🚀 Multi-Functional MPI Toolkit in C++

A command-line utility built in C++ that demonstrates several core concepts of the OpenMPI library for parallel and distributed computing. This single application can operate in different modes to showcase various message-passing techniques.

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✨ Features / Modes

This application can be run in one of the following modes:

• file_transfer: A two-process application to send a file from one host to another over the network.
• broadcast: A one-to-many communication demo where a root process sends data to all other processes.
• array_send: A point-to-point demo for sending a simple C++ array from one process to another.

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🌐 Network Architecture

This toolkit demonstrates true distributed computing across multiple physical machines:

• Process 0: Runs on Machine 1 (e.g., 192.168.100.101)
• Process 1: Runs on Machine 2 (e.g., 192.168.100.102)
• Communication: Real network communication using MPI over TCP/IP

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🚀 How to Compile and Run

1. Prerequisites

• A Linux environment.
• build-essential (for the g++ compiler).
• The OpenMPI library (sudo apt-get install libopenmpi-dev).

2. Compile the Application

Use the MPI C++ compiler wrapper (mpic++) to create a single executable from the unified source file:

mpic++ main.cpp -o mpi_app

3. Multi-Machine Network Setup (Optional)

For distributed execution across multiple physical machines:

Prerequisites for Multi-Machine Execution:

• Network Connectivity: All machines must be on the same network and able to communicate
• SSH Access: Passwordless SSH must be configured between all machines
• Synchronized Files: The compiled executable must be present on all machines in the same path
• MPI Installation: OpenMPI must be installed on all participating machines

Setting up Passwordless SSH:

# Generate SSH key (if not already done)
ssh-keygen -t rsa -b 2048

# Copy public key to remote machine
ssh-copy-id username@192.168.100.102

# Test passwordless connection
ssh username@192.168.100.102

Create Hostfile:

Create a hostfile.txt with the IP addresses of your machines:

192.168.100.101 slots=1
192.168.100.102 slots=1

• Each line represents one machine
• slots=1 indicates one process slot per machine
• Use actual IP addresses of your machines

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4. Execute a Mode

Run the compiled application using mpirun, specifying the number of processes (-np) and the desired --mode.

➤ File Transfer Mode (Requires 2 processes)

Demonstrates actual file transfer between separate machines over the network. Ensure a file named whatsapp.webp.gz exists on the sender machine.

# For local execution (single machine, 2 processes)
mpirun -np 2 ./mpi_app --mode file_transfer

# For multi-machine execution (2 separate machines)
mpirun -np 2 --hostfile hostfile.txt ./mpi_app --mode file_transfer

What happens: Process 0 (machine 1) reads a file and sends it over the network to Process 1 (machine 2), which saves it as whatsapp_received.webp.

➤ Broadcast Mode (Requires 2+ processes)

Demonstrates one-to-many communication across multiple machines.

# Local execution
mpirun -np 4 ./mpi_app --mode broadcast

# Multi-machine execution (requires 4+ machines or modify slots in hostfile)
mpirun -np 4 --hostfile hostfile.txt ./mpi_app --mode broadcast

What happens: Process 0 broadcasts a value to all other processes across the network.

➤ Array Send Mode (Requires 2 processes)

Demonstrates point-to-point array transmission between machines.

# Local execution
mpirun -np 2 ./mpi_app --mode array_send

# Multi-machine execution
mpirun -np 2 --hostfile hostfile.txt ./mpi_app --mode array_send

What happens: Process 0 sends an integer array over the network to Process 1, which receives and displays it.

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🔧 Troubleshooting

Common Issues:

• "Permission denied": Check SSH passwordless setup
• "Host unreachable": Verify network connectivity and IP addresses in hostfile
• "File not found": Ensure the input file exists on the sender machine
• "Connection refused": Check if MPI is installed on all machines
• Wrong process count: Each mode requires specific number of processes (see above)

Testing Network Setup:

# Test SSH connectivity
ssh username@192.168.100.102

# Test MPI installation on remote machine
ssh username@192.168.100.102 "mpirun --version"

# Ping test between machines
ping 192.168.100.102