running a C program on an MPI cluster - linux

Question

I have a problem running an MPI program (written in C or C++) over a cluster comprising of two nodes.
Details:
OS: Ubuntu 16.04
No. of nodes: 2 (master and slave)
Everything works well. When I run a simple mpi_hello program on the cluster with 12 as an argument (no. of processes) I see 4 mpi-hello instances running on the slave node (checked using top).
Output on master node + mpi_hello instances running on the second node (slave node)
When I try to run another program (for instance a simple program calculating and printing prime numbers in a range) it is running on the master node but i don't see any instances of it on the slave node.
#include <stdio.h>
#include<time.h>
//#include</usr/include/c++/5/iostream>
#include<mpi.h>
int main(int argc, char **argv)
{
int N, i, j, isPrime;
clock_t begin = clock();
int myrank, nprocs;
MPI_Init(&argc, &argv);
MPI_Comm_size(MPI_COMM_WORLD,&nprocs);
MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
printf("Hello from the processor %d of %d \n" , myrank, nprocs);
printf("To print all prime numbers between 1 to N\n");
printf("Enter the value of N\n");
scanf("%d",&N);
/* For every number between 2 to N, check
whether it is prime number or not */
printf("Prime numbers between %d to %d\n", 1, N);
for(i = 2; i <= N; i++){
isPrime = 0;
/* Check whether i is prime or not */
for(j = 2; j <= i/2; j++){
/* Check If any number between 2 to i/2 divides I
completely If yes the i cannot be prime number */
if(i % j == 0){
isPrime = 1;
break;
}
}
if(isPrime==0 && N!= 1)
printf("%d ",i);
}
clock_t end = clock();
double time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
printf("\nThe time spent by the program is %f\n" , time_spent);
while(1)
{}
MPI_Finalize();
return 0;
}
What could be the possible reasons behind it ?
Are there any other ways to check if it is running on the slave node as well ?
Thanks

Answer 1

Okay so here is a code I worked with. A vector containing first 500 integers. Now I want to divide them into 4 processes equally (i.e. each process gets 125 integers -- the first process gets 1-125, the second 126-250 and so on). I tried to use MPI_Scatter(). but I don't see the data equally divided or even divided. Do I have to use MPI_Recv() (I have another piece of code which is functional and uses only scatter to divide data equally).
Could you pint out any problems in the code. Thanks
int main(int argc, char* argv[])
{
int root = 0;
MPI_Init(&argc, &argv);
int myrank, nprocs;
MPI_Status status;
//variables for prime number calculation
int num1, num2, count, n;
MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
MPI_Comm_size(MPI_COMM_WORLD, &nprocs);
char name[MPI_MAX_PROCESSOR_NAME + 1];
int namelen;
MPI_Get_processor_name(name, &namelen);
cout << "Enter first number: ";
cin >> num1;
cout << "Enter second number: ";
cin >> num2;
int size = 500;
int size1 = num2 / nprocs;
cout << "The size of each small vector is " << size1 << endl;
auto start = get_time::now(); //start measuring the time
vector<int> sendbuffer(size), recbuffer(size1); //vectors/buffers involved in the processing
cout << "The prime numbers between " << num1 << " and " << num2 << " are: " << endl;
if (myrank == root)
{
for (unsigned int i = 1; i <= num2; ++i) //array containing all the numbers from which you want to find prime numbers
{
sendbuffer[i] = i;
}
cout << "Processor " << myrank << " initial data";
for (int i = 1; i <= size; ++i)
{
cout << " " << sendbuffer[i];
}
cout << endl;
MPI_Scatter(&sendbuffer.front(), 125, MPI_INT, &recbuffer.front(), 125, MPI_INT, root, MPI_COMM_WORLD);
}
cout << "Process " << myrank << " now has data ";
for (int j = 1; j <= size1; ++j)
{
cout << " " << recbuffer[j];
}
cout << endl;
auto end = get_time::now();
auto diff = end - start;
cout << "Elapsed time is : " << chrono::duration_cast<ms>(diff).count() << " microseconds " << endl;
MPI_Finalize();
return 0;
}`