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Non-Blocking Communication Non-blocking send and receive The previously introduced MPI_SEND and MPI_RECV use blocking communication, i.e. the code does not return from a call to these routines until the send or receive operations have been completed entirely. Sometimes, one can take advantage of a parallel system's ability to overlap communication and computation by: initiating communication carrying out some computation checking for the communication's completion The non-blocking variant of the send routine is invoked by: CALL MPI_ISEND(buf, count, type, iddest, itag, MPI_COMM_WORLD, ireq,ierr) where: buf - a buffer holding the data to be sent count - the number of items in the buffer type - the type of the items in the buffer. Common predefined types include MPI_INTEGER, MPI_REAL, MPI_DOUBLE_PRECISION, MPI_COMPLEX, MPI_LOGICAL with obvious Fortran equivalents. iddest - the ID of the destination process. itag - an integer tag attached to the message for identification purposes ireq - an integer request handle provided by the MPI system to track individual, non-blocking communication requests The non-blocking MPI routine that receives a message is invoked by CALL MPI_IRECV(buf, count, type, idsrc, itag, MPI_COMM_WORLD,ireq, ierr) where: buf - a buffer to hold the received data count - the number of items in the buffer type - the type of the items in the buffer. idsrc - the ID of the process from which to receive the message. A special value MPI_ANY_SOURCE allows reception from any process. itag - an integer tag attached to the message for identification purposes. A special value MPI_ANY_TAG allows reception of a message irrespective of its tag. ireq - an integer request handle provided by the MPI system to track individual, non-blocking communication requests Checking for the completion of a communication There are two options in checking for a communication's completion: Wait until the communication is complete using MPI_WAIT Test if the communication is complete using MPI_TEST. If the communication is not complete this variant allows other computations to be done while awaiting completion. CALL MPI_WAIT(ireq,istatus,ierr) ireq - an integer request handle provided by the MPI system to track individual, non-blocking communication requests istatus - an integer array that provides information about this particular request's status CALL MPI_TEST(ireq,lflag,istatus,ierr) ireq - an integer request handle provided by the MPI system to track individual, non-blocking communication requests lflag - a logical flag; .TRUE. if communication indicated by ireq is complete, .FALSE. otherwise. istatus - an integer array that provides information about this particular request's status Example To see an example of the above basic routines go to the Crank-Nicolson application.