• MPI学习笔记(三):矩阵相乘


    参考自:https://blog.csdn.net/lcx543576178/article/details/45892839

    程序稍作修改,如下:

    #include<iostream>
    using namespace std;
    #include<mpi.h>
    
    int main(int argc, char * argv[] ){
    
        double start, stop;
        int *a, *b, *c, *buffer, *ans;
        int size = 1000;
        int rank, numprocs, line;
    
        MPI_Init(NULL,NULL);
        MPI_Comm_rank(MPI_COMM_WORLD, &rank);
        MPI_Comm_size(MPI_COMM_WORLD, &numprocs);
    
        line = size/numprocs;
    
        b = new int [ size * size ];
        ans = new int [ size * line ];
    
        start = MPI_Wtime();
    
        if( rank ==0 ){
    
            a = new int [ size * size ];
            c = new int [ size * size ];
    
            for(int i=0;i<size; i++)
            for(int j=0;j<size; j++){
                a[ i*size + j ] = i*j;
                b[ i*size + j ] = i + j;
            }
    
            for(int i=1;i<numprocs;i++){// send b
                MPI_Send( b, size*size, MPI_INT, i, 0, MPI_COMM_WORLD );
            }
    
            for(int i=1;i<numprocs;i++){// send part of a
                MPI_Send( a + (i-1)*line*size, size*line, MPI_INT, i, 1, MPI_COMM_WORLD);
            }
    
            for(int i = (numprocs-1)*line;i<size;i++){// calculate block 1
                for(int j=0;j<size;j++){
                    int temp = 0;
                    for(int k=0;k<size;k++)
                        temp += a[i*size+k]*b[k*size+j];
                    c[i*size+j] = temp;
                }
            }
    
            for(int k=1;k<numprocs;k++){// recieve ans
                MPI_Recv( ans, line*size, MPI_INT, k, 3, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
                for(int i=0;i<line;i++){
                    for(int j=0;j<size;j++){
                        c[ ((k-1)*line + i)*size + j] = ans[i*size+j];
                    }
                }
            }
    
            FILE *fp = fopen("c.txt","w");
            for(int i=0;i<size;i++){
                for(int j=0;j<size;j++)
                    fprintf(fp,"%d	",c[i*size+j]);
                fputc('
    ',fp);
            }
            fclose(fp);
            
            stop = MPI_Wtime();
    
            printf("rank:%d time:%lfs
    ",rank,stop-start);
    
            delete [] a,c;
    
        }
        else{
            buffer = new int [ size * line ];
    
            MPI_Recv(b, size*size, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
            MPI_Recv(buffer, size*line, MPI_INT, 0, 1, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
            for(int i=0;i<line;i++)
            for(int j=0;j<size;j++){
                int temp=0;
                for(int k=0;k<size;k++)
                    temp += buffer[i*size+k]*b[k*size+j];
                ans[i*size+j] = temp;
            }
            MPI_Send(ans, line*size, MPI_INT, 0, 3, MPI_COMM_WORLD);
    
            delete [] buffer;
            delete [] ans;
        }
    
        delete [] b;
    
        MPI_Finalize();
    
        return 0;
    }

    线程 0 发送矩阵 b,以及 a 的分块矩阵给其他线程,然后自己做一部分矩阵乘法,然后接收其他线程的结果,然后输出最终答案。

    2. 用 MPI_Scatter 和 MPI_Gather,代码同样参考自 https://blog.csdn.net/lcx543576178/article/details/45892839

    稍作修改,如下:

    #include<iostream>
    using namespace std;
    #include<mpi.h>
    
    int main(){
    
        int my_rank;
        int num_procs;
        int size = 1000;
        double start, finish;
    
        MPI_Init(NULL,NULL);
        MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
        MPI_Comm_size(MPI_COMM_WORLD, &num_procs);
        
        int line = size / num_procs;
        cout<<" line = "<<line<<endl;
        int * local_a = new int [ line * size ];
        int * b = new int [ size * size ];
        int * ans = new int [ line * size ];
        int * a = new int [ size * size ];
        int * c = new int [ size * size ];
    
        if( my_rank == 0 ){
    
            start = MPI_Wtime();
    
            for(int i=0;i<size;i++){
                for(int j=0;j<size;j++){
                    a[ i*size + j ] = i*j;
                    b[ i*size + j ] = i + j;
                }
            }
    
            MPI_Scatter(a, line * size, MPI_INT, local_a, line * size, MPI_INT, 0, MPI_COMM_WORLD );
    
            MPI_Bcast(b, size*size, MPI_INT, 0, MPI_COMM_WORLD);
    
            for(int i= 0; i< line;i++){
                for(int j=0;j<size;j++){
                    int temp = 0;
                    for(int k=0;k<size;k++)
                        temp += a[i*size+k] * b[k*size + j];
                    ans[i*size + j ] = temp;
                }
            }
    
            MPI_Gather( ans, line * size, MPI_INT, c, line * size, MPI_INT, 0, MPI_COMM_WORLD );
    
            for(int i= num_procs *line; i< size;i++){
                for(int j=0;j<size;j++){
                    int temp = 0;
                    for(int k=0;k<size;k++)
                        temp += a[i*size+k] * b[k*size + j];
                    c[i*size + j ] = temp;
                }
            }
    
            FILE *fp = fopen("c2.txt","w");
                    for(int i=0;i<size;i++){
                            for(int j=0;j<size;j++)
                                    fprintf(fp,"%d	",c[i*size+j]);
                            fputc('
    ',fp);
                    }
                    fclose(fp);
    
            finish = MPI_Wtime();
            printf(" time: %lf s 
    ", finish - start );
        }
        else{
            int * buffer = new int [ size * line ];
            MPI_Scatter(a, line * size, MPI_INT, buffer, line * size, MPI_INT, 0, MPI_COMM_WORLD );
            MPI_Bcast( b, size * size, MPI_INT, 0, MPI_COMM_WORLD );
            /*
            cout<<" b:"<<endl;
            for(int i=0;i<size;i++){
                for(int j=0;j<size;j++){
                    cout<<b[i*size + j]<<",";
                }
                cout<<endl;
            }
            */
    
            for(int i=0;i<line;i++){
                for(int j=0;j<size;j++){
                    int temp = 0;
                    for(int k=0;k<size;k++)
                        temp += buffer[i*size+k] * b[k*size + j];
                    //cout<<"i = "<<i<<"	 j= "<<j<<"	 temp = "<<temp<<endl;
                    ans[i*size + j] = temp;
                }
            }
    
            /*
            cout<<" ans:"<<endl;
            for(int i=0;i<line;i++){
                for(int j=0;j<size;j++){
                    cout<<ans[i*size + j]<<",";
                }
                cout<<endl;
            }
            */
    
            MPI_Gather(ans, line*size, MPI_INT, c, line*size, MPI_INT, 0, MPI_COMM_WORLD );
            delete [] buffer;
        }
    
        delete [] a, local_a, b, ans, c;
    
        MPI_Finalize();
        return 0;
    }

    进程 0 向所有进程(包括自己)分发(MPI_Scatter)a 矩阵的各个分块矩阵,并广播(MPI_Bcast)b 矩阵。其他进程接收 a 的分块矩阵与 b 矩阵,做乘法,并返回结果(MPI_Gather)。进程 0 收集所有结果(包括自己的)(MPI_Gather),如果 a 矩阵分块有剩余,进程 0 就做掉相关乘法,最后输出所有结果。

    3. 测时间。下图为上面两种方法的耗时,

    4. 总结:①确实进程越多,耗时越少。

        ②由于消息传递需要成本,而且不是每个进程都同时开始和结束,所以随着进程数的上升,平均每进程的效率下降。

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  • 原文地址:https://www.cnblogs.com/luyi07/p/11110658.html
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