Don't set SO_SNDBUF on recent Windows versions that have a bigger default.

It's unnecessary to set it if the default is higher in the first place.
Furthermore, setting SO_SNDBUF disables the so-called "dynamic send
buffering" feature, which hurts performance further. This can be seen
especially when the network between the client and the server has high
latency.

Chen Huajun

diff --git a/src/backend/libpq/pqcomm.c b/src/backend/libpq/pqcomm.c
index a4b37ed5a263c5e2f7e66526ac3a2446b1101725..9f92e4efae7660f7e49688860f4c54f1ba0785bf 100644 (file)
--- a/src/backend/libpq/pqcomm.c
+++ b/src/backend/libpq/pqcomm.c
@@ -726,6 +726,11 @@ StreamConnection(pgsocket server_fd, Port *port)
    if (!IS_AF_UNIX(port->laddr.addr.ss_family))
    {
        int         on;
+#ifdef WIN32
+       int         oldopt;
+       int         optlen;
+       int         newopt;
+#endif
 
 #ifdef TCP_NODELAY
        on = 1;
@@ -747,16 +752,43 @@ StreamConnection(pgsocket server_fd, Port *port)
 #ifdef WIN32
 
        /*
-        * This is a Win32 socket optimization.  The ideal size is 32k.
-        * http://support.microsoft.com/kb/823764/EN-US/
+        * This is a Win32 socket optimization.  The OS send buffer should be
+        * large enough to send the whole Postgres send buffer in one go, or
+        * performance suffers.  The Postgres send buffer can be enlarged if a
+        * very large message needs to be sent, but we won't attempt to
+        * enlarge the OS buffer if that happens, so somewhat arbitrarily
+        * ensure that the OS buffer is at least PQ_SEND_BUFFER_SIZE * 4.
+        * (That's 32kB with the current default).
+        *
+        * The default OS buffer size used to be 8kB in earlier Windows
+        * versions, but was raised to 64kB in Windows 2012.  So it shouldn't
+        * be necessary to change it in later versions anymore.  Changing it
+        * unnecessarily can even reduce performance, because setting
+        * SO_SNDBUF in the application disables the "dynamic send buffering"
+        * feature that was introduced in Windows 7.  So before fiddling with
+        * SO_SNDBUF, check if the current buffer size is already large enough
+        * and only increase it if necessary.
+        *
+        * See https://support.microsoft.com/kb/823764/EN-US/ and
+        * https://msdn.microsoft.com/en-us/library/bb736549%28v=vs.85%29.aspx
         */
-       on = PQ_SEND_BUFFER_SIZE * 4;
-       if (setsockopt(port->sock, SOL_SOCKET, SO_SNDBUF, (char *) &on,
-                      sizeof(on)) < 0)
+       optlen = sizeof(oldopt);
+       if (getsockopt(server_fd, SOL_SOCKET, SO_SNDBUF, (char *) &oldopt,
+                      &optlen) < 0)
        {
-           elog(LOG, "setsockopt(SO_SNDBUF) failed: %m");
+           elog(LOG, "getsockopt(SO_SNDBUF) failed: %m");
            return STATUS_ERROR;
        }
+       newopt = PQ_SEND_BUFFER_SIZE * 4;
+       if (oldopt < newopt)
+       {
+           if (setsockopt(port->sock, SOL_SOCKET, SO_SNDBUF, (char *) &newopt,
+                          sizeof(newopt)) < 0)
+           {
+               elog(LOG, "setsockopt(SO_SNDBUF) failed: %m");
+               return STATUS_ERROR;
+           }
+       }
 #endif
 
        /*