settings: zms: add cache for linked list hash

Increase the load performance by adding an optional cache for the linked
list hashes.
This is used only when the settings_load is called with NULL parameter
and we need to load all Settings that exist in the persistent storage.
Cache is enabled using SETTINGS_ZMS_LL_CACHE and the size of the cache
is set using SETTINGS_ZMS_LL_CACHE_SIZE.
Each cache entry will add 8 bytes of RAM usage.

Signed-off-by: Riadh Ghaddab <[email protected]>

Author: rghaddab

subsys/settings/Kconfig

@@ -47,6 +47,25 @@ config SETTINGS_ZMS
 	help
 	  Use ZMS as settings storage backend.
 
+if SETTINGS_ZMS
+
+config SETTINGS_ZMS_LL_CACHE
+	bool "ZMS linked list lookup cache"
+	help
+	  Enable ZMS lookup cache for linked list, used to reduce the
+	  Settings load time by having most linked list elements already
+	  in cache.
+
+config SETTINGS_ZMS_LL_CACHE_SIZE
+	int "ZMS linked list lookup cache size"
+	default 128
+	range 1 $(UINT32_MAX)
+	depends on SETTINGS_ZMS_LL_CACHE
+	help
+	  Number of entries in Settings ZMS linked list cache.
+
+endif # SETTINGS_ZMS
+
 config SETTINGS_FCB
 	bool "FCB"
 	depends on FCB

subsys/settings/include/settings/settings_zms.h

@@ -68,20 +68,25 @@ extern "C" {
 #define ZMS_COLLISION_NUM(x)     ((x & ZMS_COLLISIONS_MASK) >> 1)
 #define ZMS_NAME_ID_FROM_HASH(x) ((x & ZMS_HASH_TOTAL_MASK) | BIT(31))
 
+struct settings_hash_linked_list {
+	uint32_t previous_hash;
+	uint32_t next_hash;
+};
+
 struct settings_zms {
 	struct settings_store cf_store;
 	struct zms_fs cf_zms;
 	const struct device *flash_dev;
+#if CONFIG_SETTINGS_ZMS_LL_CACHE
+	struct settings_hash_linked_list ll_cache[CONFIG_SETTINGS_ZMS_LL_CACHE_SIZE];
+	uint32_t ll_cache_next;
+	bool ll_has_changed;
+#endif /* CONFIG_SETTINGS_ZMS_LL_CACHE */
 	uint32_t last_hash_id;
 	uint32_t second_to_last_hash_id;
 	uint8_t hash_collision_num;
 };
 
-struct settings_hash_linked_list {
-	uint32_t previous_hash;
-	uint32_t next_hash;
-};
-
 #ifdef __cplusplus
 }
 #endif

subsys/settings/src/settings_zms.c

@@ -135,6 +135,9 @@ static int settings_zms_delete(struct settings_zms *cf, uint32_t name_hash)
 	}
 
 #ifndef CONFIG_SETTINGS_ZMS_NO_LL_DELETE
+#ifdef CONFIG_SETTINGS_ZMS_LL_CACHE
+	cf->ll_has_changed = true;
+#endif
 	rc = settings_zms_unlink_ll_node(cf, name_hash);
 	if (rc < 0) {
 		return rc;
@@ -220,16 +223,28 @@ static int settings_zms_load(struct settings_store *cs, const struct settings_lo
 	}
 #endif /* CONFIG_SETTINGS_ZMS_LOAD_SUBTREE_PATH */
 
+#ifdef CONFIG_SETTINGS_ZMS_LL_CACHE
+	uint32_t ll_cache_index = 0;
+
+	if (cf->ll_has_changed) {
+		/* reload the linked list in cache */
+		ret = settings_zms_get_last_hash_ids(cf);
+		if (ret < 0) {
+			return ret;
+		}
+	}
+	settings_element = cf->ll_cache[ll_cache_index++];
+#else
 	ret = zms_read(&cf->cf_zms, ZMS_LL_HEAD_HASH_ID, &settings_element,
 		       sizeof(struct settings_hash_linked_list));
 	if (ret < 0) {
 		return ret;
 	}
+#endif /* CONFIG_SETTINGS_ZMS_LL_CACHE */
 	ll_hash_id = settings_element.next_hash;
 
 	/* If subtree is NULL then we must load all found Settings */
 	while (ll_hash_id) {
-
 		/* In the ZMS backend, each setting item is stored in two ZMS
 		 * entries one for the setting's name and one with the
 		 * setting's value.
@@ -254,12 +269,24 @@ static int settings_zms_load(struct settings_store *cs, const struct settings_lo
 				return ret;
 			}
 #endif /* CONFIG_SETTINGS_ZMS_NO_LL_DELETE */
-
-			ret = zms_read(&cf->cf_zms, ll_hash_id, &settings_element,
-				       sizeof(struct settings_hash_linked_list));
-			if (ret < 0) {
-				return ret;
+#ifdef CONFIG_SETTINGS_ZMS_LL_CACHE
+			if (ll_cache_index < cf->ll_cache_next) {
+				settings_element = cf->ll_cache[ll_cache_index++];
+			}  else if (ll_hash_id == cf->second_to_last_hash_id) {
+				/* The last ll node is not stored in the cache as it is already
+				 * in the cf->last_hash_id.
+				 */
+				settings_element.next_hash = cf->last_hash_id;
+			} else {
+#endif
+				ret = zms_read(&cf->cf_zms, ll_hash_id, &settings_element,
+					       sizeof(struct settings_hash_linked_list));
+				if (ret < 0) {
+					return ret;
+				}
+#ifdef CONFIG_SETTINGS_ZMS_LL_CACHE
 			}
+#endif
 			/* update next ll_hash_id */
 			ll_hash_id = settings_element.next_hash;
 			continue;
@@ -276,12 +303,22 @@ static int settings_zms_load(struct settings_store *cs, const struct settings_lo
 			break;
 		}
 
-		/* update next ll_hash_id */
-		ret = zms_read(&cf->cf_zms, ll_hash_id, &settings_element,
-			       sizeof(struct settings_hash_linked_list));
-		if (ret < 0) {
-			return ret;
+#ifdef CONFIG_SETTINGS_ZMS_LL_CACHE
+		if (ll_cache_index < cf->ll_cache_next) {
+			settings_element = cf->ll_cache[ll_cache_index++];
+		} else if (ll_hash_id == cf->second_to_last_hash_id) {
+			settings_element.next_hash = cf->last_hash_id;
+		} else {
+#endif
+			ret = zms_read(&cf->cf_zms, ll_hash_id, &settings_element,
+				       sizeof(struct settings_hash_linked_list));
+			if (ret < 0) {
+				return ret;
+			}
+#ifdef CONFIG_SETTINGS_ZMS_LL_CACHE
 		}
+#endif
+		/* update next ll_hash_id */
 		ll_hash_id = settings_element.next_hash;
 	}
 
@@ -418,6 +455,7 @@ static int settings_zms_save(struct settings_store *cs, const char *name, const
 		if (rc < 0) {
 			return rc;
 		}
+
 		/* Now update the previous linked list element */
 		settings_element.next_hash = name_hash | 1;
 		settings_element.previous_hash = cf->second_to_last_hash_id;
@@ -428,6 +466,12 @@ static int settings_zms_save(struct settings_store *cs, const char *name, const
 		}
 		cf->second_to_last_hash_id = cf->last_hash_id;
 		cf->last_hash_id = name_hash | 1;
+#ifdef CONFIG_SETTINGS_ZMS_LL_CACHE
+		if (cf->ll_cache_next < CONFIG_SETTINGS_ZMS_LL_CACHE_SIZE) {
+			cf->ll_cache[cf->ll_cache_next] = settings_element;
+			cf->ll_cache_next = cf->ll_cache_next + 1;
+		}
+#endif
 	}
 #ifdef CONFIG_SETTINGS_ZMS_NO_LL_DELETE
 no_ll_update:
@@ -441,6 +485,9 @@ static int settings_zms_get_last_hash_ids(struct settings_zms *cf)
 	uint32_t ll_last_hash_id = ZMS_LL_HEAD_HASH_ID;
 	int rc = 0;
 
+#ifdef CONFIG_SETTINGS_ZMS_LL_CACHE
+	cf->ll_cache_next = 0;
+#endif
 	cf->hash_collision_num = 0;
 	do {
 		rc = zms_read(&cf->cf_zms, ll_last_hash_id, &settings_element,
@@ -461,6 +508,13 @@ static int settings_zms_get_last_hash_ids(struct settings_zms *cf)
 			return rc;
 		}
 
+#ifdef CONFIG_SETTINGS_ZMS_LL_CACHE
+		if ((cf->ll_cache_next < CONFIG_SETTINGS_ZMS_LL_CACHE_SIZE) &&
+		    (settings_element.next_hash)) {
+			cf->ll_cache[cf->ll_cache_next] = settings_element;
+			cf->ll_cache_next = cf->ll_cache_next + 1;
+		}
+#endif
 		/* increment hash collision number if necessary */
 		if (ZMS_COLLISION_NUM(ll_last_hash_id) > cf->hash_collision_num) {
 			cf->hash_collision_num = ZMS_COLLISION_NUM(ll_last_hash_id);
@@ -470,6 +524,9 @@ static int settings_zms_get_last_hash_ids(struct settings_zms *cf)
 		ll_last_hash_id = settings_element.next_hash;
 	} while (settings_element.next_hash);
 
+#ifdef CONFIG_SETTINGS_ZMS_LL_CACHE
+	cf->ll_has_changed = false;
+#endif
 	return 0;
 }