MPI partition array into blocks and Send - c

Question

I am trying to find a maximum element of an array using MPI in C language. I have to compare the time it takes to send and calculation of the maximum using vs MPI_Scatter functions. MPI_Send: Here' the code for the MPI_Scatter function it works great:
#include "mpi.h"
#include <stdio.h>
#include <math.h>
#include <sys/time.h>
#define lim 20
//returns "a-b" in seconds
double timeval_diff(struct timeval *a, struct timeval *b)
{
return
(double)(a->tv_sec + (double)a->tv_usec/1000000) -
(double)(b->tv_sec + (double)b->tv_usec/1000000);
}
//Array to be divided among the processes
int buf[lim]=
{27,24,3,8,45,10,50,15,10,11,9,48,69,25,19,29,61,72,93,20};
int buf2[lim];
int buf3[lim];
int max;
int main(int argc, char *argv[])
{
struct timeval t_ini, t_fin;
double secs;
int n, myid, numprocs, i,j;
int namelen;
char processor_name[MPI_MAX_PROCESSOR_NAME];
MPI_Init(&argc,&argv);
MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
MPI_Comm_rank(MPI_COMM_WORLD,&myid);
MPI_Get_processor_name(processor_name,&namelen);
fprintf(stderr,"Process %d in %s\n",myid, processor_name);
/*Check Border Conditions */
n=lim/numprocs;
gettimeofday(&t_ini, NULL); //take the time before sending the buffer with Scatter
MPI_Scatter(buf,n, MPI_INT,buf2,n,MPI_INT, 0, MPI_COMM_WORLD);
gettimeofday(&t_fin, NULL);//take the time to complete the send routine
secs = timeval_diff(&t_fin, &t_ini);
MPI_Reduce(buf2,buf3,n, MPI_INT, MPI_MAX, 0,MPI_COMM_WORLD);
if (myid == 0)
{ max = buf3[0];
for (i=1; i<n ; i++)
if (max < buf3[i]) max = buf3[i];
for (i=0; i<n ; i++)
printf("Buf3[%d]= %d \n", i, buf3[i]);
printf("Max number of the array is: %d \n", max);
}
for (i=0; i<n ; i++){
printf("%d,Buf2[%d]= %d \n",myid, i,buf2[i]);}
printf("%.16g milliseconds\n", secs * 1000.0);
MPI_Finalize();
return 0;
}
The problem comes when I try to do the same procedure with the MPI_Send function because I calculated the maximum array elements, what am I doing wrong?:
#include "mpi.h"
#include <stdio.h>
#include <math.h>
#include <sys/time.h>
#define lim 20
//returns "a-b" in seconds
double timeval_diff(struct timeval *a, struct timeval *b)
{
return
(double)(a->tv_sec + (double)a->tv_usec/1000000) -
(double)(b->tv_sec + (double)b->tv_usec/1000000);
}
//Array to be divided among the processes
int buf[lim]=
{27,24,3,8,45,10,50,15,10,11,9,48,69,25,19,29,61,72,93,20};
int buf2[lim];
int buf3[lim];
int max;
int main(int argc, char *argv[])
{
struct timeval t_ini, t_fin;
double secs;
int n, myid, numprocs, i,j;
int namelen;
char processor_name[MPI_MAX_PROCESSOR_NAME];
MPI_Init(&argc,&argv);
MPI_Comm_size(MPI_COMM_WORLD,&numprocs);
MPI_Comm_rank(MPI_COMM_WORLD,&myid);
MPI_Get_processor_name(processor_name,&namelen);
fprintf(stderr,"Process %d in %s\n",myid, processor_name);
/*Check Border Conditions */
n=lim/numprocs;
gettimeofday(&t_ini, NULL); //take the time before sending the buffer with Scatter
for (j=0;j<n;j++){
MPI_Send(buf, lim, MPI_INT, 1, 111, MPI_COMM_WORLD);
}
gettimeofday(&t_fin, NULL);//take the time to complete the send routine
secs = timeval_diff(&t_fin, &t_ini);
if (myid == 0)
{ max = buf3[0];
for (i=1; i<n ; i++)
if (max < buf3[i]) max = buf3[i];
for (i=0; i<n ; i++)
printf("Buf3[%d]= %d \n", i, buf3[i]);
printf("Max number of the array is: %d \n", max);
}
for (i=0; i<n ; i++){
printf("%d,Buf2[%d]= %d \n",myid, i,buf2[i]);}
printf("%.16g milliseconds\n", secs * 1000.0);
MPI_Finalize();
return 0;
}
I wasted some hours watching Where is the fault but I can not see it ... Any help?

Answer 1

you are missing the MPI_Recv call on the other end of your MPI_Send call, these kind of functions are more low level as opposed to the collective scatter, gather, reduce and broadcast functions