MPI聚合通信之MPI_Barrier函数

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MPI_Barrier函数用于一个通信子中所有进程的同步，调用函数时进程将处于等待状态，直到通信子中所有进程都调用了该函数后才继续执行。

如：

#include "stdio.h"#include "string.h"#include "mpi.h"int main(int agc,char *agv[]){int comm_size;int my_rank;MPI_Init(&agc,&agv);MPI_Comm_size(MPI_COMM_WORLD,&comm_size);MPI_Comm_rank(MPI_COMM_WORLD,&my_rank);for(int i=0;i<5;i++)printf("process %d: %d\n",my_rank,i);printf("waiting.....\n");//MPI_Barrier(MPI_COMM_WORLD);for(int i=5;i<10;i++)printf("process %d: %d\n",my_rank,i);MPI_Finalize();return 0;}

编译运行结果为：

process 3: 0process 3: 1process 3: 2process 3: 3process 3: 4waiting.....process 3: 5process 3: 6process 3: 7process 3: 8process 3: 9process 0: 0process 0: 1process 0: 2process 0: 3process 0: 4waiting.....process 0: 5process 0: 6process 0: 7process 0: 8process 0: 9process 1: 0process 1: 1process 1: 2process 1: 3process 1: 4waiting.....process 1: 5process 1: 6process 1: 7process 1: 8process 1: 9process 2: 0process 2: 1process 2: 2process 2: 3process 2: 4waiting.....process 2: 5process 2: 6process 2: 7process 2: 8process 2: 9

可以看到进程直接将0-9打印出来，中间并没有等待。现在将MPI_Barrier的注释去掉，即：

#include "stdio.h"#include "string.h"#include "mpi.h"int main(int agc,char *agv[]){int comm_size;int my_rank;MPI_Init(&agc,&agv);MPI_Comm_size(MPI_COMM_WORLD,&comm_size);MPI_Comm_rank(MPI_COMM_WORLD,&my_rank);for(int i=0;i<5;i++)printf("process %d: %d\n",my_rank,i);printf("waiting.....\n");MPI_Barrier(MPI_COMM_WORLD);for(int i=5;i<10;i++)printf("process %d: %d\n",my_rank,i);MPI_Finalize();return 0;}

打印输出如下：

process 3: 0process 3: 1process 3: 2process 3: 3process 3: 4waiting.....process 0: 0process 0: 1process 0: 2process 0: 3process 0: 4waiting.....process 1: 0process 1: 1process 1: 2process 1: 3process 1: 4waiting.....process 2: 0process 2: 1process 2: 2process 2: 3process 2: 4waiting.....process 1: 5process 1: 6process 1: 7process 1: 8process 1: 9process 3: 5process 3: 6process 3: 7process 3: 8process 3: 9process 0: 5process 0: 6process 0: 7process 0: 8process 0: 9process 2: 5process 2: 6process 2: 7process 2: 8process 2: 9

可以看出来进程在输出0-4之后等待其它进程完成打印0-4操作之后，再打印5-9.

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