Hello, I followed a simple MPI example to do a ring communication.
Here's the figure that illustrates this example with 7 processes : http://i.imgur.com/Wrd6acv.png Here the code : -------------------------------------------------------------------------------------------------------------------------- program ring implicit none include 'mpif.h' integer, dimension( MPI_STATUS_SIZE ) :: status integer, parameter :: tag=100 integer :: nb_procs, rank, value, & num_proc_previous,num_proc_next,code call MPI_INIT (code) call MPI_COMM_SIZE ( MPI_COMM_WORLD ,nb_procs,code) call MPI_COMM_RANK ( MPI_COMM_WORLD ,rank,code) num_proc_next=mod(rank+1,nb_procs) num_proc_previous=mod(nb_procs+rank-1,nb_procs) if (rank == 0) then call MPI_SEND (1000,1, MPI_INTEGER ,num_proc_next,tag, & MPI_COMM_WORLD ,code) call MPI_RECV (value,1, MPI_INTEGER ,num_proc_previous,tag, & MPI_COMM_WORLD ,status,code) else call MPI_RECV (value,1, MPI_INTEGER ,num_proc_previous,tag, & MPI_COMM_WORLD ,status,code) call MPI_SEND (rank+1000,1, MPI_INTEGER ,num_proc_next,tag, & MPI_COMM_WORLD ,code) end if print *,'Me, process ',rank,', I have received ',value,' from process ',num_proc_previous call MPI_FINALIZE (code) end program ring -------------------------------------------------------------------------------------------------------------------------- At the execution, I expect to always have : Me, process 1 , I have received 1000 from process 0 Me, process 2 , I have received 1001 from process 1 Me, process 3 , I have received 1002 from process 2 Me, process 4 , I have received 1003 from process 3 Me, process 5 , I have received 1004 from process 4 Me, process 6 , I have received 1005 from process 5 Me, process 0 , I have received 1006 from process 6 But sometimes, I have the reception of process 0 from process 6 which is not the last reception, like this : Me, process 1 , I have received 1000 from process 0 Me, process 2 , I have received 1001 from process 1 Me, process 3 , I have received 1002 from process 2 Me, process 4 , I have received 1003 from process 3 Me, process 5 , I have received 1004 from process 4 Me, process 0 , I have received 1006 from process 6 Me, process 6 , I have received 1005 from process 5 where reception of process 0 from process 6 happens before the reception of process 6 from process 5 or like on this result : Me, process 1 , I have received 1000 from process 0 Me, process 2 , I have received 1001 from process 1 Me, process 3 , I have received 1002 from process 2 Me, process 4 , I have received 1003 from process 3 Me, process 0 , I have received 1006 from process 6 Me, process 5 , I have received 1004 from process 4 Me, process 6 , I have received 1005 from process 5 where process 0 receives between the reception of process 4 and 5. How can we explain this strange result ? I thought that standard use of MPI_SEND and MPI_RECV were blocking by default and, with this result, it seems to be not blocking. I tested this example on Debian 7.0 with open-mpi package. Thanks for your help
