# 10.5. Launching only on the local node

It is common to develop MPI applications on a single workstation or laptop, and then move to a larger parallel / HPC environment once the MPI application is ready.

Open MPI supports running multi-process MPI jobs on a single machine. In such cases, you can simply avoid listing a hostfile or remote hosts, and simply list a number of MPI processes to launch. For example:

    shell$ mpirun -n 6 mpi-hello-world
    Hello world, I am 0 of 6 (running on my-laptop))
    Hello world, I am 1 of 6 (running on my-laptop)
    ...
    Hello world, I am 5 of 6 (running on my-laptop)

If you do not specify the `-n` option, `mpirun` will default to launching as many MPI processes as there are processor cores (not hyperthreads) on the machine.

## 10.5.1. MPI communication

When running on a single machine, Open MPI will most likely use the `ob1` PML and the following BTLs for MPI communication between peers:

- `self`: used for sending and receiving loopback MPI messages — where the source and destination MPI process are the same.

- `sm`: used for sending and receiving MPI messages where the source and destination MPI processes can share memory (e.g., via SYSV or POSIX shared memory mechanisms).

  Note

  For more information about using shared memory MPI communication, see the Shared Memory page.