MPICommunicators.c File Reference

Functions to create and destroy MPI subcommunicators. More...

#include <stdlib.h>
#include <arrayfunctions.h>
#include <mpivars.h>

Go to the source code of this file.

Functions
int	MPICreateCommunicators (int ndims, void *m)
int	MPIFreeCommunicators (int ndims, void *m)

Detailed Description

Functions to create and destroy MPI subcommunicators.

Author

Debojyoti Ghosh

Definition in file MPICommunicators.c.

Function Documentation

int MPICreateCommunicators	(	int	ndims,
		void *	m
	)

Create subcommunicators from MPI_WORLD, where each subcommunicator contains MPI ranks along a spatial dimension. Consider a two-dimensional problem, partitioned on 21 MPI ranks as follows:

This function will create 10 subcommunicators with the following ranks:

• 0,1,2,3,4,5,6
• 7,8,9,10,11,12,13
• 14,15,16,17,18,19,20
• 0,7,14
• 1,8,15
• 2,9,16
• 3,10,17
• 4,11,18
• 5,12,19
• 6,13,20

These subcommunicators are useful for parallel computations along grid lines. For example, a compact finite-difference scheme solves implicit systems along grid lines in every spatial dimension. Thus, the subcommunicator may be passed on to the parallel systems solver instead of MPI_WORLD.

Parameters

ndims	Number of spatial dimensions
m	MPI object of type MPIVariables

Definition at line 35 of file MPICommunicators.c.

{
  MPIVariables *mpi = (MPIVariables*) m;
#ifdef serial
  mpi->comm = NULL;
#else
  int          i,n,color,key;
  int          *ip,*iproc;

  mpi->comm = (MPI_Comm*) calloc (ndims, sizeof(MPI_Comm));
  if (ndims == 1) MPI_Comm_dup(mpi->world,mpi->comm);
  else {
    ip    = (int*) calloc (ndims-1,sizeof(int));
    iproc = (int*) calloc (ndims-1,sizeof(int));
    for (n=0; n<ndims; n++) {
      int tick=0; 
      for (i=0; i<ndims; i++) {
        if (i != n) {
          ip[tick]    = mpi->ip[i];
          iproc[tick] = mpi->iproc[i];
          tick++;
        }
      }
      _ArrayIndex1D_(ndims-1,iproc,ip,0,color); 
      key   = mpi->ip[n];
      MPI_Comm_split(mpi->world,color,key,&mpi->comm[n]);
    }
    free(ip);
    free(iproc);
  }
#endif
  return(0);
}

int MPIFreeCommunicators	(	int	ndims,
		void *	m
	)

Free the subcommunicators created in MPICreateCommunicators().

Parameters

ndims	Number of spatial dimensions
m	MPI object of type MPIVariables

Definition at line 75 of file MPICommunicators.c.

{
#ifndef serial
  MPIVariables *mpi = (MPIVariables*) m;
  int          n;
  for (n=0; n<ndims; n++) MPI_Comm_free(&mpi->comm[n]);
  free(mpi->comm);
  if (mpi->IOParticipant) MPI_Comm_free(&mpi->IOWorld);
  MPI_Comm_free(&mpi->world);
#endif
  return(0);
}