MPI Asynchronous: Example

- buffers declared in isend/irecv can be (re-)used only after the communication has actually completed.
- Requests should be freed (mpi_test, mpi_wait, mpi_request_free) for all the isend calls in the program, otherwise mpi_finalize might hang

include ‘mpif.h’

integer stat(MPI_STATUS_SIZE,10)

integer req(10)

real B1(NB1,10)

if(mype.eq.0) then ! Master receiving from all slaves

do ip=1,npes-1

call mpi_irecv(B1(ip),NB1,MPI_REAL,

ip,MPI_ANY_TAG,MPI_COMM_WORLD,req(ip),info)

enddo

nreq = npes

else ! Slave send to master

call mpi_isend(B1(mype),NB1,MPI_REAL,0,itag,MPI_COMM_WORLD,req,info)

nreq = 1

endif

… ! Some unrelated calculations

call mpi_waitall(nreq,req,stat,ierr)

… ! Data is available in B1 in the master process

… ! Buffer B1 can be reused in the slave processes

Buffer management with asynchronous communcation: