Index: winsup/cygwin/net.cc
===================================================================
RCS file: /cvs/src/src/winsup/cygwin/net.cc,v
retrieving revision 1.320
diff -u -p -r1.320 net.cc
--- winsup/cygwin/net.cc	13 Oct 2014 08:18:18 -0000	1.320
+++ winsup/cygwin/net.cc	19 Nov 2014 18:02:18 -0000
@@ -621,13 +621,16 @@ fdsock (cygheap_fdmanip& fd, const devic
      this is no problem on 64 bit.  So we set the default buffer size to
      the default values in current 3.x Linux versions.
 
-     (*) Maximum normal TCP window size.  Coincidence?  */
+     (*) Maximum normal TCP window size.  Coincidence?  
+
+     NOTE 3. Setting the window size to 65535 results in extremely bad performance for apps that send data in multiples of Kb, as they eventually end up sending 1 byte on the network and naggle + delay ack kicks in. For example, iperf on a 10Gb network gives only 10 Mbits/sec with a 65535 send buffer. We want this to be a multiple of 1k, but since 64k breaks WSADuplicateSocket we use 63Kb.
+*/
 #ifdef __x86_64__
   ((fhandler_socket *) fd)->rmem () = 212992;
   ((fhandler_socket *) fd)->wmem () = 212992;
 #else
-  ((fhandler_socket *) fd)->rmem () = 65535;
-  ((fhandler_socket *) fd)->wmem () = 65535;
+  ((fhandler_socket *) fd)->rmem () = 64512;
+  ((fhandler_socket *) fd)->wmem () = 64512;
 #endif
   if (::setsockopt (soc, SOL_SOCKET, SO_RCVBUF,
 		    (char *) &((fhandler_socket *) fd)->rmem (), sizeof (int)))