[erts|kernel] New default values for the 'buffer' option

The socket option 'buffer' has new default values; 8192.
The major difference now is that its a unique value for each
type of socket (TCP, UDP and SCTP).

erts/emulator/drivers/common/inet_drv.c

@@ -1,7 +1,7 @@
 /*
  * %CopyrightBegin%
  *
- * Copyright Ericsson AB 1997-2024. All Rights Reserved.
+ * Copyright Ericsson AB 1997-2025. All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -1032,8 +1032,17 @@ static size_t my_strnlen(const char *s, size_t maxlen)
 
 #define INET_MAX_OPT_BUFFER (64*1024)
 
-#define INET_DEF_BUFFER     1460        /* default buffer size */
-#define INET_MIN_BUFFER     1           /* internal min buffer */
+#if !defined(INET_BUFFER_TCP_DEFAULT)
+#define INET_BUFFER_TCP_DEFAULT  8192
+#endif
+#if !defined(INET_BUFFER_UDP_DEFAULT)
+#define INET_BUFFER_UDP_DEFAULT  8192
+#endif
+#if !defined(INET_BUFFER_SCTP_DEFAULT)
+#define INET_BUFFER_SCTP_DEFAULT 8192
+#endif
+#define INET_BUFFER_MIN          1
+#define INET_UDP_THEORETICAL_MAX_MTU (1 << 16)
 
 #define INET_HIGH_WATERMARK (1024*8) /* 8k pending high => busy  */
 #define INET_LOW_WATERMARK  (1024*4) /* 4k pending => allow more */
@@ -1877,7 +1886,11 @@ static void end_caller_ref(CallerRef *cref_p) {
 #else
 #   define IS_SCTP(desc) 0
 #endif
-#   define ANC_BUFF_SIZE   INET_DEF_BUFFER/2 /* XXX: not very good... */
+/*
+ * Can we "calculate" this dynamically?
+ * On linux: sysctl net.core.optmem_max
+ */
+#   define ANC_BUFF_SIZE        1024
 
 
 #ifdef HAVE_UDP
@@ -6904,7 +6917,7 @@ static int inet_set_opts(inet_descriptor* desc, char* ptr, int len)
                  ("INET-DRV-DBG[%d][" SOCKET_FSTR ",%T] "
                   "inet_set_opts(buffer) -> %d\r\n",
                   __LINE__, desc->s, driver_caller(desc->port), ival) );
-	    if (ival < INET_MIN_BUFFER) ival = INET_MIN_BUFFER;
+	    if (ival < INET_BUFFER_MIN) ival = INET_BUFFER_MIN;
 	    desc->bufsz = ival;
             desc->flags |= INET_FLG_BUFFER_SET;
 	    continue;
@@ -7268,14 +7281,39 @@ static int inet_set_opts(inet_descriptor* desc, char* ptr, int len)
                   "inet_set_opts(rcvbuf) -> %d\r\n",
                   __LINE__, desc->s, driver_caller(desc->port), ival) );
             if (!(desc->flags & INET_FLG_BUFFER_SET)) {
-                /* make sure we have desc->bufsz >= SO_RCVBUF */
-                if (ival > (1 << 16) && desc->stype == SOCK_DGRAM && !IS_SCTP(desc))
+
+                /* BUFFER has not previously been explicitly set.
+                 * Make sure we have desc->bufsz >= SO_RCVBUF,
+                 * except if rcvbuf is larger than max when type = DGRAM.
+                 */
+
+                if ((ival > INET_UDP_THEORETICAL_MAX_MTU) &&
+                    (desc->stype == SOCK_DGRAM) &&
+                    !IS_SCTP(desc)) {
+
                     /* For UDP we don't want to automatically
-                       set the buffer size to be larger than
-                       the theoretical max MTU */
-                    desc->bufsz = 1 << 16;
-                else if (ival > desc->bufsz)
+                     * set the buffer size to be larger than
+                     * the theoretical max MTU. */
+
+                    DDBG(desc,
+                         ("INET-DRV-DBG[%d][" SOCKET_FSTR ",%T] "
+                          "inet_set_opts(rcvbuf) -> "
+                          "force update of 'buffer' to max-mtu (%d)\r\n",
+                          __LINE__, desc->s, driver_caller(desc->port),
+                          INET_UDP_THEORETICAL_MAX_MTU) );
+
+                    desc->bufsz = INET_UDP_THEORETICAL_MAX_MTU;
+
+                } else if (ival > desc->bufsz) {
+
+                    DDBG(desc,
+                         ("INET-DRV-DBG[%d][" SOCKET_FSTR ",%T] "
+                          "inet_set_opts(rcvbuf) -> "
+                          "force update of 'buffer'\r\n",
+                          __LINE__, desc->s, driver_caller(desc->port)) );
+
                     desc->bufsz = ival;
+                }
             }
 	    break;
 
@@ -7999,8 +8037,8 @@ static int sctp_set_opts(inet_descriptor* desc, char* ptr, int len)
 
                 desc->bufsz  = ival;
 
-                if (desc->bufsz < INET_MIN_BUFFER)
-                    desc->bufsz = INET_MIN_BUFFER;
+                if (desc->bufsz < INET_BUFFER_MIN)
+                    desc->bufsz = INET_BUFFER_MIN;
                 desc->flags |= INET_FLG_BUFFER_SET;
                 res = 0;   /* This does not affect the kernel buffer size */
             }
@@ -9024,6 +9062,10 @@ static ErlDrvSSizeT inet_fill_opts(inet_descriptor* desc,
 
 	switch(opt) {
 	case INET_LOPT_BUFFER:
+            DDBG(desc,
+                 ("INET-DRV-DBG[%d][" SOCKET_FSTR ",%T] "
+                  "inet_fill_opts -> buffer: %d\r\n",
+                  __LINE__, desc->s, driver_caller(desc->port), desc->bufsz) );
 	    *ptr++ = opt;
 	    put_int32(desc->bufsz, ptr);
 	    continue;
@@ -10749,7 +10791,18 @@ static ErlDrvData inet_start(ErlDrvPort port, int size, int protocol)
     desc->dport = driver_mk_port(port);
     desc->state = INET_STATE_CLOSED;
     desc->prebound = 0;
-    desc->bufsz = INET_DEF_BUFFER; 
+
+    if (protocol == IPPROTO_TCP)
+        desc->bufsz = INET_BUFFER_TCP_DEFAULT;
+    else if (protocol == IPPROTO_UDP)
+        desc->bufsz = INET_BUFFER_UDP_DEFAULT;
+#if defined(HAVE_SCTP)
+    else if (protocol == IPPROTO_SCTP)
+        desc->bufsz = INET_BUFFER_SCTP_DEFAULT;
+#endif
+    else /* We only support tcp, udp and SCTP so this "should not" happen... */
+        desc->bufsz = 9999;
+
     desc->hsz = 0;                     /* list header size */
     desc->htype = TCP_PB_RAW;          /* default packet type */
     desc->psize = 0;                   /* no size check */

lib/kernel/src/inet_udp.erl

@@ -1,7 +1,7 @@
 %%
 %% %CopyrightBegin%
 %% 
-%% Copyright Ericsson AB 1997-2024. All Rights Reserved.
+%% Copyright Ericsson AB 1997-2025. All Rights Reserved.
 %% 
 %% Licensed under the Apache License, Version 2.0 (the "License");
 %% you may not use this file except in compliance with the License.
@@ -32,7 +32,7 @@
 -define(FAMILY, inet).
 -define(PROTO,  udp).
 -define(TYPE,   dgram).
--define(RECBUF, (8*1024)).
+-define(RECBUF, (8*1024)). % No longer needed when opening a *new* socket
 
 
 %% inet_udp port lookup
@@ -52,7 +52,7 @@ open(Port) -> open(Port, []).
 -spec open(_, _) -> {ok, port()} | {error, atom()}.
 open(Port, Opts) ->
     case inet:udp_options(
-	   [{port,Port}, {recbuf, ?RECBUF} | Opts], 
+	   [{port,Port} | Opts], 
 	   ?MODULE) of
 	{error, Reason} -> exit(Reason);
 	{ok,