Speed up a common case in iostream._merge_prefix

diff --git a/tornado/iostream.py b/tornado/iostream.py
index 02016a0f3b6033fb047ce033c981b8de403ff998..01df805445212acb35691f175ffe6b29f259ef14 100644 (file)
--- a/tornado/iostream.py
+++ b/tornado/iostream.py
@@ -516,6 +516,8 @@ def _merge_prefix(deque, size):
     >>> _merge_prefix(d, 100); print d
     deque(['abcdefghij'])
     """
+    if len(deque) == 1 and len(deque[0]) < size:
+        return
     prefix = []
     remaining = size
     while deque and remaining > 0: