使用 MPI_Scatter 和 MPI_Gather 实现 Master-Worker-Model

Question

我已经为图像处理过滤器编写了一个 MPI 程序。该程序的主要目标是使用并行计算将给定的过滤器应用于图像文件。

MPI 程序运行良好，没有任何问题。

现在我想为我的 MPI 程序使用 Master-Worker-Model。经过一些研究，我发现 MPI_Scatter 和 MPI_Gather 是我需要的命令。

但我不太明白它们是如何工作的。这是我第一次尝试编码：

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

int main(int argc, char** argv)
{

    // Filters

    MPI_Init(&argc, &argv);

    if(parent == MPI_COMM_NULL)
    {
        MPI_Scatter(); // What are the parameters?

        // Master reads in the image file and distribute it to the workers.
        MPI_File_open();
        MPI_File_read();
        MPI_File_close();
    }
    else
    {
        // After receiving data from the master, worker processes begin their job.

        /* Normally here would be the entire logic of this program.*/
        /* For simplicity I just skip it for readability.*/
        /* The calculcation is exclusively up to the workers.*/
    }

    // After finishing, the results will be sent back to the master.

    MPI_Gather(); // What are the parameters?

    MPI_Finalize(); 
    return 0; 
}

不会运行因为它只是第一个原型。我想知道的是我是否理解了Master-Worker-Model的概念。

对代码的一些反馈会很有帮助。

提前致谢。

Answer 1

这是一个示例程序，展示了如何执行 scatter/gather 操作。请注意，在您的情况下，由于主任务没有做任何工作，因此内部通信器是最合适的（即使内部通信器上的集体操作语义并不总是很直观）因此您不需要 MPI_Intercomm_merge().

#include <stdio.h>
#include <stdbool.h>

#include <mpi.h>

int main(int argc, char *argv[]) {
    MPI_Comm parent, intercomm;
    const int slaves = 2;
    bool master;
    int data[slaves];
    int mydata;
    int i, rank, root;

    MPI_Init(&argc, &argv);
    MPI_Comm_get_parent(&parent);
    master = parent == MPI_COMM_NULL;

    if (master) {
        int errcodes[slaves];
        /* spawn the slaves */
        MPI_Comm_spawn(argv[0], MPI_ARGV_NULL, slaves, MPI_INFO_NULL, 0, MPI_COMM_WORLD, &intercomm, errcodes);

        /* prepare data to be scattered to the slaves */
        for (i=0; i<slaves; i++) {
            data[i] = i;
        }

        /* here we assume there is only one master */
        root = MPI_ROOT;
    } else {
        intercomm = parent;
        root = 0;
    }

    /* scatter data from master to slaves */
    MPI_Scatter(data, 1, MPI_INT, &my-data, 1, MPI_INT, root, intercomm);

    if (!master) {
        /* slaves do their work */
        mydata = mydata + 1;
    }

    /* gather data from slaves to master */
    MPI_Gather(&mydata, 1, MPI_INT, data, 1, MPI_INT, root, intercomm);

    if (master) {
        int i;
        for (i=0; i<slaves; i++) {
            printf("Slave %d returned %d\n", i, data[i]);
        }
    }

    MPI_Comm_disconnect(&intercomm);

    MPI_Finalize();
    return 0;
}

使用 MPI_Scatter 和 MPI_Gather 实现 Master-Worker-Model

Using MPI_Scatter and MPI_Gather to implement Master-Worker-Model

c

image-processing

parallel-processing

mpi

broadcast