PATCH: add support for TCP_NODELAY on network streams

[Eric Marsden <eric.marsden@risk-engineering.org>]

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Hi,

The attached patch adds support for the socket option TCP_NODELAY,
to disable Nagle's algorithm. This means that network segments are
sent as soon as possible, even when they contain little data. This
reduces network latency on the network connection, but can lead to
many small packets being sent.

I have only tested on Linux/AMD64, but support for this socket option
is very widespread.

My motivation for adding this support is that Emacs is very slow
when connecting to the PostgreSQL database over the network, using
my pg-el library, and is unable to saturate CPU when sending a
stream of messages. It is much faster when using a local Unix
connection and saturates CPU.

With this patch, a test case that takes 911 seconds goes down to
76 seconds (x 12 speedup).

Eric

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diff --git a/doc/lispref/processes.texi b/doc/lispref/processes.texi
index 79ef959ae65..93ac5a6aa7d 100644
--- a/doc/lispref/processes.texi
+++ b/doc/lispref/processes.texi
@@ -3090,6 +3090,13 @@ Network Options
 may be a period of time after the last use of that port (by any
 process on the host) where it is not possible to make a new server on
 that port.
+
+@item :nodelay @var{nodelay-flag}
+If @var{nodelay-flag} is non-@code{nil}, the @code{TCP_NODELAY} option
+is enabled on the socket. This disables the Nagle algorithm, meaning
+that network segments are sent as soon as possible, even when they
+contain little data. This reduces network latency on the network
+connection, but can lead to many small packets being sent.
 @end table
 
 @defun set-network-process-option process option value &optional no-error
diff --git a/lisp/emacs-lisp/bytecomp.el b/lisp/emacs-lisp/bytecomp.el
index f058fc48cc7..07eb4690fce 100644
--- a/lisp/emacs-lisp/bytecomp.el
+++ b/lisp/emacs-lisp/bytecomp.el
@@ -6049,8 +6049,8 @@ bytecomp--check-keyword-args
             :buffer :host :service :type :family :local :remote :coding
             :nowait :noquery :stop :filter :filter-multibyte :sentinel
             :log :plist :tls-parameters :server :broadcast :dontroute
-            :keepalive :linger :oobinline :priority :reuseaddr :bindtodevice
-            :use-external-socket)
+            :keepalive :linger :oobinline :priority :reuseaddr :nodelay
+            :bindtodevice :use-external-socket)
           '(:name :service))))
 
 (provide 'byte-compile)
diff --git a/src/process.c b/src/process.c
index b71ba3daf2d..0b994d6c5b7 100644
--- a/src/process.c
+++ b/src/process.c
@@ -38,6 +38,7 @@ Copyright (C) 1985-1988, 1993-1996, 1998-1999, 2001-2024 Free Software
 #include <sys/socket.h>
 #include <netdb.h>
 #include <netinet/in.h>
+#include <netinet/tcp.h>
 #include <arpa/inet.h>
 
 #else
@@ -2860,6 +2861,9 @@ DEFUN ("set-process-datagram-address", Fset_process_datagram_address, Sset_proce
 #endif
 #ifdef SO_REUSEADDR
     { ":reuseaddr", SOL_SOCKET, SO_REUSEADDR, SOPT_BOOL, OPIX_REUSEADDR },
+#endif
+#ifdef TCP_NODELAY
+    { ":nodelay", IPPROTO_TCP, TCP_NODELAY, SOPT_BOOL, OPIX_MISC },
 #endif
     { 0, 0, 0, SOPT_UNKNOWN, OPIX_NONE }
   };