sendcount

Sendcount is a term used in the Message Passing Interface (MPI) to denote the number of elements of a given datatype that are sent in a single communication operation. In the standard blocking send routine MPI_Send(buffer, count, datatype, dest, tag, comm), count specifies the sendcount. The datatype indicates the kind of elements in the buffer, and the actual data volume is count times the size of datatype. The concept applies to both basic and derived datatypes.

For a derived datatype, the sendcount still counts elements of that datatype, not raw bytes. Therefore, sending an array of 100 elements of MPI_DOUBLE uses a sendcount of 100 with datatype MPI_DOUBLE; the total bytes equals 100 times the size of a double. The same concept applies to non-contiguous and derived datatypes, where the effective number of bytes transmitted depends on the datatype description.

On the receiving side, MPI_Recv defines a receive buffer with a capacity specified by recvcount. The operation copies up to recvcount elements of the same datatype. The actual number received can be obtained from the status object (e.g., via MPI_Get_count). If the incoming message is larger than the receive buffer, the operation may trigger an error such as MPI_ERR_TRUNCATE.

Practical considerations include ensuring matching datatype usage and consistent counts between sender and receiver, avoiding deadlock in blocking patterns, and choosing appropriate sendcount values for performance. Non-blocking variants and buffered sends have similar count semantics, and programmers must not reuse buffers until communications complete.