POINT-TO-POINT COMMUNICATION ROUTINES BASIC INFORMATION

MPI point-to-point communication routines are utilized for data communications between a sending processor and a receiving processor. The argument list of these routines takes one of the following formats:

Blocking communication

MPI-Send (buffer, count, type, dest, tag, comm)

MPI-Recv (buffer, count, type, source, tag, comm, status)

Non-blocking communication

MPI-Isend (buffer, count, type, dest, tag, comm, request)

MPI-Irecv (buffer, count, type, source, tag, comm, request)

The arguments of these communication routines are explained as

follows:

A buffer is the application address space that references the data to be sent or received. In most cases, this is simply the variable name that is to be sentheceived. For C programs this argument is passed by the reference and usually must be pre-pended with an ampersand.

A data count indicates the number of data elements of a particular type to be sent or received. A data type must use the data types predefined by MPI, though programmers can also create their derived types. We should note that the MPI types, MPI-BYTE, and MPIPACKED do not correspond to standard C or Fortran types.

A dest indicates the send process where a message should be delivered, it is specified as the rank of the receiving process. A source indicates the originating process of the message; as the counterpart of the dest, it also specifies the rank of the sending process.

The source may be set to the wild card MPI-ANY-SOURCE to receive a message from any task. The tag is an arbitrary non-negative integer assigned by the programmer to uniquely identify a message. The send and receive operations should match message tags.

For a receive operation, the wild card MPIANYTAG can be used to receive any message regardless of its tag. The MPI standard guarantees that integers 0-32767 can be used as tags, but most implementations allow much wider range than this.

A comm represents the designated communicator and indicates the communication context, or the set of processes for which the source or destination fields are valid. The predefined communicator MPI-COMM-WORLD is usually used, unless a programmer is explicitly creating new communicators.

A status indicates the source of the message and the tag of the message for a receive operation. In C, this argument is a pointer to a predefined structure MPI-Status. In Fortran, it is an integer array of size MPI STATUS SIZE. Additionally the actual number of bytes received is obtainable fromstatus via the MPI-GET-COUNT routine.

A request is used by non-blocking send and receive operations. Since nonblocking operations may return before the requested system buffer space is obtained, the system issues a unique request number. The programmer uses this system assigned handle later to determine completion of the nonblocking operation. In C, this argument is a pointer to a predefined structure MPI-Request. In Fortran, it is an integer.

In the following, we present an example of point-to-point communication. Suppose that in the distributed load flow computation, SACCi and SACCj will exchange boundary state variables. For simplicity but without losing generality, we assume that SACCi will only send the magnitude of one boundary voltage variable.

If the ranks of SACCi and SACC, are assigned by the distributed system are x and y, respectively, then SACCi and SACC, can use the following function calls to realize synchronous communication:

SACCi

SACCj

SACCi and SACC, can use the following function calls to realize the asynchronous communication:

MPI-Send (vk, 1, MPI-FLOAT, y, 8, comm)

MPI-Recv (vb, 1, MPI-FLOAT, x, 8, comm, status)

SACCi

SACCj

MPI-Isend (vk, 1, MPI-FLOAT, y, 8, comm, 5)

MPI-Irecv (vb, 1, MPI-FLOAT, x, 8, comm, 7)