[config] server.listen-backlog option (fixes #1825, #2116)

See doc/config/lighttpd.conf for explanation of listen() backlog queue

Additionally, mod_fastcgi and mod_scgi backend servers can now also be
configured with separate listen-backlog settings per server

x-ref:
  "add server.listen-backlog option instead of hard-coded value (128 * 8) for listen()"
  https://redmine.lighttpd.net/issues/2116
  "Don't disable backend when overloaded"
  https://redmine.lighttpd.net/issues/1825

github:
Closes #50

1 files changed, 29 insertions, 0 deletions

diff --git a/doc/config/lighttpd.conf b/doc/config/lighttpd.conf
index 68cfea94..79122d7e 100644
--- a/doc/config/lighttpd.conf
+++ b/doc/config/lighttpd.conf
@@ -207,6 +207,35 @@ server.network-backend = "sendfile"
 server.max-fds = 2048
 
 ##
+## listen-backlog is the size of the listen() backlog queue requested when
+## the lighttpd server ask the kernel to listen() on the provided network
+## address.  Clients attempting to connect() to the server enter the listen()
+## backlog queue and wait for the lighttpd server to accept() the connection.
+##
+## The out-of-box default on many operating systems is 128 and is identified
+## as SOMAXCONN.  This can be tuned on many operating systems.  (On Linux,
+## cat /proc/sys/net/core/somaxconn)  Requesting a size larger than operating
+## system limit will be silently reduced to the limit by the operating system.
+##
+## When there are too many connection attempts waiting for the server to
+## accept() new connections, the listen backlog queue fills and the kernel
+## rejects additional connection attempts.  This can be useful as an
+## indication to an upstream load balancer that the server is busy, and
+## possibly overloaded.  In that case, configure a smaller limit for
+## server.listen-backlog.  On the other hand, configure a larger limit to be
+## able to handle bursts of new connections, but only do so up to an amount
+## that the server can keep up with responding in a reasonable amount of
+## time.  Otherwise, clients may abandon the connection attempts and the
+## server will waste resources servicing abandoned connections.
+##
+## It is best to leave this setting at its default unless you have modelled
+## your traffic and tested that changing this benefits your traffic patterns.
+##
+## Default: 1024
+##
+#server.listen-backlog = 128
+
+##
 ## Stat() call caching.
 ##
 ## lighttpd can utilize FAM/Gamin to cache stat call.