store: track HashPosition for first and last elements

this produces a huge performance improvement for queues with large internal tables.

an internal table of large size may appear if the array had lots of elements inserted into it and later deleted.
this resulted in major performance losses for the reader of the elements, as zend_hash_internal_pointer_reset_ex() had to scan through many IS_UNDEF offsets to find the actual first element.

there are two ways to attack this problem:
1) reallocate the internal table as elements are deleted to reduce the internal table size - this proved to be relatively ineffective
2) track the start and end of the hashtable to avoid repeated scans during every shift() call - this is the approach taken in this commit, and provides major performance benefits

the test case written in #42 now runs to completion substantially faster, without any performance degradation.

more tests are needed to ensure that this works fully as intended, but I chose to take the safe route with invalidating vs updating the offsets, so I think it should be good.

Author: dktapps

src/store.c

@@ -41,6 +41,8 @@ void pmmpthread_store_init(pmmpthread_store_t* store) {
 	zend_hash_init(
 		&store->hash, 8, NULL,
 		(dtor_func_t)pmmpthread_store_storage_dtor, 1);
+	store->first = HT_INVALID_IDX;
+	store->last = HT_INVALID_IDX;
 } /* }}} */
 
 /* {{{ */
@@ -286,6 +288,10 @@ int pmmpthread_store_delete(zend_object *object, zval *key) {
 		if (result == SUCCESS && was_pmmpthread_object) {
 			_pmmpthread_store_bump_modcount_nolock(threaded);
 		}
+		//TODO: it would be better if we can update this, if we deleted the first element
+		ts_obj->props.first = HT_INVALID_IDX;
+		ts_obj->props.last = HT_INVALID_IDX;
+
 		//TODO: sync local properties?
 		pmmpthread_monitor_unlock(&ts_obj->monitor);
 	} else result = FAILURE;
@@ -414,6 +420,8 @@ static inline zend_bool pmmpthread_store_update_shared_property(pmmpthread_objec
 			}
 		}
 	}
+	ts_obj->props.first = HT_INVALID_IDX;
+	ts_obj->props.last = HT_INVALID_IDX;
 
 	return result;
 }
@@ -538,6 +546,12 @@ int pmmpthread_store_write(zend_object *object, zval *key, zval *write, zend_boo
 		if (result == SUCCESS && was_pmmpthread_object) {
 			_pmmpthread_store_bump_modcount_nolock(threaded);
 		}
+		if (key) {
+			//only invalidate position if an arbitrary key was used
+			//if the item was appended, the first element was either unchanged or the position was invalid anyway
+			ts_obj->props.first = HT_INVALID_IDX;
+			ts_obj->props.last = HT_INVALID_IDX;
+		}
 		//this isn't necessary for any specific property write, but since we don't have any other way to clean up local
 		//cached ThreadSafe references that are dead, we have to take the opportunity
 		pmmpthread_store_sync_local_properties(object);
@@ -576,12 +590,20 @@ int pmmpthread_store_shift(zend_object *object, zval *member) {
 
 	if (pmmpthread_monitor_lock(&ts_obj->monitor)) {
 		zval key;
-		HashPosition position;
 		zval *zstorage;
 
-		zend_hash_internal_pointer_reset_ex(&ts_obj->props.hash, &position);
-		if ((zstorage = zend_hash_get_current_data_ex(&ts_obj->props.hash, &position))) {
-			zend_hash_get_current_key_zval_ex(&ts_obj->props.hash, &key, &position);
+		if (ts_obj->props.first == HT_INVALID_IDX) {
+			zend_hash_internal_pointer_reset_ex(&ts_obj->props.hash, &ts_obj->props.first);
+		}
+		if ((zstorage = zend_hash_get_current_data_ex(&ts_obj->props.hash, &ts_obj->props.first))) {
+			zend_hash_get_current_key_zval_ex(&ts_obj->props.hash, &key, &ts_obj->props.first);
+
+			//the new first element will now be the next item, if there is one
+			if (zend_hash_move_forward_ex(&ts_obj->props.hash, &ts_obj->props.first) == FAILURE) {
+				ts_obj->props.first = HT_INVALID_IDX;
+				ts_obj->props.last = HT_INVALID_IDX;
+			}
+
 			zend_bool may_be_locally_cached;
 
 			pmmpthread_store_restore_zval_ex(member, zstorage, &may_be_locally_cached);
@@ -617,17 +639,22 @@ int pmmpthread_store_chunk(zend_object *object, zend_long size, zend_bool preser
 	pmmpthread_object_t *ts_obj = threaded->ts_obj;
 
 	if (pmmpthread_monitor_lock(&ts_obj->monitor)) {
-		HashPosition position;
 		zval *zstorage;
 
 		array_init(chunk);
-		zend_hash_internal_pointer_reset_ex(&ts_obj->props.hash, &position);
+		if (ts_obj->props.first == HT_INVALID_IDX) {
+			zend_hash_internal_pointer_reset_ex(&ts_obj->props.hash, &ts_obj->props.first);
+		}
 		zend_bool stale_local_cache = 0;
 		while((zend_hash_num_elements(Z_ARRVAL_P(chunk)) < size) &&
-			(zstorage = zend_hash_get_current_data_ex(&ts_obj->props.hash, &position))) {
+			(zstorage = zend_hash_get_current_data_ex(&ts_obj->props.hash, &ts_obj->props.first))) {
 			zval key, zv;
 
-			zend_hash_get_current_key_zval_ex(&ts_obj->props.hash, &key, &position);
+			zend_hash_get_current_key_zval_ex(&ts_obj->props.hash, &key, &ts_obj->props.first);
+			if (zend_hash_move_forward_ex(&ts_obj->props.hash, &ts_obj->props.first) == FAILURE) {
+				ts_obj->props.first = HT_INVALID_IDX;
+				ts_obj->props.last = HT_INVALID_IDX;
+			}
 
 			zend_bool may_be_locally_cached;
 			pmmpthread_store_restore_zval_ex(&zv, zstorage, &may_be_locally_cached);
@@ -651,8 +678,6 @@ int pmmpthread_store_chunk(zend_object *object, zend_long size, zend_bool preser
 				}
 				zend_string_release(Z_STR(key));
 			}
-
-			zend_hash_internal_pointer_reset_ex(&ts_obj->props.hash, &position);
 		}
 		if (stale_local_cache) {
 			_pmmpthread_store_bump_modcount_nolock(threaded);
@@ -673,12 +698,18 @@ int pmmpthread_store_pop(zend_object *object, zval *member) {
 
 	if (pmmpthread_monitor_lock(&ts_obj->monitor)) {
 		zval key;
-		HashPosition position;
 		zval *zstorage;
 
-		zend_hash_internal_pointer_end_ex(&ts_obj->props.hash, &position);
-		if ((zstorage = zend_hash_get_current_data_ex(&ts_obj->props.hash, &position))) {
-			zend_hash_get_current_key_zval_ex(&ts_obj->props.hash, &key, &position);
+		if (ts_obj->props.last == HT_INVALID_IDX) {
+			zend_hash_internal_pointer_end_ex(&ts_obj->props.hash, &ts_obj->props.last);
+		}
+		if ((zstorage = zend_hash_get_current_data_ex(&ts_obj->props.hash, &ts_obj->props.last))) {
+			zend_hash_get_current_key_zval_ex(&ts_obj->props.hash, &key, &ts_obj->props.last);
+
+			if (zend_hash_move_backwards_ex(&ts_obj->props.hash, &ts_obj->props.last) == FAILURE) {
+				ts_obj->props.first = HT_INVALID_IDX;
+				ts_obj->props.last = HT_INVALID_IDX;
+			}
 
 			zend_bool may_be_locally_cached;
 			pmmpthread_store_restore_zval_ex(member, zstorage, &may_be_locally_cached);
@@ -1313,9 +1344,15 @@ void pmmpthread_store_reset(zend_object *object, HashPosition *position) {
 	pmmpthread_object_t *ts_obj = PMMPTHREAD_FETCH_TS_FROM(object);
 
 	if (pmmpthread_monitor_lock(&ts_obj->monitor)) {
-		zend_hash_internal_pointer_reset_ex(&ts_obj->props.hash, position);
-		if (zend_hash_has_more_elements_ex(&ts_obj->props.hash, position) == FAILURE) { //empty
-			*position = HT_INVALID_IDX;
+		if (ts_obj->props.first == HT_INVALID_IDX) {
+			zend_hash_internal_pointer_reset_ex(&ts_obj->props.hash, position);
+			if (zend_hash_has_more_elements_ex(&ts_obj->props.hash, position) == FAILURE) { //empty
+				*position = HT_INVALID_IDX;
+			} else {
+				ts_obj->props.first = *position;
+			}
+		} else {
+			*position = ts_obj->props.first;
 		}
 		pmmpthread_monitor_unlock(&ts_obj->monitor);
 	}

src/store.h

@@ -32,6 +32,8 @@
 typedef struct _pmmpthread_store_t {
 	HashTable hash;
 	zend_long modcount;
+	HashPosition first;
+	HashPosition last;
 } pmmpthread_store_t;
 
 void pmmpthread_store_init(pmmpthread_store_t* store);