mpirun actual number of processors used

Question

I am starting programming on an OpenMPI managed cluster. I use the following command to run my executable:

mpirun -np 32 file

Now what I understand is that 32 specifies the number of processes that should be created. They may be created on the same processor. Am I right?

I am noticing increasing time for execution with increase in the number of processes. Could the above be a reason for this?

How do I find out the execution and scheduling policy of the cluster?

Is it correct to assume that typically the cluster I am working on will have many processes running on each node just as they run on my PC.

Answer 1

I would expect your job management system (which is ?) to allocate 1 MPI process per core. But that is a configuration matter and your cluster may not be configured as I expect. Can you see what processes are running on the various cores of your cluster at run time ?

There are many explanations for increasing execution time with increasing numbers of processes, several good ones which include the possibility of one-process-per-core. But multiple processes per core is a potential explanation.

You find out about the policies of your cluster by asking the cluster administrator.

No, I think it is atypical for cluster processors (or cores) to execute multiple MPI processes simultaneously.