C - mpi programming

Hi,

i am trying to implement a program using (open) mpi that sends groups of numbers to each process which calculate the sum and return it to the master which in turn calculates to the total sum.
i am new to mpi and c programming.

can anyone tell me what is wrong in my code please.

Code:

---

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

main(int argc, char ** argv)
{
        int my_rank;
        int source, dest;
        int tag=1234;
        int namelen;
        char msg[100];
        char processor_name[MPI_MAX_PROCESSOR_NAME];
       
       

        int startInt = 1;
        int endInt = 1000;

        int nb = (endInt-startInt)+1;
          int numbers[nbInt];
        int test[nbInt];
 
          int i, j, k, n, s;
        int part_sum;
       
        MPI_Status status;
        MPI_Init(&argc, &argv);
        MPI_Comm_size(MPI_COMM_WORLD, &processes);
        MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
        MPI_Get_processor_name(processor_name,&namelen);
       
          for(i=0, k=startInt; i<nb; i++, k++)
          {
                numbers[i] = k;
          }
                   
        if(my_rank != 0)
        {
               
                MPI_Recv(&numbers[n], 100, MPI_INTEGER, source, tag, MPI_COMM_WORLD, &status);
                part_sum = 0;
               
                dest = 0; 
                for(i=0; i<s; i++)
                {
                        part_sum = part_sum + numbers[i];
                }
                MPI_Send(&part_sum, 1, MPI_INTEGER, dest, tag, MPI_COMM_WORLD);
        }
        else
        {               
               
                s = nb/processes;
                 
               
               
                for(j=0, n=0; j<processes; j++, n=n+s)
                {
                                MPI_Send(&numbers[n], s, MPI_INTEGER, j, tag, MPI_COMM_WORLD);       
                }

                int total_sum = 0;

                for(i=0; i<processes; i++)
                {
                        MPI_Recv(&part_sum, 100, MPI_INTEGER, i, tag, MPI_COMM_WORLD, &status);
                        total_sum += part_sum;       
                }
               
                printf("%i\n",total_sum);
        }
        MPI_Finalize();
        return 0;
}

---

thanks in advance.

It would help if you told us what happens in relation to what you expect to see.

--
Mats

Basically, let's say if I have 5 "slaves" and numbers from 1-20.
The master will send 5 chunks to each slave.
E.g. the first slave will get the numbers 1,2,3,4, the second slave the numbers 5,6,7,8 ......
Slave one will calculate 1+2+3+4, slave two will calculate 5+6+7+8 and all the slave processes return their addition results to the master which the sums up all the partial sums and prints them on the standard output (screen).

if(my_rank != 0) block contains the code for the slave and the else block for the master.

Yes, I understand the concept of distributed computing. But to be able to help you, we'd need to understand what part of your code is or isn't working. Are you getting the result you expect?

--
Mats

when i run the code i get errors with the recv functions. I am not sure if i am using them in the correct way.

the function header for the methods are:

Code:

---

int MPI Send(void *buf, int count, MPI Datatype type, int dest, int tag,
MPI Comm comm)
int MPI Recv(void *buf, int count, MPI Datatype type, int src, int tag, MPI Comm
comm, MPI Status *status)

---

Getting more and more tired of the "twenty questions"... ;)

And do you think it would help us help you if you told us WHAT those errors are?

--
Mats

Sorry about that.
Those are the error message that I get when I try to run the code:

*** An error occurred in MPI_Recv
*** on communicator MPI_COMM_WORLD
*** MPI_ERR_TRUNCATE: message truncated
*** MPI_ERRORS_ARE_FATAL (goodbye)

So your receive process expects to receive 100, and you are sending s - is s 100?

--
Mats

Quote:

---

Originally Posted by matsp

So your receive process expects to receive 100, and you are sending s - is s 100?

---

This is exactly the problem. If you run with <10 processes, it looks like you're sending more than 100 int's to each process, and the message gets truncated at the receiver. MPI defines that this is an error. It would likely be better to MPI_Recv the right number of elements, since even the compute processes can perform the same s=... computation as MPI_COMM_WORLD rank 0 (because they all got the same value for "processes").

Also, shouldn't s = nb/(processes - 1) since you're not sending to MCW rank 0?

Hope that helps (yay google alerts for finding this question for me ;-) ).