Hi All,
 
I'm having this weird problem when running a very simple OpenMPI application. The application sends an integer from the rank 0 process to the rank 1 process. The sequence of code that I use to accomplish this is the following:
        if (rank == 0)
        {
                printf("Process %d - Sending...\n", rank);
                MPI_Send(&sent, 1, MPI_INT, 1, 1, MPI_COMM_WORLD);
                printf("Process %d - Sent.\n", rank);
        }
        if (rank == 1)
        {
                 printf("Process %d - Receiving...\n", rank);
                MPI_Recv(&received, 1, MPI_INT, 0, 1, MPI_COMM_WORLD, &stat);
                printf("Process %d - Received.\n", rank");
        }
 
        printf("Process %d - Barrier reached.\n", rank);
        MPI_Barrier(MPI_COMM_WORLD);
        printf("Process %d - Barrier passed.\n", rank");
 
Like I said, a very simple program.
When launching this application with SLURM (using "salloc -N2 mpirun ./<my_app>"), it hangs at the barrier. However, it passes the barrier if I launch it without SLURM (using "mpirun -np 2 ./<my_app>"). I first noticed this problem when my application hanged if I tried to send two successive messages from a process to another. Only the first MPI_Send would work. The second MPI_Send would block indefinitely. I was wondering whether any of you have encountered a similar problem, or may have an ideea as to what is causing the Send/Receive pair to block when using SLURM. The exact output in my console is as follows:
 
        salloc: Granted job allocation 1138
        Process 0 - Sending...
        Process 1 - Receiving...
        Process 1 - Received.
        Process 1 - Barrier reached.
        Process 0 - Sent.
        Process 0 - Barrier reached.
        (it just hangs here)
 
I am new to MPI programming and to OpenMPI and would greatly appreciate any help. My OpenMPI version is 1.4.4 (although I have also tried it on 1.5.4), my SLURM version is 0.3.3-1 (slurm-llnl 2.1.0-1), the operating system on the cluster on which I tried to run my application is Ubuntu 10.04 LTS Server x64. If anyone is willing to help me out, I will happily provide any other info requested (as long as the request comes with instructions on how to get that info).
 
Your answers will be of great help! Thanks!
 
Adrian