Re: [OMPI users] OpenMPI how large its buffer size ?

[Eugene Loh]

Jack Bryan wrote: The master node can receive message ( the same size)  from 50 worker nodes.  But, it cannot receive message from 51 nodes. It caused "truncate error". How big was the buffer that the program specified in the receive call?  How big was the message that was sent? MPI_ERR_TRUNCATE means that you posted a receive with an application buffer that turned out to be too small to hold the message that was received.  It's a user application error that has nothing to do with MPI's internal buffers.  MPI's internal buffers don't need to be big enough to hold that message.  MPI could require the sender and receiver to coordinate so that only part of the message is moved at a time. 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 ? 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). This "rendezvous" protocol" can work automatically in background without programmer indicates in his program ? Right.  MPI actually allows you to force such synchronization with MPI_Ssend, but typically MPI implementations use it automatically for "plain" long sends as well even if the user didn't not use MPI_Ssend. The "acknowledgement " can be generated by the receiver only when the corresponding mpi_irecv is posted by the receiver ?  Right.