Installing and Running OpenMP with Python on macOS

In this tutorial, we will walk you through the process of installing OpenMP (Open Multi-Processing) for Python on macOS, specifically using the mpi4py library. We will also cover how to run a simple host-worker script and troubleshoot common issues encountered during the installation and execution process.

etietopdemas from frankincents.ai

Requirements:

• macOS (with M1 or M2 processor)
• Python 3.x installed
• Homebrew package manager installed

Steps:

1. Install Open MPI with Homebrew: To use mpi4py, we need to install the Open MPI library first. Open a terminal and run the following command:

brew install open-mpi

2. Install mpi4py using pip: Once the Open MPI library is installed, you can install the mpi4py library using pip:

pip install mpi4py

3. Prepare the host-worker script: Create two Python scripts, host.py and worker.py, to demonstrate a simple host-worker communication using mpi4py.

host.py

import sys
from mpi4py import MPI as mpi

if (mpi.COMM_WORLD.Get_size() != 1):
    if (mpi.COMM_WORLD.Get_rank() == 1):
        print("Must be only one Host!")
else:
    N = 3
    comm = mpi.COMM_WORLD.Spawn(sys.executable, args=['worker.py'], maxprocs=N)

    for i in range(N):
        message = comm.recv()
        print("Rank {}, message: {}".format(comm.Get_rank(), message))

    comm.Disconnect()

worker.py

from mpi4py import MPI as mpi

comm = mpi.Comm.Get_parent()
rank = comm.Get_rank()
host = 0

print("Host created {} worker!".format(rank))
comm.send(rank, dest=host)

comm.Disconnect()

4. Run the host-worker script: To execute the host-worker script using mpiexec, first, find the path of your Python interpreter using the following command:

which python3

Now, run the host-worker script using mpiexec:

mpiexec -n 1 /path/to/your/python3 host.py

Replace /path/to/your/python3 with the path obtained from the which python3 command.

5. Troubleshooting:

If you encounter issues during the installation or execution process, review the following tips:

• Make sure you are using the correct Python interpreter path when running mpiexec.
• Ensure that the Open MPI and mpi4py libraries are installed correctly.
• Verify that the host.py and worker.py scripts are placed in the correct directory and have the appropriate file permissions.
• If you encounter issues related to your M1 or M2 processor, try installing the libraries using Homebrew with Rosetta.
• If you encounter the “Error: open-mpi: Failed to download resource ‘gmp_bottle_manifest’” error, try installing Open MPI from source to ensure compatibility with the Apple Silicon processor:

brew install --build-from-source open-mpi

• To make sure mpi4py is installed for the correct Python version, try installing it directly using the Python interpreter you are using:

/opt/homebrew/bin/python3 -m pip install mpi4py

• Configuration issue or firewall blocking communication between MPI processes. During our setup, we encountered an issue where the communication between the host and worker processes was blocked. This problem could have been caused by a configuration issue or a firewall blocking the communication between MPI processes.

To resolve this, we set the mca parameter as follows:

mpiexec -n 1 --mca btl_tcp_if_include lo0 /opt/homebrew/bin/python3 host.py

This command includes the loopback interface (lo0) for communication, ensuring that the communication between MPI processes occurs correctly. It’s essential to note that this solution is for a single-node setup where all processes are running on the same machine. If you are working with a multi-node setup, you will need to configure the appropriate network interfaces.

Conclusion

In this tutorial, we have demonstrated how to install OpenMP with Python on macOS using the mpi4py library. We also covered how to run a simple host-worker script and troubleshoot common issues. With this knowledge, you can now harness the power of parallel processing in your Python projects!

Results from host-worker:

INFO:root:Host created 2 worker!
INFO:root:Host created 0 worker!
INFO:root:Host created 1 worker!
INFO:root:Rank 0, message: 0
INFO:root:Rank 0, message: 2
INFO:root:Rank 0, message: 1