settings: zms: add a cache for hash collisions

adding a lookup cache for hash collisions improves write speed as it
doesn't look for the name in the storage if no hash collision exists

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_NAME_CACHE
+	bool "ZMS name lookup cache"
+	select SYS_HASH_FUNC32
+	help
+	  Enable ZMS lookup cache, used to reduce the Settings name
+	  lookup time by checking if its hash had a collision.
+
+config SETTINGS_ZMS_NAME_CACHE_SIZE
+	int "ZMS name lookup cache size"
+	default 128
+	range 1 $(UINT32_MAX)
+	depends on SETTINGS_ZMS_NAME_CACHE
+	help
+	  Number of entries in Settings ZMS name cache.
+
+endif # SETTINGS_ZMS
+
 config SETTINGS_FCB
 	bool "FCB"
 	depends on FCB

subsys/settings/include/settings/settings_zms.h

@@ -71,6 +71,14 @@ struct settings_zms {
 	struct settings_store cf_store;
 	struct zms_fs cf_zms;
 	const struct device *flash_dev;
+#if CONFIG_SETTINGS_ZMS_NAME_CACHE
+	struct {
+		uint32_t name_hash;
+		bool     has_collision;
+	} cache[CONFIG_SETTINGS_ZMS_NAME_CACHE_SIZE];
+
+	uint32_t cache_next;
+#endif
 	uint32_t last_hash_id;
 	uint32_t second_to_last_hash_id;
 	uint8_t hash_collision_num;

subsys/settings/src/settings_zms.c

@@ -146,6 +146,41 @@ static int settings_zms_delete(struct settings_zms *cf, uint32_t name_hash)
 	return rc;
 }
 
+#if CONFIG_SETTINGS_ZMS_NAME_CACHE
+static void settings_zms_cache_add(struct settings_zms *cf, uint32_t name_hash, bool has_collision)
+{
+	cf->cache[cf->cache_next].name_hash = name_hash;
+	cf->cache[cf->cache_next++].has_collision = has_collision;
+
+	cf->cache_next %= CONFIG_SETTINGS_ZMS_NAME_CACHE_SIZE;
+}
+
+static bool settings_zms_cache_match(struct settings_zms *cf, uint32_t name_hash)
+{
+	int cache_index;
+
+	if (!cf->cache_next) {
+		cache_index = CONFIG_SETTINGS_ZMS_NAME_CACHE_SIZE - 1;
+	} else {
+		cache_index = cf->cache_next - 1;
+	}
+
+	for (int i = 0; i < CONFIG_SETTINGS_ZMS_NAME_CACHE_SIZE; i++) {
+		/* we check cache from recent values to old values */
+		if (cf->cache[cache_index--].name_hash != name_hash) {
+			if (cache_index < 0) {
+				cache_index = CONFIG_SETTINGS_ZMS_NAME_CACHE_SIZE - 1;
+			}
+			continue;
+		}
+
+		return !cf->cache[i].has_collision;
+	}
+
+	return false;
+}
+#endif /* CONFIG_SETTINGS_ZMS_NAME_CACHE */
+
 static int settings_zms_load(struct settings_store *cs, const struct settings_load_arg *arg)
 {
 	int ret = 0;
@@ -199,6 +234,11 @@ static int settings_zms_load(struct settings_store *cs, const struct settings_lo
 			break;
 		}
 
+#if CONFIG_SETTINGS_ZMS_NAME_CACHE
+		settings_zms_cache_add(cf, ll_hash_id & ZMS_HASH_MASK,
+				       ZMS_COLLISION_NUM(ll_hash_id) > 0);
+#endif
+
 		/* update next ll_hash_id */
 		ret = zms_read(&cf->cf_zms, ll_hash_id, &settings_element,
 			       sizeof(struct settings_hash_linked_list));
@@ -236,6 +276,17 @@ static int settings_zms_save(struct settings_store *cs, const char *name, const
 	/* MSB is always 1 */
 	name_hash |= BIT(31);
 
+#if CONFIG_SETTINGS_ZMS_NAME_CACHE
+	bool name_in_cache = false;
+
+	name_in_cache = settings_zms_cache_match(cf, name_hash & ZMS_HASH_MASK);
+	if (name_in_cache) {
+		write_name = false;
+		hash_collision = false;
+		goto no_hash_collision;
+	}
+#endif
+
 	/* Let's find out if there is no hash collisions in the storage */
 	write_name = true;
 	hash_collision = true;
@@ -342,6 +393,13 @@ static int settings_zms_save(struct settings_store *cs, const char *name, const
 		cf->last_hash_id = name_hash | 1;
 	}
 
+#if CONFIG_SETTINGS_ZMS_NAME_CACHE
+	if (!name_in_cache) {
+		settings_zms_cache_add(cf, name_hash & ZMS_HASH_MASK,
+				       ZMS_COLLISION_NUM(name_hash) > 0);
+	}
+#endif
+
 	return 0;
 }