Sum of an array using MPI - GeeksforGeeks

28 Jun, 2019
Prerequisite: MPI – Distributed Computing made easy
Message Passing Interface(MPI) is a library of routines that can be used to create parallel programs in C or Fortran77. It allows users to build parallel applications by creating parallel processes and exchange information among these processes.MPI uses two basic communication routines:
MPI_Send, to send a message to another process.
MPI_Recv, to receive a message from another process.
The syntax of MPI_Send and MPI_Recv is:
int MPI_Send(void *data_to_send, 
             int send_count, 
             MPI_Datatype send_type, 
             int destination_ID, 
             int tag, 
             MPI_Comm comm); 

int MPI_Recv(void *received_data, 
             int receive_count, 
             MPI_Datatype receive_type,
             int sender_ID, 
             int tag, 
             MPI_Comm comm, 
             MPI_Status *status);

To reduce the time complexity of the program, parallel execution of sub-arrays is done by parallel processes running to calculate their partial sums and then finally, the master process(root process) calculates the sum of these partial sums to return the total sum of the array.
Examples:
Input : {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}
Output : Sum of array is 55

Input : {1, 3, 5, 10, 12, 20, 4, 50, 100, 1000}
Output : Sum of array is 1205

Note – You must have MPI installed on your Linux based system for executing the following program. For details to do so, please refer MPI – Distributed Computing made easy
Compile and run the program using following code:
mpicc program_name.c -o object_file
mpirun -np [number of processes] ./object_file

Below is the implementation of the above topic:
#include <mpi.h>#include <stdio.h>#include <stdlib.h>#include <unistd.h>  // size of array#define n 10  int a[] = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };  // Temporary array for slave processint a2[1000];  int main(int argc, char* argv[]){      int pid, np,        elements_per_process,        n_elements_recieved;    // np -> no. of processes    // pid -> process id      MPI_Status status;      // Creation of parallel processes    MPI_Init(&argc, &argv);      // find out process ID,    // and how many processes were started    MPI_Comm_rank(MPI_COMM_WORLD, &pid);    MPI_Comm_size(MPI_COMM_WORLD, &np);      // master process    if (pid == 0) {        int index, i;        elements_per_process = n / np;          // check if more than 1 processes are run        if (np > 1) {            // distributes the portion of array            // to child processes to calculate            // their partial sums            for (i = 1; i < np - 1; i++) {                index = i * elements_per_process;                  MPI_Send(&elements_per_process,                         1, MPI_INT, i, 0,                         MPI_COMM_WORLD);                MPI_Send(&a[index],                         elements_per_process,                         MPI_INT, i, 0,                         MPI_COMM_WORLD);            }              // last process adds remaining elements            index = i * elements_per_process;            int elements_left = n - index;              MPI_Send(&elements_left,                     1, MPI_INT,                     i, 0,                     MPI_COMM_WORLD);            MPI_Send(&a[index],                     elements_left,                     MPI_INT, i, 0,                     MPI_COMM_WORLD);        }          // master process add its own sub array        int sum = 0;        for (i = 0; i < elements_per_process; i++)            sum += a[i];          // collects partial sums from other processes        int tmp;        for (i = 1; i < np; i++) {            MPI_Recv(&tmp, 1, MPI_INT,                     MPI_ANY_SOURCE, 0,                     MPI_COMM_WORLD,                     &status);            int sender = status.MPI_SOURCE;              sum += tmp;        }          // prints the final sum of array        printf("Sum of array is : %d\n", sum);    }    // slave processes    else {        MPI_Recv(&n_elements_recieved,                 1, MPI_INT, 0, 0,                 MPI_COMM_WORLD,                 &status);          // stores the received array segment        // in local array a2        MPI_Recv(&a2, n_elements_recieved,                 MPI_INT, 0, 0,                 MPI_COMM_WORLD,                 &status);          // calculates its partial sum        int partial_sum = 0;        for (int i = 0; i < n_elements_recieved; i++)            partial_sum += a2[i];          // sends the partial sum to the root process        MPI_Send(&partial_sum, 1, MPI_INT,                 0, 0, MPI_COMM_WORLD);    }      // cleans up all MPI state before exit of process    MPI_Finalize();      return 0;}
Output:
Sum of array is 55

Below is the snapshot of the processes calculating their partial sums:
Arrays
Articles
C Language
GBlog
Arrays
Writing code in comment?
Please use ide.geeksforgeeks.org,
generate link and share the link here.
SQL Interview Questions
Mutex vs Semaphore
Time Complexity and Space Complexity
SQL | Views
SQL | GROUP BY
Arrays in C/C++
Dynamic Memory Allocation in C using malloc(), calloc(), free() and realloc()
std::sort() in C++ STL
Bitwise Operators in C/C++
Substring in C++