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
 
                /*