MPI Broadcasting dynamic 2D array to other processors

Question

I have searched many explanations, but I think I could not figure out how I can do this situation. I want to do something like that: Using master processor, I am creating a dynamic 2D array. Then I want to;

1- send this array to other processors. And each processor prints this 2D array 2- send part of this array to others. And each processor prints their part to the screen.

For example; I have 2D array 11*11 and 4 processors. rank 0 is master. Others are slaves. For the first situation, I want to send all array to rank 1, rank 2, and rank 3. And for the second situation, I want to share rows to slaves. 11/3 = 3. Therefore rank 1 takes 3 rows, rank 2 takes 3 rows, and rank 3 takes 5 rows.

Here is my code:

int processorID;  
int numberOfProcessors;  


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

    MPI_Init(&argc, &argv);  
    MPI_Comm_size(MPI_COMM_WORLD ,&numberOfProcessors);  
    MPI_Comm_rank(MPI_COMM_WORLD ,&processorID);

    double **array;

    if(MASTER){
        array = (double**) malloc(11*sizeof(double *));
        for(i=0; i<11; i++){
            array[i] =  (double *) malloc(11*sizeof(double));
        }

        for(i=0; i<11; i++){
            for(j=0; j<11; j++){
                array[i][j] = i*j;
            }

        }
    }
    MPI_Bcast(array, 11*11, MPI_DOUBLE, 0, MPI_COMM_WORLD);
    if(SLAVE){
        for(i=0; i<11; i++){
            for(j=0; j<11; j++){
                printf("%f ", array[i][j]);
            }

        }
    }

    MPI_Finalize();  
    return 0;  
}

according to these link ; MPI_Bcast a dynamic 2d array ; I need to create my array as;

if (MASTER){

    array = (double**) malloc(121*sizeof(double))

        for(i=0; i<11; i++){
            for(j=0; j<11; j++){
                array[i][j] = i*j;  // this is not working.
            }
        }
}

but if I do this, I cannot initialize each member in the array. Inner for loops are not working. I could not find any way to solve it.

And for my second question, I followed this link sending blocks of 2D array in C using MPI . I think I need to change the inside of if(SLAVE). I should create 2D subArrays for each slave processor. And I need to use MPI_Scatterv. But I could not understand completely.

int main() {

    ...
    ...

    MPI_Scatterv() // what must be here?
    if(SLAVE){
        if(processorID = numberOfProcessor-1){
            subArray = (double**) malloc(5*sizeof(double *)); // beacuse number of row for last processor is 5
            for(i=0; i<11; i++){
                array[i] =  (double *) malloc(11*sizeof(double));
            }
        }
        else {
            subArray = (double**) malloc(3*sizeof(double *));
            for(i=0; i<11; i++){
                array[i] =  (double *) malloc(11*sizeof(double));
            }
        }
    }
}

What is your exact compiler error message? – lethal-guitar Jan 12 '13 at 11:07 — lethal-guitar, Jan 12 '13 at 11:07

talonmies · Answer 1 · 2013-01-12T17:36:15.097

1

You can't use an array of pointers (what you are incorrectly calling a "2D array"), because each row pointer is not portable to another node's address space.

The code you quoted for creating a 2D array in a linear memory allocation is perfectly correct, all you need to do is index that memory in row major order, so that the loops become:

double* array = (double*) malloc(121*sizeof(double)); 
if (MASTER){
    for(i=0; i<11; i++){
        for(j=0; j<11; j++){
            array[j+i*11] = i*j;  // row major indexing here
        }
    }
}

/* Scatter code follows */

You can safely scatter this sort of array to multiple nodes.

edited Jan 12 '13 at 17:36

answered Jan 12 '13 at 11:10

talonmies

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But I think instead of double**, double* should take its place. Am I correct? – Burak Keceli Jan 12 '13 at 16:24
I did what you said. but now I am getting "process exited without calling finalize" error. Here is my new code. if(MASTER){ //thing you said } MPI_Bcast(array, 11*11, MPI_FLOAT, 0, MPI_COMM_WORLD); if(SLAVE){ //there is nothing here } MPI_Finalize(); return 0; } code finishes here. But I could not solve it – Burak Keceli Jan 12 '13 at 16:52
Yes sorry, I copied that from your answer without noticing it. See my edit – talonmies Jan 12 '13 at 17:36

score 1 · Accepted Answer · answered Jan 12 '13 at 11:11

1

C does not really have multidimensional arrays. I would recommend storing your values in a regular 1D buffer, and then calculating correct indices from 1D values. Like so:

double* data = (double*)malloc(sizeof(double)*11*11);

// Now, to access data[i][j], simply do:
data[j + i*11] = ...; // this "maps" 2D indices into 1D

This will spare you all the trouble with the hierarchical malloc-ing, and it can easily be passed to the MPI APIs.

answered Jan 12 '13 at 11:11

lethal-guitar

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I did what you said. but now I am getting "process exited without calling finalize" error. Here is my new code. if(MASTER){ //thing you said } MPI_Bcast(array, 11*11, MPI_FLOAT, 0, MPI_COMM_WORLD); if(SLAVE){ //there is nothing here } MPI_Finalize(); return 0; } code finishes here. But I could not solve it – Burak Keceli Jan 12 '13 at 17:04
You have to allocate memory for the array also on the slaves, otherwise they will segfault and exit early. So the allocation (but not filling the array) should come before the `if (MASTER)`-section. Maybe this is the problem? Otherwise, check the return value of MPI_Bcast for any error codes. – lethal-guitar Jan 12 '13 at 18:22

MPI Broadcasting dynamic 2D array to other processors

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