_type, str, strlen(str) + 1);
+	struct ast_event_ie_str_payload *str_payload;
+	size_t payload_len;
+
+	payload_len = sizeof(*str_payload) + strlen(str);
+	str_payload = alloca(payload_len);
+
+	strcpy(str_payload->str, str);
+	str_payload->hash = ast_str_hash(str);
+
+	return ast_event_append_ie_raw(event, ie_type, str_payload, payload_len);
 }
 
 int ast_event_append_ie_uint(struct ast_event **event, enum ast_event_ie_type ie_type,
@@ -850,10 +956,11 @@ void ast_event_destroy(struct ast_event *event)
 	ast_free(event);
 }
 
-static void ast_event_ref_destroy(struct ast_event_ref *event_ref)
+static void ast_event_ref_destroy(void *obj)
 {
+	struct ast_event_ref *event_ref = obj;
+
 	ast_event_destroy(event_ref->event);
-	ast_free(event_ref);
 }
 
 static struct ast_event *ast_event_dup(const struct ast_event *event)
@@ -863,9 +970,10 @@ static struct ast_event *ast_event_dup(const struct ast_event *event)
 
 	event_len = ast_event_get_size(event);
 
-	if (!(dup_event = ast_calloc(1, event_len)))
+	if (!(dup_event = ast_calloc(1, event_len))) {
 		return NULL;
-	
+	}
+
 	memcpy(dup_event, event, event_len);
 
 	return dup_event;
@@ -876,139 +984,122 @@ struct ast_event *ast_event_get_cached(enum ast_event_type type, ...)
 	va_list ap;
 	enum ast_event_ie_type ie_type;
 	struct ast_event *dup_event = NULL;
-	struct ast_event_ref *event_ref;
-	struct ast_event_ie_val *cache_arg;
-	AST_LIST_HEAD_NOLOCK_STATIC(cache_args, ast_event_ie_val);
+	struct ast_event_ref *cached_event_ref;
+	struct ast_event *cache_arg_event;
+	struct ast_event_ref tmp_event_ref = {
+		.event = NULL,
+	};
+	struct ao2_container *container = NULL;
 
 	if (type >= AST_EVENT_TOTAL) {
 		ast_log(LOG_ERROR, "%u is an invalid type!\n", type);
 		return NULL;
 	}
 
+	if (!(container = ast_event_cache[type].container)) {
+		ast_log(LOG_ERROR, "%u is not a cached event type\n", type);
+		return NULL;
+	}
+
+	if (!(cache_arg_event = ast_event_new(type, AST_EVENT_IE_END))) {
+		return NULL;
+	}
+
 	va_start(ap, type);
 	for (ie_type = va_arg(ap, enum ast_event_type);
 		ie_type != AST_EVENT_IE_END;
 		ie_type = va_arg(ap, enum ast_event_type))
 	{
-		cache_arg = alloca(sizeof(*cache_arg));
-		memset(cache_arg, 0, sizeof(*cache_arg));
-		cache_arg->ie_type = ie_type;
-		cache_arg->ie_pltype = va_arg(ap, enum ast_event_ie_pltype);
-		if (cache_arg->ie_pltype == AST_EVENT_IE_PLTYPE_UINT)
-			cache_arg->payload.uint = va_arg(ap, uint32_t);
-		else if (cache_arg->ie_pltype == AST_EVENT_IE_PLTYPE_STR)
-			cache_arg->payload.str = ast_strdupa(va_arg(ap, const char *));
-		else if (cache_arg->ie_pltype == AST_EVENT_IE_PLTYPE_RAW) {
+		enum ast_event_ie_pltype ie_pltype;
+
+		ie_pltype = va_arg(ap, enum ast_event_ie_pltype);
+
+		switch (ie_pltype) {
+		case AST_EVENT_IE_PLTYPE_UINT:
+			ast_event_append_ie_uint(&cache_arg_event, ie_type, va_arg(ap, uint32_t));
+			break;
+		case AST_EVENT_IE_PLTYPE_STR:
+			ast_event_append_ie_str(&cache_arg_event, ie_type, va_arg(ap, const char *));
+			break;
+		case AST_EVENT_IE_PLTYPE_RAW:
+		{
 			void *data = va_arg(ap, void *);
 			size_t datalen = va_arg(ap, size_t);
-			cache_arg->payload.raw = alloca(datalen);
-			memcpy(cache_arg->payload.raw, data, datalen);
-			cache_arg->raw_datalen = datalen;
+			ast_event_append_ie_raw(&cache_arg_event, ie_type, data, datalen);
+		}
+		case AST_EVENT_IE_PLTYPE_EXISTS:
+			ast_log(LOG_WARNING, "PLTYPE_EXISTS not supported by this function\n");
+			break;
+		case AST_EVENT_IE_PLTYPE_UNKNOWN:
+			break;
 		}
-		AST_LIST_INSERT_TAIL(&cache_args, cache_arg, entry);
 	}
 	va_end(ap);
 
-	if (AST_LIST_EMPTY(&cache_args)) {
-		ast_log(LOG_ERROR, "Events can not be retrieved from the cache without "
-			"specifying at least one IE type!\n");
-		return NULL;
-	}
+	tmp_event_ref.event = cache_arg_event;
 
-	AST_RWLIST_RDLOCK(&ast_event_cache[type]);
-	AST_RWLIST_TRAVERSE_SAFE_BEGIN(&ast_event_cache[type], event_ref, entry) {
-		AST_LIST_TRAVERSE(&cache_args, cache_arg, entry) {
-			if (!match_ie_val(event_ref->event, cache_arg, NULL))
-				break;	
-		}
-		if (!cache_arg) {
-			/* All parameters were matched on this cache entry, so return it */
-			dup_event = ast_event_dup(event_ref->event);
-			break;
-		}
+	cached_event_ref = ao2_find(container, &tmp_event_ref, OBJ_POINTER);
+
+	ast_event_destroy(cache_arg_event);
+	cache_arg_event = NULL;
+
+	if (cached_event_ref) {
+		dup_event = ast_event_dup(cached_event_ref->event);
+		ao2_ref(cached_event_ref, -1);
+		cached_event_ref = NULL;
 	}
-	AST_RWLIST_TRAVERSE_SAFE_END
-	AST_RWLIST_UNLOCK(&ast_event_cache[type]);
 
 	return dup_event;
 }
 
+static struct ast_event_ref *alloc_event_ref(void)
+{
+	return ao2_alloc(sizeof(struct ast_event_ref), ast_event_ref_destroy);
+}
+
 /*! \brief Duplicate an event and add it to the cache
  * \note This assumes this index in to the cache is locked */
-static int ast_event_dup_and_cache(const struct ast_event *event)
+static int attribute_unused ast_event_dup_and_cache(const struct ast_event *event)
 {
 	struct ast_event *dup_event;
 	struct ast_event_ref *event_ref;
 
-	if (!(dup_event = ast_event_dup(event)))
+	if (!(dup_event = ast_event_dup(event))) {
 		return -1;
-	if (!(event_ref = ast_calloc(1, sizeof(*event_ref))))
+	}
+
+	if (!(event_ref = alloc_event_ref())) {
+		ast_event_destroy(dup_event);
 		return -1;
-	
+	}
+
 	event_ref->event = dup_event;
 
-	AST_LIST_INSERT_TAIL(&ast_event_cache[ntohs(event->type)], event_ref, entry);
+	ao2_link(ast_event_cache[ast_event_get_type(event)].container, event_ref);
+
+	ao2_ref(event_ref, -1);
 
 	return 0;
 }
 
-int ast_event_queue_and_cache(struct ast_event *event, ...)
+int ast_event_queue_and_cache(struct ast_event *event)
 {
-	va_list ap;
-	enum ast_event_type ie_type;
-	uint16_t host_event_type;
-	struct ast_event_ref *event_ref;
-	int res;
-	struct ast_event_ie_val *cache_arg;
-	AST_LIST_HEAD_NOLOCK_STATIC(cache_args, ast_event_ie_val);
-
-	host_event_type = ntohs(event->type);
-
-	/* Invalid type */
-	if (host_event_type >= AST_EVENT_TOTAL) {
-		ast_log(LOG_WARNING, "Someone tried to queue an event of invalid "
-			"type '%d'!\n", host_event_type);
-		return -1;
+	struct ao2_container *container;
+	struct ast_event_ref tmp_event_ref = {
+		.event = event,
+	};
+
+	if (!(container = ast_event_cache[ast_event_get_type(event)].container)) {
+		ast_log(LOG_WARNING, "cache requested for non-cached event type\n");
+		goto queue_event;
 	}
 
-	va_start(ap, event);
-	for (ie_type = va_arg(ap, enum ast_event_type);
-		ie_type != AST_EVENT_IE_END;
-		ie_type = va_arg(ap, enum ast_event_type))
-	{
-		cache_arg = alloca(sizeof(*cache_arg));
-		memset(cache_arg, 0, sizeof(*cache_arg));
-		cache_arg->ie_type = ie_type;
-		cache_arg->ie_pltype = va_arg(ap, enum ast_event_ie_pltype);
-		if (cache_arg->ie_pltype == AST_EVENT_IE_PLTYPE_RAW)
-			cache_arg->raw_datalen = va_arg(ap, size_t);
-		AST_LIST_INSERT_TAIL(&cache_args, cache_arg, entry);
-	}
-	va_end(ap);
-
-	if (AST_LIST_EMPTY(&cache_args)) {
-		ast_log(LOG_ERROR, "Events can not be cached without specifying at "
-			"least one IE type!\n");
-		return ast_event_queue(event);
-	}
- 
-	AST_RWLIST_WRLOCK(&ast_event_cache[host_event_type]);
-	AST_RWLIST_TRAVERSE_SAFE_BEGIN(&ast_event_cache[host_event_type], event_ref, entry) {
-		AST_LIST_TRAVERSE(&cache_args, cache_arg, entry) {
-			if (!match_ie_val(event_ref->event, cache_arg, event))
-				break;	
-		}
-		if (!cache_arg) {
-			/* All parameters were matched on this cache entry, so remove it */
-			AST_LIST_REMOVE_CURRENT(entry);
-			ast_event_ref_destroy(event_ref);
-		}
-	}
-	AST_RWLIST_TRAVERSE_SAFE_END;
-	res = ast_event_dup_and_cache(event);
-	AST_RWLIST_UNLOCK(&ast_event_cache[host_event_type]);
+	/* Remove matches from the cache */
+	ao2_callback(container, OBJ_POINTER | OBJ_UNLINK | OBJ_MULTIPLE | OBJ_NODATA,
+			ast_event_cmp, &tmp_event_ref);
 
-	return (ast_event_queue(event) || res) ? -1 : 0;
+queue_event:
+	return ast_event_queue(event);
 }
 
 static int handle_event(void *data)
@@ -1024,22 +1115,25 @@ static int handle_event(void *data)
 	AST_RWDLLIST_TRAVERSE(&ast_event_subs[host_event_type], sub, entry) {
 		struct ast_event_ie_val *ie_val;
 		AST_LIST_TRAVERSE(&sub->ie_vals, ie_val, entry) {
-			if (!match_ie_val(event_ref->event, ie_val, NULL))
+			if (!match_ie_val(event_ref->event, ie_val, NULL)) {
 				break;
+			}
 		}
-		if (ie_val)
+		if (ie_val) {
 			continue;
+		}
 		sub->cb(event_ref->event, sub->userdata);
 	}
 	AST_RWDLLIST_UNLOCK(&ast_event_subs[host_event_type]);
 
 	/* Now to subscribers to all event types */
 	AST_RWDLLIST_RDLOCK(&ast_event_subs[AST_EVENT_ALL]);
-	AST_RWDLLIST_TRAVERSE(&ast_event_subs[AST_EVENT_ALL], sub, entry)
+	AST_RWDLLIST_TRAVERSE(&ast_event_subs[AST_EVENT_ALL], sub, entry) {
 		sub->cb(event_ref->event, sub->userdata);
+	}
 	AST_RWDLLIST_UNLOCK(&ast_event_subs[AST_EVENT_ALL]);
 
-	ast_event_ref_destroy(event_ref);
+	ao2_ref(event_ref, -1);
 
 	return 0;
 }
@@ -1059,29 +1153,149 @@ int ast_event_queue(struct ast_event *event)
 	}
 
 	/* If nobody has subscribed to this event type, throw it away now */
-	if (ast_event_check_subscriber(host_event_type, AST_EVENT_IE_END) 
-		== AST_EVENT_SUB_NONE) {
+	if (ast_event_check_subscriber(host_event_type, AST_EVENT_IE_END)
+			== AST_EVENT_SUB_NONE) {
 		ast_event_destroy(event);
 		return 0;
 	}
 
-	if (!(event_ref = ast_calloc(1, sizeof(*event_ref))))
+	if (!(event_ref = alloc_event_ref())) {
 		return -1;
+	}
 
 	event_ref->event = event;
 
 	return ast_taskprocessor_push(event_dispatcher, handle_event, event_ref);
 }
 
-void ast_event_init(void)
+static int ast_event_hash_mwi(const void *obj, const int flags)
+{
+	const struct ast_event *event = obj;
+	const char *mailbox = ast_event_get_ie_str(event, AST_EVENT_IE_MAILBOX);
+	const char *context = ast_event_get_ie_str(event, AST_EVENT_IE_CONTEXT);
+
+	return ast_str_hash_add(context, ast_str_hash(mailbox));
+}
+
+/*!
+ * \internal
+ * \brief Hash function for AST_EVENT_DEVICE_STATE
+ *
+ * \param[in] obj an ast_event
+ * \param[in] flags unused
+ *
+ * \return hash value
+ */
+static int ast_event_hash_devstate(const void *obj, const int flags)
+{
+	const struct ast_event *event = obj;
+
+	return ast_str_hash(ast_event_get_ie_str(event, AST_EVENT_IE_DEVICE));
+}
+
+/*!
+ * \internal
+ * \brief Hash function for AST_EVENT_DEVICE_STATE_CHANGE
+ *
+ * \param[in] obj an ast_event
+ * \param[in] flags unused
+ *
+ * \return hash value
+ */
+static int ast_event_hash_devstate_change(const void *obj, const int flags)
+{
+	const struct ast_event *event = obj;
+
+	return ast_str_hash(ast_event_get_ie_str(event, AST_EVENT_IE_DEVICE));
+}
+
+static int ast_event_hash(const void *obj, const int flags)
+{
+	const struct ast_event_ref *event_ref;
+	const struct ast_event *event;
+	ao2_hash_fn *hash_fn;
+
+	event_ref = obj;
+	event = event_ref->event;
+
+	if (!(hash_fn = ast_event_cache[ast_event_get_type(event)].hash_fn)) {
+		return 0;
+	}
+
+	return hash_fn(event, flags);
+}
+
+/*!
+ * \internal
+ * \brief Compare two events
+ *
+ * \param[in] obj the first event, as an ast_event_ref
+ * \param[in] arg the second event, as an ast_event_ref
+ * \param[in] flags unused
+ *
+ * \pre Both events must be the same type.
+ * \pre The event type must be declared as a cached event type in ast_event_cache
+ *
+ * \details This function takes two events, and determines if they are considered
+ * equivalent.  The values of information elements specified in the cache arguments
+ * for the event type are used to determine if the events are equivalent.
+ *
+ * \retval 0 No match
+ * \retval CMP_MATCH The events are considered equivalent based on the cache arguments
+ */
+static int ast_event_cmp(void *obj, void *arg, int flags)
+{
+	struct ast_event_ref *event_ref, *event_ref2;
+	struct ast_event *event, *event2;
+	int res = CMP_MATCH;
+	int i;
+	enum ast_event_ie_type *cache_args;
+
+	event_ref = obj;
+	event = event_ref->event;
+
+	event_ref2 = arg;
+	event2 = event_ref2->event;
+
+	cache_args = ast_event_cache[ast_event_get_type(event)].cache_args;
+
+	for (i = 0; i < ARRAY_LEN(ast_event_cache[0].cache_args) && cache_args[i]; i++) {
+		struct ast_event_ie_val ie_val = {
+			.ie_type = cache_args[i],
+		};
+
+		if (!match_ie_val(event, &ie_val, event2)) {
+			res = 0;
+			break;
+		}
+	}
+
+	return res;
+}
+
+int ast_event_init(void)
 {
 	int i;
 
-	for (i = 0; i < AST_EVENT_TOTAL; i++)
+	for (i = 0; i < AST_EVENT_TOTAL; i++) {
 		AST_RWDLLIST_HEAD_INIT(&ast_event_subs[i]);
+	}
 
-	for (i = 0; i < AST_EVENT_TOTAL; i++)
-		AST_RWLIST_HEAD_INIT(&ast_event_cache[i]);
+	for (i = 0; i < AST_EVENT_TOTAL; i++) {
+		if (!ast_event_cache[i].hash_fn) {
+			/* This event type is not cached. */
+			continue;
+		}
 
-	event_dispatcher = ast_taskprocessor_get("core_event_dispatcher", 0);
+		if (!(ast_event_cache[i].container = ao2_container_alloc(NUM_CACHE_BUCKETS,
+				ast_event_hash, ast_event_cmp))) {
+			return -1;
+		}
+	}
+
+	if (!(event_dispatcher = ast_taskprocessor_get("core_event_dispatcher", 0))) {
+		return -1;
+	}
+
+	return 0;
 }