# 如何使用MPI计算数组的总和？代码实现

2021年3月28日12:18:30 发表评论 1,518 次浏览

MPI使用两个基本的通信例程：

• MPI_Send, 将消息发送到另一个进程。
• MPI_Recv, 以接收来自另一个进程的消息。

MPI_Send和MPI_Recv的语法为：

``````int MPI_Send(void *data_to_send, int send_count, MPI_Datatype send_type, int destination_ID, int tag, MPI_Comm comm);

``````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``````

``````mpicc program_name.c -o object_file
mpirun -np [number of processes] ./object_file``````

``````#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;
}``````

``Sum of array is 55``