Mpi Spawn and Cray's Aprun

Question

Suppose I have a master program, which is basically a 1 rank mpi which uses MPI spawn to spawn 5 worker programs.

Now, if I execute my master using the following command

aprun -n 1 -N 1 master

The total number of ranks after spawning will be 6. But will all the 6 ranks be running on the same node? Is there anyway I can distribute the 6 among 3 nodes?

I can exactly one copy of the master process and 5 worker processes.

Accepted Answer

Cray MPI has not supported MPI_Comm_spawn until recently, and its solution to managing resources for spawned MPI jobs is unique. A place-holder job is launched using aprun to manage the resources used to host spawned jobs, i.e., the cores/nodes that will be hosting the spawned MPI ranks. The set of resources managed by the place-holder job is called a "rank pool", in analogy to a memory pool. Here's how you would set up and use a rank pool:

Must Read

rankpool.c

int main(int argc, char **argv) {
    MPI_Init(&argc, &argv);
    /* Name this rank pool "all_nodes", which will be
     * used by MPI_Comm_spawn to identify it. */
    MPIX_Comm_rankpool(MPI_COMM_WORLD, "all_nodes", /* 60 seconds timeout */ 60);
    MPI_Finalize();
}

Mpi Spawn and Cray's Aprun

1 Answer

Must Read

rankpool.c

Sophia Al-Mansoor