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Subject: [OMPI users] issue with column type in language C
From: Christian Perrier (christian01.perrier_at_[hidden])
Date: 2012-08-19 00:59:02


I have a problem with MPI_Senrecv communication where I send columns on
edges between processes.
For debugging, I show you below a basic example where I initialize a 10x10
matrix ("x0" array) with x_domain=4
and y_domain=4. For the test, I simply initialize the 2D array values with
x0[i][j] = i+j . After, in updateBound.c", I'm
using the MPI_Sendrecv functions for the North-South and Est-West process.

Here's the main program "example.c" :


#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include "mpi.h"

int main(int argc, char *argv[])
      /* size of the discretization */

      double** x;
      double** x0;

      int i,j,k,l;
      int nproc;
      int ndims;
      int S=0, E=1, N=2, W=3;
      int NeighBor[4];
      int xcell, ycell, size_tot_x, size_tot_y;
      int *xs,*ys,*xe,*ye;
      int size_x = 4;
      int size_y = 4;
      int me;
      int x_domains=2;
      int y_domains=2;

      MPI_Comm comm, comm2d;
      int dims[2];
      int periods[2];
      int reorganisation = 0;
      int row;
      MPI_Datatype column_type;



      MPI_Init(&argc, &argv);
      comm = MPI_COMM_WORLD;

      x = malloc(size_tot_y*sizeof(double*));
      x0 = malloc(size_tot_y*sizeof(double*));

      for(j=0;j<=size_tot_y-1;j++) {
        x[j] = malloc(size_tot_x*sizeof(double));
        x0[j] = malloc(size_tot_x*sizeof(double));

      xs = malloc(nproc*sizeof(int));
      xe = malloc(nproc*sizeof(int));
      ys = malloc(nproc*sizeof(int));
      ye = malloc(nproc*sizeof(int));

      /* Create 2D cartesian grid */
      periods[0] = 0;
      periods[1] = 0;

      ndims = 2;

      MPI_Cart_create(comm, ndims, dims, periods, reorganisation, &comm2d);

      /* Identify neighbors */

      NeighBor[0] = MPI_PROC_NULL;
      NeighBor[1] = MPI_PROC_NULL;
      NeighBor[2] = MPI_PROC_NULL;
      NeighBor[3] = MPI_PROC_NULL;

      /* Left/West and right/Est neigbors */


      /* Bottom/South and Upper/North neigbors */


      /* coordinates of current cell with me rank */





         { for(j=0;j<=(y_domains-1);j++)

          { for(j=0;j<=size_tot_x-1;j++)
            { x0[i][j]= i+j;
        // printf("%f\n",x0[i][j]);

      /* Create column data type to communicate with South and North
neighbors */

      MPI_Type_vector( ycell, 1, size_tot_x, MPI_DOUBLE, &column_type);

      updateBound(x0, NeighBor, comm2d, column_type, me, xs, ys, xe, ye,





        return 0;

and the second file "updateBound.c" which sends the columns and rows


#include "mpi.h"
#include <stdio.h>

/* Update Bounds of subdomain with me process */

  void updateBound(double** x,int NeighBor[], MPI_Comm comm2d, MPI_Datatype
column_type , int me, int* xs, int* ys, int* xe, int* ye, int xcell)

  int S=0, E=1, N=2, W=3;
  int flag;
  MPI_Status status;

  int i,j;

         if(me==0) {printf("verif_update_before\n");
                    { for(j=xs[me]-1;j<=xe[me]+1;j++)
                      { printf("%f ",x[i][j]);

/********* North/South communication **********************************/
  flag = 1;
  /*Send my boundary to North and receive from South*/
  MPI_Sendrecv(&x[ys[me]][xs[me]], xcell, MPI_DOUBLE, NeighBor[N], flag,
&x[ye[me]+1][xs[me]], xcell, MPI_DOUBLE, NeighBor[S], flag, comm2d,

  /*Send my boundary to South and receive from North*/
  MPI_Sendrecv(&x[ye[me]][xs[me]], xcell, MPI_DOUBLE, NeighBor[S], flag,
&x[ys[me]-1][xs[me]], xcell, MPI_DOUBLE, NeighBor[N], flag, comm2d,

/********* Est/West communication ************************************/
  flag = 2;
  /*Send my boundary to Est and receive from West*/
  MPI_Sendrecv(&x[ys[me]][xe[me]], 1, column_type, NeighBor[E], flag,
&x[ys[me]][xs[me]-1], 1, column_type, NeighBor[W], flag, comm2d, &status);

  /*Send my boundary to West and receive from Est*/
  MPI_Sendrecv(&x[ys[me]][xs[me]], 1, column_type, NeighBor[W], flag,
&x[ys[me]][xe[me]+1], 1, column_type, NeighBor[E], flag, comm2d, &status);

         if(me==0) {printf("verif_update_after\n");
                    { for(j=xs[me]-1;j<=xe[me]+1;j++)
                      { printf("%f ",x[i][j]);


Running with nproc=4, I print the values of the subarray with rank=0 (so at
left bottom of the grid) and I get before and after the
bounds udpate :

6.000000 7.000000 8.000000 9.000000
7.000000 8.000000 9.000000 10.000000
8.000000 9.000000 10.000000 11.000000
9.000000 10.000000 11.000000 12.000000

6.000000 5.000000 6.000000 9.000000
7.000000 8.000000 9.000000 12.000000
8.000000 9.000000 10.000000 *11.000000 *
9.000000 10.000000 11.000000 12.000000

As you can see, after the udpate, I don't have the correct value ( in
underligned bold : 11.0 ) at the second element
of the column coming from the Est. I expected 13.0 instead of 11.0.

So there's a problem with the column datatype which only send the first
element of this column.

In "example.c", I define the column as following :

      MPI_Type_vector( ycell, 1, size_tot_x, MPI_DOUBLE, &column_type);

However, It seems ok and the computation of begin and end coordinates as a
function of rank "me" is also good.

I make you notice there's no problem between the exchange of rows between
the North and the South, only
between columns.

If you could help me, I don't know what to do.