MPI Basic Usage

mpi_basic_usage.cc

#include <iostream>
#include <array>

#include <mpi.h>

using message_t = std::array<int, 3>;

void printMessage(const int rank, const message_t& message)
{
    std::cout << "Process " << rank << ": ";
    for (auto i : message) {
      std::cout << i << " ";
    }
    std::cout << std::endl;
}

int main(int argc, char** argv)
{
  // Initialize MPI; should come as early in the program as possible
  MPI_Init(&argc, &argv);

  // Get the number of processes in the global process group (communicator)
  int numProc;
  MPI_Comm_size(MPI_COMM_WORLD, &numProc);

  // Get the unique identifier of the current process inside the process group
  int myRank;
  MPI_Comm_rank(MPI_COMM_WORLD, &myRank);

  std::cout << "Hello! I am process " << myRank + 1
            << " of " << numProc
            << std::endl;

  MPI_Barrier(MPI_COMM_WORLD);

  // Blocking send/receive between processes 0 and 1
  using message_t = std::array<int, 3>;

  if (myRank == 0) {
    const int messageTag = 111;
    const int destinationRank = 1;
    const auto message = message_t{{1, 2, 3}};

    MPI_Send(message.data(), message.size(), MPI_INT, destinationRank,
             messageTag, MPI_COMM_WORLD);

    std::cout << "Process " << myRank << " sent message " << messageTag << std::endl;
  } else if (myRank == 1) {
    const int messageTag = 111;
    const int sourceRank = 0;
    message_t message;
    MPI_Status status;

    MPI_Recv(message.data(), message.size(), MPI_INT, sourceRank,
             messageTag, MPI_COMM_WORLD, &status);

    std::cout << "Process " << myRank << " received message " << messageTag << std::endl;
    printMessage(myRank, message);
  }


  // Non-blocking send/receive between processes 0 and 1
  if (myRank == 0) {
    const int messageTag = 112;
    const int destinationRank = 1;
    const auto message = message_t{{4, 5, 6}};
    MPI_Request request;

    // Begin send
    MPI_Isend(message.data(), message.size(), MPI_INT, destinationRank,
              messageTag, MPI_COMM_WORLD, &request);

    MPI_Status status;
    MPI_Wait(&request, &status); // This call ensures that sending the message has completed

    std::cout << "Process " << myRank << " sent message " << messageTag << std::endl;
  } else if (myRank == 1) {
    const int messageTag = 112;
    const int sourceRank = 0;
    message_t message;
    MPI_Request request;

    // Begin receive
    MPI_Irecv(message.data(), message.size(), MPI_INT, sourceRank,
              messageTag, MPI_COMM_WORLD, &request);

    MPI_Status status;
    MPI_Wait(&request, &status); // This call ensures that sending the message has completed

    std::cout << "Process " << myRank << " received message " << messageTag << std::endl;
    printMessage(myRank, message);
  }


  // Reduction on process 0: sum the rank (unique id) of all processes and print on process 0
  int result;
  MPI_Reduce(&myRank, &result, 1, MPI_INT, MPI_SUM, 0, MPI_COMM_WORLD);

  if (myRank == 0)
  {
    // Result should be (numProc - 1) * numProc / 2
    std::cout << "Process " << myRank << " obtained result: " << result << std::endl;
  }


  // Shutdown MPI; ideally, it's the last thing before exiting
  MPI_Finalize();

  return 0;
}