c++ - Dynamic Memory Allocation in MPI

Question

I am new to MPI. I wrote a simple code to display a matrix using multiple process. Say if I have a matrix of 8x8 and launching the MPI program with 4 processes, the 1st 2 rows will be printed my 1st process the 2nd set of 2 rows will be printed by 2nd thread so on by dividing itself equally.

#define S 8

MPI_Status status;

int main(int argc, char *argv[])
{
int numtasks, taskid;
int i, j, k = 0;

MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &taskid);
MPI_Comm_size(MPI_COMM_WORLD, &numtasks);

int rows, offset, remainPart, orginalRows, height, width;
int **a;
//  int a[S][S];

if(taskid == 0)
{
    cout<<taskid<<endl;
    height = width = S;

    a = (int **)malloc(height*sizeof(int *));
    for(i=0; i<height; i++)
        a[i] =  (int *)malloc(width*sizeof(int));

    for(i=0; i<S; i++)
        for(j=0; j<S; j++)
            a[i][j] = ++k;

    rows = S/numtasks;
    offset = rows;
    remainPart = S%numtasks;

    cout<<"Num Rows : "<<rows<<endl;

    for(i=1; i<numtasks; i++)
        if(remainPart > 0)
        {
            orginalRows = rows;
            rows++;
            remainPart--;

            MPI_Send(&offset, 1, MPI_INT, i, 1, MPI_COMM_WORLD);
            MPI_Send(&rows, 1, MPI_INT, i, 1, MPI_COMM_WORLD);
            MPI_Send(&width, 1, MPI_INT, i, 1, MPI_COMM_WORLD);
            MPI_Send(&a[offset][0], rows*S, MPI_INT,i,1, MPI_COMM_WORLD);

            offset += rows;
            rows = orginalRows;
        }
        else
        {
            MPI_Send(&offset, 1, MPI_INT, i, 1, MPI_COMM_WORLD);
            MPI_Send(&rows, 1, MPI_INT, i, 1, MPI_COMM_WORLD);
            MPI_Send(&width, 1, MPI_INT, i, 1, MPI_COMM_WORLD);
            MPI_Send(&a[offset][0], rows*S, MPI_INT,i,1, MPI_COMM_WORLD);

            offset += rows;
        }

        //Processing
        rows = S/numtasks;
        for(i=0; i<rows; i++)
        {
            for(j=0; j<width; j++)
                cout<<a[i][j]<<"";
            cout<<endl;
        }
}else
{
    cout<<taskid<<endl;

    MPI_Recv(&offset, 1, MPI_INT, 0, 1, MPI_COMM_WORLD, &status);
    MPI_Recv(&rows, 1, MPI_INT, 0, 1, MPI_COMM_WORLD, &status);
    MPI_Recv(&width, 1, MPI_INT, 0, 1, MPI_COMM_WORLD, &status);
    a = (int **)malloc(rows*sizeof(int *));
    for(i=0; i<rows; i++)
        a[i] =  (int *)malloc(width*sizeof(int));
    MPI_Recv(&a, rows*width, MPI_INT, 0, 1, MPI_COMM_WORLD, &status);
    cout<<"Offset : "<<offset<<"
Rows : "<<rows<<"
Width : "<<width<<endl;

    for(i=0; i<rows; i++)
    {
        for(j=0; j<width; j++)
            cout<<a[i][j]<<"";
        cout<<endl;
    }
}

getch();
MPI_Finalize();

return 0;
}

This is my complete code, here I have allocated the memory dynamically for 'a', while printing a[i][j], under the else part, I am getting runtime error. If I change the dynamic memory allocation to static like changing int **a to int a[N][N] and removing

    a = (int **)malloc(rows*sizeof(int));
    for(i=0; i<rows; i++)
        a[i] =  (int *)malloc(width*sizeof(int));

it works perfectly.

Answer 1

There are at least two ways to dynamically allocate a 2D array.

The first one is the one of @HRoid : each row is allocated one at a time. Look here for getting an scheme.

The second one is suggested by @Claris, and it will ensure that the data is contiguous in memory. This is required by many MPI operations...it is also required by libraries like FFTW (2D fast fourier transform) or Lapack (dense matrices for linear algebra). Your program may fail at

MPI_Send(&a[offset][0], rows*S, MPI_INT,i,1, MPI_COMM_WORLD);

if S>1, this program will try to send items that are after the end of the line n°offset...That may trigger a segmentation fault or undefined behavior.

You may allocate your array this way :

a = malloc(rows * sizeof(int *));
if(a==NULL){fprintf(stderr,"out of memory...i will fail
");}
int *t = malloc(rows * width * sizeof(int));
if(t==NULL){fprintf(stderr,"out of memory...i will fail
");}
for(i = 0; i < rows; ++i)
  a[i] = &t[i * width];

Watch out : malloc does not initialize memory to 0 !

It seems that you want to spread a 2D array over many process. Look at MPI_Scatterv() here. Look at this question too.

If you want to know more about 2D arrays and MPI, look here.

You may find a basic example of MPI_Scatterv here.

I changed #define S 8 for #define SQUARE_SIZE 42. It's always better to give descriptive names.

And here is a working code using MPI_Scatterv() !

#include <mpi.h>
#include <iostream> 
#include <cstdlib>

using namespace std;

#define SQUARE_SIZE 42

MPI_Status status;

int main(int argc, char *argv[])
{
    int numtasks, taskid;
    int i, j, k = 0;

    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &taskid);
    MPI_Comm_size(MPI_COMM_WORLD, &numtasks);

    int rows, offset, remainPart, orginalRows, height, width;
    int **a;

    height = width = SQUARE_SIZE;

    //on rank 0, let's build a big mat of int
    if(taskid == 0){ 
        a=new int*[height]; 
        int *t =new int[height * width];
        for(i = 0; i < height; ++i)
            a[i] = &t[i * width];
        for(i=0; i<height; i++)
            for(j=0; j<width; j++)
                a[i][j] = ++k;
    }

    //for everyone, lets compute numbers of rows, numbers of int and displacements for everyone. Only 0 will use these arrays, but it's a practical way to get `rows` 
    int nbrows[numtasks];
    int sendcounts[numtasks];
    int displs[numtasks];
    displs[0]=0;
    for(i=0;i<numtasks;i++){
        nbrows[i]=height/numtasks;
        if(i<height%numtasks){
            nbrows[i]=nbrows[i]+1;
        }
        sendcounts[i]=nbrows[i]*width;
        if(i>0){
            displs[i]=displs[i-1]+sendcounts[i-1];
        }
    }
    rows=nbrows[taskid];

    //scattering operation. 
    //The case of the root is particular, since the communication is not to be done...Hence, the flag MPI_IN_PLACE is used.
    if(taskid==0){
        MPI_Scatterv(&a[0][0],sendcounts,displs,MPI_INT,MPI_IN_PLACE,0,MPI_INT,0,MPI_COMM_WORLD);
    }else{
        //allocation of memory for the piece of mat on the other nodes.
        a=new int*[rows];
        int *t =new int[rows * width];
        for(i = 0; i < rows; ++i)
            a[i] = &t[i * width];

        MPI_Scatterv(NULL,sendcounts,displs,MPI_INT,&a[0][0],rows*width,MPI_INT,0,MPI_COMM_WORLD);
    }
    //printing, one proc at a time
    if(taskid>0){
        MPI_Status status;
        MPI_Recv(NULL,0,MPI_INT,taskid-1,0,MPI_COMM_WORLD,&status);
    }
    cout<<"rank"<< taskid<<" Rows : "<<rows<<" Width : "<<width<<endl;

    for(i=0; i<rows; i++)
    {
        for(j=0; j<width; j++)
            cout<<a[i][j]<<"";
        cout<<endl;
    }
    if(taskid<numtasks-1){
        MPI_Send(NULL,0,MPI_INT,taskid+1,0,MPI_COMM_WORLD);
    }

    //freeing the memory !

    delete[] a[0];
    delete[] a;

    MPI_Finalize();

    return 0;
}

To compile : mpiCC main.cpp -o main

To run : mpiexec -np 3 main