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pydevd_comm.py start_client() does not set TCP_NODELAY on the connections it opens #2054
Description
Environment data
- debugpy version: 1.8.20
- OS and version: Linux and MacOS
- Python version (& distribution if applicable, e.g. Anaconda): 3.11 and later
- Using VS Code or Visual Studio: No
Actual behavior
TCP_NODELAY is not set on the connections debugpy opens
Expected behavior
TCP_NODELAY is set on the connections debugpy opens
Steps to reproduce:
A connection established with debugpy --connect does not set TCP_NODELAY on the socket connection, which can cause latency when sending responses and events. This is can be verified simply by inspecting the pydevd_comm.py source code, or by using external tools to inspect the connection once it has been established.
We patched our copy of debugpy 1.8.20 as follows:
% diff --git a/3rdparty/debugpy/DEBUGPY/src/debugpy/_vendored/pydevd/_pydevd_bundle/pydevd_comm.py b/3rdparty/debugpy/DEBUGPY/src/debugpy/_vendored/pydevd/_pydevd_bundle/pydevd_comm.py
index 64ac454f29..812bf59eb0 100644
--- a/3rdparty/debugpy/DEBUGPY/src/debugpy/_vendored/pydevd/_pydevd_bundle/pydevd_comm.py
+++ b/3rdparty/debugpy/DEBUGPY/src/debugpy/_vendored/pydevd/_pydevd_bundle/pydevd_comm.py
@@ -524,6 +524,15 @@ def start_client(host, port):
except (AttributeError, OSError):
pass # May not be available everywhere.
+ try:
+ # TVT-46072: "Missing TCP_NODELAY on debugpy's DAP tracer connection
+ # causes Nagle-induced latency in Python debugging". Disable Nagle's
+ # algorithm so small DAP messages are sent immediately instead of
+ # being buffered, reducing round-trip latency.
+ s.setsockopt(socket_module.IPPROTO_TCP, socket_module.TCP_NODELAY, 1)
+ except (AttributeError, OSError):
+ pass # May not be available everywhere.
+
try:
# 10 seconds default timeout
timeout = int(os.environ.get("PYDEVD_CONNECT_TIMEOUT", 10))
%
Activity
nightcityblade commentedon Aug 16, 2026
Hi, I'd like to work on this. I'll submit a PR shortly.