I am trying to learn how to use MPI_Scatter and MPI_Gather multiple times, and print out the result after waiting for these two MPI functions to complete. At process 0, top of the program, I want to use a while loop that call Scatter and Gather. Once they are done with all the calculation, I want to send this array back to these functions to do more calculations. I have explained in the code bellow what I am trying to do. Comments in /*.....*/ are the tasks I am trying to achieve.
Following code is run using 4 processors. :$ mpicc test.c -o test:$ mpirun -np 4 test
#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv) {
int size, rank;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
int globaldata[8];
int localdata[2];
int counter, i;
if (rank == 0)
{
for (i=0; i<size*2; i++)//initializing array to all zeros, one time
globaldata[i] = 0;
/*counter=0;
do
{
counter++; */
printf("Sending at Processor %d has data: ", rank);
for (i=0; i<size*2; i++)
printf("%d ", globaldata[i]);
printf("\n");
/*After MPI_Gather is done, I want to get the newly assined array here.
Now the globaldata array should hold values: 0 0 1 1 2 2 3 3
Therefore, in the next iteration of this while loop, these array values need
to be send for a new calculation with Scatter & Gather
}while(counter<2);*/
//Following need to be executed after all the scatter and gather has completed
printf("Finally at Processor %d has data: ", rank);
for (i=0; i<size*2; i++)//Here the result should be: 0 0 2 2 3 3 4 4
printf("%d ", globaldata[i]);
printf("\n");
}
MPI_Scatter(globaldata, 2, MPI_INT, &localdata, 2, MPI_INT, 0, MPI_COMM_WORLD);
localdata[0]= localdata[0]+rank;
localdata[1]= localdata[1]+rank;
MPI_Gather(&localdata, 2, MPI_INT, globaldata, 2, MPI_INT, 0, MPI_COMM_WORLD);
if (rank == 0) {//Currently I can only see the newly assinged array values if I print out the result at the bottom
printf("At the bottom, Processor %d has data: ", rank);
for (i=0; i<size*2; i++)
printf("%d ", globaldata[i]);
printf("\n");
}
MPI_Finalize();
return 0;
}
More explanation on what I am trying to do above:
I am wanting to send my globaldata array to all processors. Then get an updated globaldata array. once I get the updated array, I want to resend this array back to all other process again to do more calculations. I have written the following code that does a similar job using MPI_Send and MPI_Recv. Here I am using MPI_Send to send my array to all processors. Then this array will change it and send back to the root/process 0. Once I get the modified array, do while loop will run again and perform more calculation. What I am trying to do is: use MPI_Scatter and MPI_Gather in a similar way. Where I get a updated globaldata array and send it back to MPI_Scatter and MPI_Gather to change that array again
#include <mpi.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char **argv) {
int size, rank;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD, &size);
MPI_Comm_rank(MPI_COMM_WORLD, &rank);
int globaldata[8];
int counter, i;
if (rank == 0)
{
for (i=0; i<size*2; i++)
globaldata[i] = 0;
counter=0;
do
{ /*becase of this do while loop "globaldata" array will always be updated and resend for more caculations*/
counter++;
printf("Sending at Processor %d has data: ", rank);
for (i=0; i<size*2; i++)
printf("%d ", globaldata[i]);
printf("\n");
for(i = 0; i < 4; i++)
{
MPI_Send(&globaldata, 8, MPI_INT, i, 0, MPI_COMM_WORLD);
}
for(i = 1; i < 4; i++)
{
MPI_Recv(&globaldata, 8, MPI_INT, i, 99, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
}
}while(counter<2);
/*Following executes after all the above calculations has completed*/
printf("Finally at Processor %d has data: ", rank);
for (i=0; i<size*2; i++)
printf("%d ", globaldata[i]);
printf("\n");
}
counter=0;
do
{
counter++;
MPI_Recv(&globaldata, 8, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
globaldata[rank]=globaldata[rank]+rank;
globaldata[rank+1]=globaldata[rank+1]+rank;
MPI_Send(&globaldata, 8, MPI_INT, 0, 99, MPI_COMM_WORLD);
}while(counter<2);
MPI_Finalize();
return 0;
}
