It's at least the message header (communicator, tag, etc.) so that the
receiver can figure out if this is the expected message or not. In
practice, there is probably also some data in there as well. The
amount of that portion depends on the MPI implementation and, in
practice, the interconnect the message traveled over,
MPI-implementation-dependent environment variables set by the user,
etc. E.g., with OMPI over shared memory by default it's about 4Kbytes
(if I remember correctly).
I used the same buffer to get the message in 50 node case.
About ""rendezvous" protocol", what is the meaning of "the
sender sends a short portion "?
What is the "short portion", is it a small mart of the message
of the sender ?