How to determine MPI rank/process number local to a socket/node

Question

Say, I run a parallel program using MPI. Execution command

mpirun -n 8 -npernode 2 <prg>

launches 8 processes in total. That is 2 processes per node and 4 nodes in total. (OpenMPI 1.5). Where a node comprises 1 CPU (dual core) and network interconnect between nodes is InfiniBand.

Now, the rank number (or process number) can be determined with

int myrank;
MPI_Comm_rank(MPI_COMM_WORLD, &myrank);

This returns a number between 0 and 7.

But, How can I determine the node number (in this case a number between 0 and 3) and the process number within a node (number between 0 and 1)?

Answer 1

I believe you can achieve that with MPI-3 in this manner:

MPI_Comm shmcomm;
MPI_Comm_split_type(MPI_COMM_WORLD, MPI_COMM_TYPE_SHARED, 0,
                    MPI_INFO_NULL, &shmcomm);
int shmrank;
MPI_Comm_rank(shmcomm, &shmrank);

Answer 2

It depends on the MPI implementation - and there is no standard for this particular problem.

Open MPI has some environment variables that can help. OMPI_COMM_WORLD_LOCAL_RANK will give you the local rank within a node - ie. this is the process number which you are looking for. A call to getenv will therefore answer your problem - but this is not portable to other MPI implementations.

See this for the (short) list of variables in OpenMPI.

I don't know of a corresponding "node number".