Sum of an array using MPI
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 process int 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; } |
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Output:
Sum of array is 55
Below is the snapshot of the processes calculating their partial sums:
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