Question

I developed this simple function to perform the sum of n number in parallel environment with mpi, using the strategy in which each processor calculates its partial sum and then at pairwise distinct the processor communicate between them the amounts calculated , creating a binary tree. Of course, this strategy can be applied if the numbers of processor is multiple of 2^n.

Parameters: menum = id processor, nproc = total numbers of processor , sum = partial sum

  void Second_Strategy( int menum ,int nproc,int sum, MPI_Status status)
  {
    int a,b,p,i,sumtmp;
    double t_tot, t1, t2, diff;

    p=log(nproc)/log(2);
    t1 = MPI_Wtime();

    for(i=1;i<=p;i++)
    {
       b=pow(2,i-1);
       a=pow(2,i);
       if ( (menum % a) ==0) 
       {
          MPI_Recv(&sumtmp,1,MPI_INT,(menum+b),i,MPI_COMM_WORLD,&status);
          sum=sum+sumtmp;

        }else{
               if ( ( menum % b) ==0){MPI_Send(&sum,1,MPI_INT,(menum-b),i,MPI_COMM_WORLD); }

            }
      }
        t2 = MPI_Wtime();
        diff = t2-t1;
        MPI_Reduce(&diff, &t_tot, 1, MPI_DOUBLE, MPI_MAX, 0, MPI_COMM_WORLD);

        if(menum==0) 
        {

          printf("The sum is : %d \n ",sum);
          printf("Time  : %f seconds\n", t_tot);

        }      

 }

In this case the process P0 will the result of the sum. But in the case in which the user is to select the processor that will have the sum introducing a parameter “choice”. How to do? I have problem with the creation of the binary tree for the exchange of data between processes.

Answer 1

The most straightforward solution would be to create a new communicator in which the process chosen by the user receives rank 0. Then simply use that communicator instead of MPI_COMM_WORLD in your tree-building algorithm.

I am assuming of course that you’re doing this for an exercise in parallel communications rather than the purpose of summing numbers, since obtaining a sum can be easily done with MPI_Reduce().