MPI_Gatherv: create and collect arrays of variable size (MPI+C)

问题

I am new to MPI and I am trying to manage arrays of different size in parallel and then pass them to the main thread, unsuccessfully so far.

I have learned that

MPI_Gatherv(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
        void *recvbuf, const int *recvcounts, const int *displs,
        MPI_Datatype recvtype, int root, MPI_Comm comm)

is the way to go in this case.

Here is my sample code, which doesn't work because of memory issues (I think).

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <mpi.h>

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

MPI_Init(&argc, &argv);
int world_size,*sendarray;
int rank, *rbuf=NULL, count;
int *displs=NULL,i,*rcounts=NULL;

MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &world_size);

if(rank==0){
    rbuf = malloc(10*sizeof(int));
    displs = malloc(world_size*sizeof(int));
    rcounts=malloc(world_size*sizeof(int));
    rcounts[0]=1;
    rcounts[1]=3;
    rcounts[2]=6;

    displs[0]=1;
    displs[1]=3;
    displs[2]=6;

    sendarray=malloc(1*sizeof(int));
    for(int i=0;i<1;i++)sendarray[i]=1;
    count=1;
}

if(rank==1){
    sendarray=malloc(3*sizeof(int));
    for(int i=0;i<3;i++)sendarray[i]=2;
    count=3;
}

if(rank==2){
    sendarray=malloc(6*sizeof(int));
    for(int i=0;i<6;i++)sendarray[i]=3;
    count=6;
}
MPI_Barrier(MPI_COMM_WORLD);


MPI_Gatherv(sendarray, count, MPI_INT, rbuf, rcounts,
            displs, MPI_INT, 0, MPI_COMM_WORLD);

if(rank==0){
    int SIZE=10;
    for(int i=0;i<SIZE;i++)printf("(%d) %d ",i, rbuf[i]);

    free(rbuf);
    free(displs);
    free(rcounts);
}

if(rank!=0)free(sendarray);
MPI_Finalize();

}

Specifically, when I run it, I get

(0) 0 (1) 1 (2) 0 (3) 2 (4) 2 (5) 2 (6) 3 (7) 3 (8) 3 (9) 3

Instead of something like this

(0) 1 (1) 2 (2) 2 (3) 2 (4) 3 (5) 3 (6) 3 (7) 3 (8) 3 (9) 3

Why is that?

What is even more interesting, is that it seems like missing elements are stored in 11th and 12th element of the rbuf, even though those are supposed to not even exist at the first place.

回答1:

Your program is very close to working. If you change these lines:

displs[0]=1;
displs[1]=3;
displs[2]=6;

to this:

displs[0]=0;
displs[1]=displs[0]+rcounts[0];
displs[2]=displs[1]+rcounts[1];

you will get the expected output. The variable displs is the offset into the receiving buffer to place the data from process i.

来源：https://stackoverflow.com/questions/50514981/mpi-gatherv-create-and-collect-arrays-of-variable-size-mpic

标签

memory-leaks

mpi

communication

dynamic-memory-allocation