Hi, when I run the program from below on 1 node on BGQ (Vesta), the message is not received (flag is 0). However on a Ubuntu, the message is received (flag is non-zero). If I add another Iprobe (uncomment the Iprobe in the code below) the message is received on both BGQ and Ubuntu. Note that the program sleeps for 1 second after the Isend. Is it a bug? This happens for both MPICH-3.1.3 and MPICH-3.1. #include "mpi.h" #include <stdio.h> #include <unistd.h> int main(int argc, char **argv) { int send_int, recv_int, tag, flag; MPI_Status status; MPI_Request req; MPI_Init(&argc, &argv); tag = 0; send_int = 100; MPI_Isend(&send_int, 1, MPI_INT, 0, tag, MPI_COMM_WORLD, &req ); sleep(1); MPI_Iprobe(MPI_ANY_SOURCE , MPI_ANY_TAG, MPI_COMM_WORLD, &flag, &status ); //MPI_Iprobe(MPI_ANY_SOURCE , tag, MPI_COMM_WORLD, &flag, &status ); if (flag) { MPI_Recv( &recv_int, 1, MPI_INT, MPI_ANY_SOURCE, tag, MPI_COMM_WORLD, &status); printf("Received = %d\n", recv_int); } else printf("Message not received yet"); MPI_Waitall(1, &req, MPI_STATUSES_IGNORE); MPI_Finalize(); return 0; } Thanks Florin