如何使用所有处理器在MPI中发送/接收 [英] How to Send/Receive in MPI using all processors

问题描述

此程序使用C Lagrange和MPI编写.我是MPI的新手，并且希望使用所有处理器进行一些计算，包括进程0.要学习此概念，我编写了以下简单程序.但是，该程序在接收到来自进程0的输入后挂在底部，并且不会将结果发送回进程0.

This program written using C Lagrange and MPI. I am new to MPI and want to use all processors to do some calculations, including process 0. To learn this concept, I have written the following simple program. But this program hangs at the bottom after receiving input from the process 0 and won't send results back to the process 0.

#include <mpi.h>
#include <stdio.h>

int main(int argc, char** argv) {    
    MPI_Init(&argc, &argv);
    int world_rank;
    MPI_Comm_rank(MPI_COMM_WORLD, &world_rank);
    int world_size;
    MPI_Comm_size(MPI_COMM_WORLD, &world_size);

    int number;
    int result;
    if (world_rank == 0) 
    {
        number = -2;
        int i;
        for(i = 0; i < 4; i++)
        {
            MPI_Send(&number, 1, MPI_INT, i, 0, MPI_COMM_WORLD);
        }
        for(i = 0; i < 4; i++)
        {           /*Error: can't get result send by other processos bellow*/
            MPI_Recv(&number, 1, MPI_INT, i, 99, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
            printf("Process 0 received number %d from i:%d\n", number, i);
        }
    } 
    /*I want to do this without using an else statement here, so that I can use process 0 to do some calculations as well*/

    MPI_Recv(&number, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE); 
    printf("*Process %d received number %d from process 0\n",world_rank, number);
    result = world_rank + 1;
    MPI_Send(&result, 1, MPI_INT, 0, 99, MPI_COMM_WORLD);  /* problem happens here when trying to send result back to process 0*/

    MPI_Finalize();
}

运行并获得结果:

:$ mpicc test.c -o test
:$ mpirun -np 4 test

*Process 1 received number -2 from process 0
*Process 2 received number -2 from process 0
*Process 3 received number -2 from process 0
/* hangs here and will not continue */

如果可以的话，请举例说明我，或在可能的情况下编辑以上代码.

If you can, please show me with an example or edit the above code if possible.

推荐答案

实际上，进程1-3确实将结果发送回了处理器0.但是，处理器0停留在此循环的第一次迭代中:

Actually, processes 1-3 are indeed sending the result back to processor 0. However, processor 0 is stuck in the first iteration of this loop:

for(i=0; i<4; i++)
{      
    MPI_Recv(&number, 1, MPI_INT, i, 99, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
    printf("Process 0 received number %d from i:%d\n", number, i);
}

在第一个MPI_Recv调用中，处理器0将阻止等待从其自身接收带有标签99的消息，该消息尚未0发送.

In the first MPI_Recv call, processor 0 will block waiting to receive a message from itself with tag 99, a message that 0 did not send yet.

通常，处理器向自身发送/接收消息是一个坏主意，特别是使用阻塞调用. 0已在内存中具有该值.它不需要将其发送给自己.

Generally, it is a bad idea for a processor to send/receive messages to itself, especially using blocking calls. 0 already have the value in memory. It does not need to send it to itself.

但是，一种解决方法是从i=1

However, a workaround is to start the receive loop from i=1

for(i=1; i<4; i++)
{           
    MPI_Recv(&number, 1, MPI_INT, i, 99, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
    printf("Process 0 received number %d from i:%d\n", number, i);
}

现在运行代码将为您提供:

Running the code now will give you:

Process 1 received number -2 from process 0
Process 2 received number -2 from process 0
Process 3 received number -2 from process 0
Process 0 received number 2 from i:1
Process 0 received number 3 from i:2
Process 0 received number 4 from i:3
Process 0 received number -2 from process 0

请注意，如Gilles所述，使用MPI_Bcast和MPI_Gather是一种更高效，更标准的数据分发/收集方式.

Note that using MPI_Bcast and MPI_Gather as mentioned by Gilles is a much more efficient and standard way for data distribution/collection.

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