MPI Send and Receive

The main concept in MPI is sending messages between processes. Two basic functions: MPI_Send() and MPI_Recv() provide this functionality. You provide a buffer that contains the data you want to send / receive.

  • MPI_Send() and MPI_Recv() are a blocking send/receive.

  • For the sending code, the program resumes when it is safe to reuse the buffer.

  • For the receiving code, the program resumes when the message was received.

This may cause network contention if the destination process is busy doing its own communication. There are non-blocking sends that we can use in this case.

Basic example

Here’s a basic example that mimics the ghost cell filling process used in hydrodynamics codes.

  1. On each processor we allocate an integer vector of 5 elements

  2. We fill the middle 3 element with a sequence based on the processor number. Processor 0 gets 0, 1, 2, processor 1 gets 3, 4, 5, and so on.

  3. We send messages to the left and right element with the corresponding element from the neighboring processors.

This looks like:

_images/send_recv_diagram.png

Fig. 19 Sharing data at the boundary of our vectors

Here’s the implementation:

Listing 107 sendrecv_pairs.cpp
#include <iostream>
#include <vector>

#include <mpi.h>

int main() {

    MPI_Init(nullptr, nullptr);

    std::vector<int> data(5, 0);

    int mype{-1};
    MPI_Comm_rank(MPI_COMM_WORLD, &mype);

    int num_procs{0};
    MPI_Comm_size(MPI_COMM_WORLD, &num_procs);

    // fill the interior 3 data elements

    data[1] = 3 * mype;
    data[2] = 3 * mype + 1;
    data[3] = 3 * mype + 2;

    // send to the left

    if (mype > 0) {
        MPI_Send(&data[1], 1, MPI_INT, mype-1, mype, MPI_COMM_WORLD);
    }

    MPI_Status status;
    if (mype <  num_procs-1) {
        MPI_Recv(&data[4], 1, MPI_INT, mype+1, mype+1, MPI_COMM_WORLD, &status);
    }

    // send to the right

    if (mype < num_procs-1) {
        MPI_Send(&data[3], 1, MPI_INT, mype+1, mype, MPI_COMM_WORLD);
    }

    if (mype > 0) {
        MPI_Recv(&data[0], 1, MPI_INT, mype-1, mype-1, MPI_COMM_WORLD, &status);
    }

    std::cout << "PE: " << mype << " data: ";
    for (int i = 0; i < data.size(); ++i) {
        std::cout << data[i] << " ";
    }
    std::cout << std::endl;

    MPI_Finalize();
}

Notice that at the two ends, we don’t send anything in one case (for processor 0, there is no left, and for processor \(N-1\), there is no right).

MPI_Sendrecv

Good communication performance often requires staggering the communication. A combined MPI_Sendrecv can help with this. We can use MPI_PROC_NULL when there is no destination.

Here’s the same example as above, but now using this single funciton.

Listing 108 sendrecv_single.cpp
#include <iostream>
#include <vector>

#include <mpi.h>

int main() {

    MPI_Init(nullptr, nullptr);

    std::vector<int> data(5, 0);

    int mype{-1};
    MPI_Comm_rank(MPI_COMM_WORLD, &mype);

    int num_procs{0};
    MPI_Comm_size(MPI_COMM_WORLD, &num_procs);

    // fill the interior 3 data elements

    data[1] = 3 * mype;
    data[2] = 3 * mype + 1;
    data[3] = 3 * mype + 2;

    // send to the left and receive from the right
    //
    // send:    mype -> mype - 1
    // receive: mype <- mype + 1

    int sendto{-1};
    int recvfrom{-1};

    if (mype == 0) {
        sendto = MPI_PROC_NULL;
    } else {
        sendto = mype-1;
    }

    if (mype == num_procs-1) {
        recvfrom = MPI_PROC_NULL;
    } else {
        recvfrom = mype+1;
    }

    MPI_Status status;

    MPI_Sendrecv(&data[1], 1, MPI_INT, sendto, 0,
                 &data[4], 1, MPI_INT, recvfrom, 0,
                 MPI_COMM_WORLD, &status);


    // send to the right and receive from the left
    //
    // send:    mype -> mype+1
    // receive: mype <- mype-1

    if (mype == num_procs-1) {
        sendto = MPI_PROC_NULL;
    } else {
        sendto = mype+1;
    }

    if (mype == 0) {
        recvfrom = MPI_PROC_NULL;
    } else {
        recvfrom = mype-1;
    }

    MPI_Sendrecv(&data[3], 1, MPI_INT, sendto, 1,
                 &data[0], 1, MPI_INT, recvfrom, 1,
                 MPI_COMM_WORLD, &status);

    std::cout << "PE: " << mype << " data: ";
    for (int i = 0; i < data.size(); ++i) {
        std::cout << data[i] << " ";
    }
    std::cout << std::endl;

    MPI_Finalize();
}