Issue #20540: Fix a performance regression (vs. Python 3.2) when layering a multiprocessing Connection over a TCP socket.
For small payloads, Nagle's algorithm would introduce idle delays before the entire transmission of a message.
diff --git a/Lib/multiprocessing/connection.py b/Lib/multiprocessing/connection.py
index 2a0bc2f..22589d0 100644
--- a/Lib/multiprocessing/connection.py
+++ b/Lib/multiprocessing/connection.py
@@ -395,13 +395,23 @@
return buf
def _send_bytes(self, buf):
- # For wire compatibility with 3.2 and lower
n = len(buf)
- self._send(struct.pack("!i", n))
- # The condition is necessary to avoid "broken pipe" errors
- # when sending a 0-length buffer if the other end closed the pipe.
- if n > 0:
- self._send(buf)
+ # For wire compatibility with 3.2 and lower
+ header = struct.pack("!i", n)
+ if n > 16384:
+ # The payload is large so Nagle's algorithm won't be triggered
+ # and we'd better avoid the cost of concatenation.
+ chunks = [header, buf]
+ elif n > 0:
+ # Issue # 20540: concatenate before sending, to avoid delays due
+ # to Nagle's algorithm on a TCP socket.
+ chunks = [header + buf]
+ else:
+ # This code path is necessary to avoid "broken pipe" errors
+ # when sending a 0-length buffer if the other end closed the pipe.
+ chunks = [header]
+ for chunk in chunks:
+ self._send(chunk)
def _recv_bytes(self, maxsize=None):
buf = self._recv(4)
diff --git a/Misc/NEWS b/Misc/NEWS
index 3ee0743..fd89776 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -48,6 +48,11 @@
Library
-------
+- Issue #20540: Fix a performance regression (vs. Python 3.2) when layering
+ a multiprocessing Connection over a TCP socket. For small payloads, Nagle's
+ algorithm would introduce idle delays before the entire transmission of a
+ message.
+
- Issue #16983: the new email header parsing code will now decode encoded words
that are (incorrectly) surrounded by quotes, and register a defect.