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Message Passing Interface (MPI) : MPI_Gather example

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by

Mandar Gurav
in

This post talks about Reduction operation in MPI using MPI_Gather. MPI_Gather is a collective operation in the Message Passing Interface (MPI) used to collect data from multiple processes and combine it into a single process.

Syntax for MPI_Gather

int MPI_Gather(const void *sendbuf, int sendcount, MPI_Datatype sendtype, void *recvbuf, int recvcount, MPI_Datatype recvtype, int root, MPI_Comm comm)

Input Parameters

  • sendbuf : Pointer to the data/array to be sent (for example : int *mydata)
  • sendcount : number of elements/values to be sent (for example : 10)
  • sendtype : data type of elements/values to be sent (for example : MPI_INT)
  • recvcount : number of elements/values to be received (for example : 10)
  • recvype : data type of elements/values to be received (for example : MPI_INT)
  • root : rank of root process (for example : 0)
  • comm : communicator (for example : MPI_COMM_WORLD)

Output Parameters

  • recvbuf : Pointer to the data/array to be received (for example : int *myarr)

Example code –

The code is hard-coded to run for 4 processes, but it can be easily generalized to use any number of processes.

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

#define ARRSIZE 12


int main(int argc, char **argv)
{
	int myid, size;
	int i;
	int data[ARRSIZE];
	int send_data[ARRSIZE/4];	
	
	//Initialize MPI environment 
	MPI_Init(&argc,&argv);
	
	//Get total number of processes
	MPI_Comm_size(MPI_COMM_WORLD, &size);
	
	//Get my unique identification among all processes
	MPI_Comm_rank(MPI_COMM_WORLD, &myid);
	
	//Exit the code if the number of processes is not equal to 4
	if(size!=4)
	{
		printf("\n Please use EXACTLY 4 processes!\n");
		MPI_Finalize();
		exit(0);
	}
	
	//Initialize data to some value
	for(i=0;i<ARRSIZE/4;i++)
	{
		send_data[i] = i + 3*myid;			
	}
		
	//print the data
	printf("\n myid : %d : I have %d, %d, and %d.", myid, send_data[0], send_data[1], send_data[2]);	
	
	// Wait for all the processes to print their data
	MPI_Barrier(MPI_COMM_WORLD);
	
	//Collect / Gather the data at root = 0 
	MPI_Gather(&send_data, ARRSIZE/4, MPI_INT, &data, ARRSIZE/4, MPI_INT, 0, MPI_COMM_WORLD);
	
	// Root prints the data
	if(myid==0)
	{
		printf("\nFinal data: ");
		for(i=0;i<ARRSIZE;i++)
		{
			printf(" %d", data[i]);			
		}		
	}
	//End MPI environment        
	MPI_Finalize();
}

To compile this code, use following command –

mpicc gather.c

To execute this code, run following command –

mpiexec -n 4 ./a.out

Output of this code will be something similar to the following –


 myid : 1 : I have 3, 4, and 5.
 myid : 3 : I have 9, 10, and 11.
 myid : 2 : I have 6, 7, and 8.
 myid : 0 : I have 0, 1, and 2.
Final data:  0 1 2 3 4 5 6 7 8 9 10 11

To know more about MPI, visit our dedicated page for MPI here.

If you are new to Parallel Programming / HPC, visit our dedicated page for beginners.

References :