btl tcp: Don't set socket buffer size by default

Set the default send and receive socket buffer size to 0,
which means Open MPI will not try to set a buffer size during
startup.

The default behavior since near day one of the TCP BTL has been
to set the send and receive socket buffer sizes to 128 KiB.  A
number that works great on 1 GbE, but not so great on 10 GbE
fabrics of any real size.  Modern TCP stacks, particularly on
Linux, have gotten much smarter about buffer sizes and are much
less efficient if a buffer size is set (even if set to something
large).

Signed-off-by: Brian Barrett <bbarrett@amazon.com>

opal/mca/btl/tcp/btl_tcp_component.c

@@ -250,8 +250,20 @@ static int mca_btl_tcp_component_register(void)
     mca_btl_tcp_param_register_int ("free_list_num", NULL, 8, OPAL_INFO_LVL_5,  &mca_btl_tcp_component.tcp_free_list_num);
     mca_btl_tcp_param_register_int ("free_list_max", NULL, -1, OPAL_INFO_LVL_5,  &mca_btl_tcp_component.tcp_free_list_max);
     mca_btl_tcp_param_register_int ("free_list_inc", NULL, 32, OPAL_INFO_LVL_5,  &mca_btl_tcp_component.tcp_free_list_inc);
-    mca_btl_tcp_param_register_int ("sndbuf", NULL, 128*1024, OPAL_INFO_LVL_4, &mca_btl_tcp_component.tcp_sndbuf);
-    mca_btl_tcp_param_register_int ("rcvbuf", NULL, 128*1024, OPAL_INFO_LVL_4, &mca_btl_tcp_component.tcp_rcvbuf);
+    mca_btl_tcp_param_register_int ("sndbuf",
+                                    "The size of the send buffer socket option for each connection.  "
+                                    "Modern TCP stacks generally are smarter than a fixed size and in some "
+                                    "situations setting a buffer size explicitly can actually lower "
+                                    "performance.  0 means the tcp btl will not try to set a send buffer "
+                                    "size.",
+                                    0, OPAL_INFO_LVL_4, &mca_btl_tcp_component.tcp_sndbuf);
+    mca_btl_tcp_param_register_int ("rcvbuf",
+                                    "The size of the receive buffer socket option for each connection.  "
+                                    "Modern TCP stacks generally are smarter than a fixed size and in some "
+                                    "situations setting a buffer size explicitly can actually lower "
+                                    "performance.  0 means the tcp btl will not try to set a send buffer "
+                                    "size.",
+                                    0, OPAL_INFO_LVL_4, &mca_btl_tcp_component.tcp_rcvbuf);
     mca_btl_tcp_param_register_int ("endpoint_cache",
         "The size of the internal cache for each TCP connection. This cache is"
         " used to reduce the number of syscalls, by replacing them with memcpy."