Hi, I configured two virtual machines on Amazon EC2 to run mpich-3.2. The system is Ubuntu 12.04.2 LTS. The two virtual machines can ssh to each other successfully (passwordless) and I can run a simple hello world program using the two machines. ubuntu@ip-10-169-125-85:~$ mpiexec -n 2 -f host_file ./hello_world Hello world from processor ip-10-169-125-85, rank 1 out of 2 processors Hello world from processor ip-10-235-37-156, rank 0 out of 2 processors Then I run a simple program with MPI_Send and MPI_Receive to communicate between the two vms. Following are the core code of the program. if (world_rank == 0) { // If we are rank 0, set the number to -1 and send it to process 1 number = -1; MPI_Send(&number, 1, MPI_INT, 1, 0, MPI_COMM_WORLD); } else if (world_rank == 1) { MPI_Recv(&number, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE); printf("Process 1 received number %d from process 0\n", number); } Following are the error msg I encountered. ubuntu@ip-10-169-125-85:~$ mpiexec -n 2 -f host_file ./send_recv Fatal error in MPI_Send: Unknown error class, error stack: MPI_Send(174)..............: MPI_Send(buf=0x7fff49f2759c, count=1, MPI_INT, dest=1, tag=0, MPI_COMM_WORLD) failed MPID_nem_tcp_connpoll(1835): Communication error with rank 1: Connection timed out I googled similar errors and have made sure that: 1) there is no rule in my firewall setting, 2) there is a tcp port listening on both sides when the send_recv program runs. I cannot think of any other possible way to fix this problem. BTW, the two virtual machines are on two different regions of Amazon EC2 and are not in VPCs. Please help. Thanks! Regards, Amelie _______________________________________________ discuss mailing list [email protected] To manage subscription options or unsubscribe: https://lists.mpich.org/mailman/listinfo/discuss