The Message Passing Interface (MPI) is a standard api for communication on parallel computing architectures. ABLATE uses MPI for its primary communication layer on large scale computing systems. Follow the Introduction to Parallel Computing Tutorial for an overview of parallel computing. With CLion you can debug small mpi cases. Follow the CMake guide on day 16 to link in PETSc and mpi. Create a new executable and including the following code.

#include <mpi.h>
#include <iostream>
#include <chrono>
#include <thread>

int main(int argc, char **argv) {
    // sleep the code to allow the debugger to attach
    // std::this_thread::sleep_for(std::chrono::milliseconds(20000));

    // Initialize the MPI environment
    MPI_Init(NULL, NULL);

    // Determine the number of processes
    int size;
    MPI_Comm_size(MPI_COMM_WORLD, &size);

    // Get the rank of this process
    int rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &rank);

    // Get the name of this processor
    char name[MPI_MAX_PROCESSOR_NAME];
    int nameSize;
    MPI_Get_processor_name(name, &nameSize);

    // Report from this
    std::cout << "Hello from " << rank << "/" << size << " on " << name << std::endl;

    // Cleanup mpi
    MPI_Finalize();
}

Use the following steps to debug mpi processes (shown in the attached video) in CLion.

1. Build and run this program in CLion as usual. It should create report a single process.
2. Inside the terminal in CLion ‘cd’ into the cmake-build-debug (or other cmake-build-* directory).
3. Run the day 18 executable with mpiexec. It should report 3 processes.

   mpiexec -n 3 ./day18
   

4. Inorder to debug in CLion you need to make the code wait until the debugger is attached. Comment in the std::this_thread::sleep_for(std::chrono::milliseconds(20000)); line and place a break point on the MPI_Init line. Rebuild the executable.
5. Run the day 18 executable with mpiexec as before. Immediately go to Run > Attach to Proccess... and select day18

Goals

1. Follow the Introduction to Parallel Computing Tutorial
2. Compile and debug the MPI example in CLion
3. Commit these activities and push to your private CodingAblate Repo.