Help with launching MPI application

[frizwi]

Please submit all the information below so that we can understand the working environment that is the context for your question.

• If you have a problem building or installing Open MPI, be sure to read this.
• If you have a problem launching MPI or OpenSHMEM applications, be sure to read this.
• If you have a problem running MPI or OpenSHMEM applications (i.e., after launching them), be sure to read this.

Background information

What version of Open MPI are you using? (e.g., v4.1.6, v5.0.1, git branch name and hash, etc.)

OpenMPI 4.0.7

Describe how Open MPI was installed (e.g., from a source/distribution tarball, from a git clone, from an operating system distribution package, etc.)

HPC system running PBS Torque Pro

Linux system

Details of the problem

I'm trying to figure out how to launch an MPI application that binds the first process/rank to 2 cores but 1 each to the rest.
mpirun -np 97 ... is how I usually do this and it works fine except the mpi_rank==0 launches a separate pthread which does file i/o operations. So I would really like 2 x dedicated cores (1 ea.) for the first process only, 1 x for the master thread and 1 for the child thread. The rest of the processes can have 1 core ea.

Thanks!