MPI_Type_create_subarray and MPI_Send

Question

this is my first question here in stackoverflow. I have two processes, a root 0 and a slave 1. Slave allocates a 2d array (CHUNK_ROWS+2,CHUNK_COLUMNS+2) and wants to send a [CHUNK_ROWS][CHUNK_COLUMNS] subarray. Root allocates a 2d array (ROWS,COLUMNS) and receives the subarray that wants to store from ddd[0][0] and print it. The result I get it's wrong..Why? I know this is a non sense code but it's only a simple program that should help me in more complex project. Here the code:

#include <mpi.h>
#include <iostream>
using namespace std;

#define ROWS 10
#define COLUMNS 10
#define CHUNK_ROWS 5
#define CHUNK_COLUMNS 5
#define TAG 0

int** alloca_matrice(int righe, int colonne)
{
int** matrice=NULL;
int i;

// per allocare la matrice devo fare in questo modo al fine di avere le righe contigue in memoria
// e poter così utilizzare il tipo 'colonna' che definisco con MPI_Type_Vector()
matrice = (int **)malloc(righe * sizeof(int*));

if(matrice != NULL){
    matrice[0] = (int *)malloc(righe*colonne*sizeof(int));
    if(matrice[0]!=NULL)
        for(i=1; i<righe; i++)
            matrice[i] = matrice[0]+i*colonne;
    else{
        free(matrice);
        matrice = NULL;
    }
}
else{
    matrice = NULL;
}
return matrice;
}


int main(int argc, char* argv[])
{

int my_id, numprocs,length,i,j;
int ndims, sizes[2],subsizes[2],starts[2];
int** DEBUG_CH;
int** ddd;
char name[BUFSIZ];
MPI_Datatype subarray;
MPI_Status status;
MPI_Init(&argc, &argv) ;                    // Chiamata obbligatoria di inizializzazione    
MPI_Comm_rank(MPI_COMM_WORLD, &my_id) ;     // Ottiene l'identificativo del processo
MPI_Comm_size(MPI_COMM_WORLD, &numprocs) ;  // Ottiene quanti processi sono attivi
MPI_Get_processor_name(name, &length);      // Il nome del nodo dove il processo ? in esecuzione



if(my_id==1){
    //creo una sottomatrice ripulita dalle ghost cells
    ndims=2;
    sizes[0] = CHUNK_ROWS+2;
    sizes[1] = CHUNK_COLUMNS+2;
    subsizes[0] = CHUNK_ROWS;
    subsizes[1] = CHUNK_COLUMNS;
    starts[0] = 1;
    starts[1] = 1;
    MPI_Type_create_subarray(ndims,sizes,subsizes,starts,MPI_ORDER_C,MPI_INT,&subarray);
    MPI_Type_commit(&subarray);

    DEBUG_CH = alloca_matrice(CHUNK_ROWS+2,CHUNK_COLUMNS+2);
    for(i=0; i<CHUNK_ROWS+2; i++){
        for(j=0; j<CHUNK_COLUMNS+2; j++){
            if(i==0 || i==CHUNK_ROWS+1 || j==0 || j==CHUNK_COLUMNS+1)
                DEBUG_CH[i][j] = 5;
            else
                DEBUG_CH[i][j] = 1;
        }
    }
    MPI_Send(DEBUG_CH,1,subarray,0,TAG,MPI_COMM_WORLD);
    MPI_Type_free(&subarray);
}
if(my_id==0){
    //creo una sottomatrice ripulita dalle ghost cells
    ndims=2;
    sizes[0] = ROWS;
    sizes[1] = COLUMNS;
    subsizes[0] = CHUNK_ROWS;
    subsizes[1] = CHUNK_COLUMNS;
    starts[0] = 0;
    starts[1] = 0;
    MPI_Type_create_subarray(ndims,sizes,subsizes,starts,MPI_ORDER_C,MPI_INT,&subarray);
    MPI_Type_commit(&subarray);

    ddd = alloca_matrice(ROWS,COLUMNS);
    MPI_Recv(ddd[0],1,subarray,1,TAG,MPI_COMM_WORLD,&status);
    MPI_Type_free(&subarray);
    for(i=0; i<CHUNK_ROWS; i++){
        for(j=0; j<CHUNK_COLUMNS; j++){
            printf("%d ",ddd[i][j]);
        }
        printf("\n");
    }
}
MPI_Finalize();                             // Chiusura di MPI.
return 0;
}

Thanks in advance.

Jonathan Dursi · Accepted Answer · 2011-04-07 16:09:13Z

2

Congratulazioni, your code is almost perfect, there's just one silly mistake in the MPI_Send which you got right in the MPI_Recv.

For the Send, you have

MPI_Send(DEBUG_CH,1,subarray,0,TAG,MPI_COMM_WORLD);

whereas for the Recv, you have

MPI_Recv(ddd[0],1,subarray,1,TAG,MPI_COMM_WORLD,&status);

The second is right. You need to send a pointer to where the data starts; that's not DEBUG_CH, which is a pointer to a pointer to ints, but &(DEBUG_CH[0][0]), or equivalently, DEBUG_CH[0], as you did with ddd.

answered Apr 7, 2011 at 16:09

Jonathan Dursi

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2 Comments

Riff Over a year ago

Thanks Jonathan. It works! Now I've edited my question with a similar problem. Maybe this is not the proper way to use stackoverflow. I'm trying to learn.

Jonathan Dursi Over a year ago

It would probably be better to make it a 2nd post, as it's a substantially different question now.

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