From 74f5b924205cde25c20d0b391bb2ead4a851afe7 Mon Sep 17 00:00:00 2001
From: Azat Khuzhin <a3at.mail@gmail.com>
Date: Wed, 6 Oct 2021 21:09:00 +0300
Subject: [PATCH] Add listen_backlog to documentation

Follow-up for: #29643
---
 .../settings.md                               | 26 +++++++++++++++++++
 programs/server/config.xml                    |  2 +-
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/docs/en/operations/server-configuration-parameters/settings.md b/docs/en/operations/server-configuration-parameters/settings.md
index 552854f7cff..b99625af91a 100644
--- a/docs/en/operations/server-configuration-parameters/settings.md
+++ b/docs/en/operations/server-configuration-parameters/settings.md
@@ -475,6 +475,32 @@ Examples:
 <listen_host>127.0.0.1</listen_host>
 ```
 
+## listen_backlog {#server_configuration_parameters-listen_backlog}
+
+Backlog (queue size of pending connections) of the listen socket.
+
+Default value: `4096` (as in linux [5.4+](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=19f92a030ca6d772ab44b22ee6a01378a8cb32d4)).
+
+Usually you don't need to change this, since:
+- default value is large enough,
+- and for accepting client's connections server has separate thread.
+
+So even if you have `TcpExtListenOverflows` (from `nstat`) non zero (and you
+100% sure that this counter grows for ClickHouse server) it does not mean that
+you need to increase this value, since:
+- usually if 4096 is not enough it shows some internal ClickHouse scaling
+  issue, so it is better to report an issue.
+- and it does not mean that the server can handle more connections later (and
+  even if it can, clients can already goes away / disconnect).
+
+However if you 100% sure, go ahead.
+
+Examples:
+
+``` xml
+<listen_backlog>4096</listen_backlog>
+```
+
 ## logger {#server_configuration_parameters-logger}
 
 Logging settings.
diff --git a/programs/server/config.xml b/programs/server/config.xml
index f9a3a5030d9..ba4c680d765 100644
--- a/programs/server/config.xml
+++ b/programs/server/config.xml
@@ -172,7 +172,7 @@
       -->
     <!-- <listen_reuse_port>0</listen_reuse_port> -->
 
-    <!-- <listen_backlog>64</listen_backlog> -->
+    <!-- <listen_backlog>4096</listen_backlog> -->
 
     <max_connections>4096</max_connections>