diff --git a/env.example b/env.example
index 73f2d7b7..4b86d79f 100644
--- a/env.example
+++ b/env.example
@@ -138,8 +138,10 @@ SUMMARY_LANGUAGE=English
 ### control the maximum chunk_ids stored in vector and graph db
 # MAX_SOURCE_IDS_PER_ENTITY=300
 # MAX_SOURCE_IDS_PER_RELATION=300
-### control chunk_ids limitation method: KEEP, FIFO (KEEP: Keep oldest, FIFO: First in first out)
-# SOURCE_IDS_LIMIT_METHOD=KEEP
+### control chunk_ids limitation method: FIFO, FIFO
+###    FIFO: First in first out
+###    KEEP: Keep oldest (less merge action and faster)
+# SOURCE_IDS_LIMIT_METHOD=FIFO
 ### Maximum number of file paths stored in entity/relation file_path field
 # MAX_FILE_PATHS=30
 
diff --git a/lightrag/constants.py b/lightrag/constants.py
index 7c2b2701..f4e06e11 100644
--- a/lightrag/constants.py
+++ b/lightrag/constants.py
@@ -60,9 +60,12 @@ DEFAULT_RERANK_BINDING = "null"
 # Default source ids limit in meta data for entity and relation
 DEFAULT_MAX_SOURCE_IDS_PER_ENTITY = 300
 DEFAULT_MAX_SOURCE_IDS_PER_RELATION = 300
-SOURCE_IDS_LIMIT_METHOD_KEEP = "KEEP"  # Keep oldest
-SOURCE_IDS_LIMIT_METHOD_FIFO = "FIFO"  # First In First Out (Keep newest)
-DEFAULT_SOURCE_IDS_LIMIT_METHOD = SOURCE_IDS_LIMIT_METHOD_KEEP
+### control chunk_ids limitation method: FIFO, FIFO
+###    FIFO: First in first out
+###    KEEP: Keep oldest (less merge action and faster)
+SOURCE_IDS_LIMIT_METHOD_KEEP = "KEEP"
+SOURCE_IDS_LIMIT_METHOD_FIFO = "FIFO"
+DEFAULT_SOURCE_IDS_LIMIT_METHOD = SOURCE_IDS_LIMIT_METHOD_FIFO
 VALID_SOURCE_IDS_LIMIT_METHODS = {
     SOURCE_IDS_LIMIT_METHOD_KEEP,
     SOURCE_IDS_LIMIT_METHOD_FIFO,