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How to Send Pair with Open MPI

watysom profile image Watson ・2 min read

Hey guys.
I'm a beginner of Open MPI.
When I wanted to send some std::pair onjects from a node to another, I found out the fact that there are no Datatype of std::pair.
You can use only the datatypes in Open MPI as bellow.

MPI Datatype C++ Datatype
MPI::CHAR char
MPI::SHORT signed short
MPI::INT signed int
MPI::LONG signed long
MPI::LONG_LONG signed long long
MPI::SIGNED_CHAR signed char
MPI::UNSIGNED_CHAR unsigned char
MPI::UNSIGNED_SHORT unsigned short
MPI::UNSIGNED unsigned int
MPI::UNSIGNED_LONG unsigned long int
MPI::UNSIGNED_LONG_LONG unsigned long long
MPI::FLOAT float
MPI::DOUBLE double
MPI::LONG_DOUBLE long double
MPI::BOOL bool
MPI::COMPLEX Complex
MPI::DOUBLE_COMPLEX Complex
MPI::LONG_DOUBLE_COMPLEX Complex
MPI::WCHAR wchar_t
MPI::BYTE
MPI::PACKED

You can see only basic types and it's natural to do so because developers use variety of custom types and Open MPI can't prepare for all types of them.

I think the most flexible one is MPI::BYTE.
So, I used it to send std::pair.
You can easly calculate the total size of pairs in 1 byte increments by using the "sizeof" function.

Here is the example code.

#include "mpi.h"
#include <bits/stdc++.h>
using namespace std;
int main(int argc, char **argv) {
  int rank, numprocs;
  int namelen;
  vector<pair<int, int>> send_buf = {make_pair(0, 1), make_pair(2, 3)};
  vector<pair<int, int>> recv_buf(2);

  MPI_Init(&argc, &argv);
  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
  MPI_Comm_size(MPI_COMM_WORLD, &numprocs);
  if(rank == 0){
    MPI_Send((void*)send_buf.data(), sizeof(pair<int,int>)*2, MPI_BYTE, 1, 0, MPI_COMM_WORLD);
  }

  if(rank == 1){
     MPI_Recv((void*)recv_buf.data(), sizeof(pair<int,int>)*2, MPI_BYTE, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
     cout << recv_buf[0].first << ":" << recv_buf[0].second << endl;
     cout << recv_buf[1].first << ":" << recv_buf[1].second << endl;
  }
  MPI_Finalize();
  return 0;
}

Enjoy hacking!

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watysom profile

Watson

@watysom

Distributed System / Security / WEB

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