Using ScatterV to split an array to multiple processes - mpi

Question

I am working with MPI and I have to send parts of an array to different processes. As an example, consider 3 processes. Then I need to send the red elements to the first process, the greed to the second and the black to the third process.
I know I could use Scatterv twice, but I want to minimize the communication between processes and the real array that I'm splitting apart is huge. Does anyone have a suggestion on how I can accomplish this?
Here is my attempt with a derived data type:
#include <stdio.h>
#include <stdlib.h>
#include <mpi.h>
void print_array(int *array,int n){
int i;
printf("\t[");
for (i=0; i<n; i++) {
printf(" %d",array[i]);
}
printf("]\n");
}
int main(int argc, char **argv){
int rank,world_size,i,n = 16, block_count = 2;
MPI_Init(&argc, &argv);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
MPI_Comm_size(MPI_COMM_WORLD, &world_size);
int *array = malloc(n * sizeof(int));
for (i=0; i<n; i++) { array[i]=i;}
if (rank==0) { print_array(array,n);}
int *sendcounts = malloc(world_size * sizeof(int));
int *reccounts = malloc(world_size * sizeof(int));
int *displs = malloc(world_size * sizeof(int));
sendcounts[0]=3; sendcounts[1]=3; sendcounts[2]=2;
displs[0]=0; displs[1]=3; displs[2]=6;
for (i=0; i<world_size; i++) {
reccounts[i] = sendcounts[i]*block_count;
}
int root = 0;
int *recvbuf = malloc(reccounts[rank] * sizeof(int));
MPI_Datatype newtype;
MPI_Type_contiguous(block_count, MPI_INT, &newtype);
MPI_Type_commit(&newtype);
if (rank==0) {
MPI_Scatterv(array, sendcounts, displs,
newtype, recvbuf, sendcounts[rank],
newtype, root, MPI_COMM_WORLD);
}
else {
MPI_Scatterv(NULL, sendcounts, displs,
newtype, recvbuf, reccounts[rank],
newtype, root, MPI_COMM_WORLD);
}
MPI_Type_free (&newtype);
print_array(recvbuf,reccounts[rank]);
free(array);array = NULL;
free(sendcounts);sendcounts = NULL;
free(displs);displs = NULL;
free(recvbuf);recvbuf = NULL;
MPI_Finalize();
return 0;
}

Answer 1

There is a way, but it is a bit convoluted.
The idea is you create a derived datatype with two elements at offset 0 and 8, and then resize this datatype so the upper bound is the size of one element.
Then you can MPI_Scatterv() once with counts={3,3,2} and displs={0,3,6}.
Note you also need to create a derived datatype on the receive side, otherwise MPI task 1 would receive {3, 11, 4, 12, 5, 13} when i guess you expect {3, 4, 5, 11, 12, 13}