在 Open MPI 中处理图像的块

Question

我正在尝试并行化绘制 Mandelbrot 集的 C 程序。我将每个处理器的图像部分划分为相同大小的块，如下图所示：

我尝试使用解决方案here，它几乎解决了同样的问题。但是，我在输出中只得到了部分图像：

对于高分辨率图像（例如 8000x8000 像素），应用程序崩溃并出现分段错误 11。这是我的代码：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <math.h>
#include <time.h>
#include "mpi.h"

// Main program
int main(int argc, char* argv[])
 {
    /* screen ( integer) coordinate */
    int iX,iY,i,j;
    const int iXmax = 8000; // default
    const int iYmax = 8000; // default

    /* world ( double) coordinate = parameter plane*/
    double Cx, Cy;
    const double CxMin = -2.5;
    const double CxMax = 1.5;
    const double CyMin = -2.0;
    const double CyMax = 2.0; 

    /* */
    double PixelWidth = (CxMax - CxMin)/iXmax;
    double PixelHeight = (CyMax - CyMin)/iYmax;


    int linePerProcess, remainingLines, processMinY,  processMaxY, lastProcessMaxY, result_offset;
    int my_rank, processors, iXmaxHalf;
    int startAlert = 1;
    int receivedAlert;
    unsigned char (*resultBuffer)[3] = NULL;
    unsigned char (*resultBufferTwo)[3] = NULL;
    unsigned char (*finalResultBuffer)[3] = NULL;

    MPI_Status stat;


    /* color component ( R or G or B) is coded from 0 to 255 */
    /* it is 24 bit color RGB file */
    const int MaxColorComponentValue = 255; 
    FILE * fp;
    char *filename = "Mandelbrot.ppm";
    char *comment = "# ";   /* comment should start with # */

    // RGB color array
    unsigned char color[3];

    /* Z = Zx + Zy*i;   Z0 = 0 */
    double Zx, Zy;
    double Zx2, Zy2; /* Zx2 = Zx*Zx;  Zy2 = Zy*Zy  */
    /*  */
    int Iteration;
    const int IterationMax = 2000; // default

    /* bail-out value , radius of circle ;  */
    const double EscapeRadius = 400;
    double ER2 = EscapeRadius * EscapeRadius;
    double startTime, endTime;

    MPI_Init(&argc, &argv);
    MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);
    MPI_Comm_size(MPI_COMM_WORLD, &processors);

    linePerProcess = iYmax / (processors/2);
    iXmaxHalf = iXmax / 2;

    if (my_rank % 2  == 0) {
        processMinY = (my_rank/2) * linePerProcess;
    } else {
        processMinY = ((my_rank - 1)/2) * linePerProcess;
    }

    processMaxY = processMinY + linePerProcess;


    int Rows = iYmax;          // Global array rows
    int Columns = iXmax; // Global array columns
    int sizes[2];                     // No of elements in each dimension of the whole array
    int subSizes[2];                  // No of elements in each dimension of the subarray
    int startCoords[2];               // Starting coordinates of each subarray
    MPI_Datatype recvBlock, recvMagicBlock;

        // Create a subarray (a rectangular block) datatype from a regular, 2d array
    sizes[0] = Rows;
    sizes[1] = Columns;
    subSizes[0] = linePerProcess;
    subSizes[1] = iXmaxHalf;
    startCoords[0] = 0;
    startCoords[1] = 0;

    MPI_Type_create_subarray(2, sizes, subSizes, startCoords, MPI_ORDER_C, MPI_UNSIGNED_CHAR, &recvBlock);

    MPI_Type_create_resized(recvBlock, 0, iXmaxHalf * sizeof(color), &recvMagicBlock);

    MPI_Type_commit(&recvMagicBlock);

    if (my_rank == 0) {


        // startTime = MPI_Wtime();
        // for(i=1; i<processors; i++){
        //  MPI_Send(&startAlert, 1, MPI_INT, i, 0, MPI_COMM_WORLD);
        // }
        // printf("rank; %d\n", my_rank);

        finalResultBuffer = malloc(iXmax * iYmax * sizeof(color));

        for(iY = processMinY; iY < processMaxY; iY++) {

            Cy = CyMin + (iY * PixelHeight);
            if (fabs(Cy) < (PixelHeight / 2))
            {
                Cy = 0.0; /* Main antenna */
            }

            for(iX = 0; iX < iXmaxHalf; iX++)
            {

                Cx = CxMin + (iX * PixelWidth);
                /* initial value of orbit = critical point Z= 0 */
                Zx = 0.0;
                Zy = 0.0;
                Zx2 = Zx * Zx;
                Zy2 = Zy * Zy;

            /* */
                for(Iteration = 0; Iteration < IterationMax && ((Zx2 + Zy2) < ER2); Iteration++)
                {
                    Zy = (2 * Zx * Zy) + Cy;
                    Zx = Zx2 - Zy2 + Cx;
                    Zx2 = Zx * Zx;
                    Zy2 = Zy * Zy;
                };

            /* compute  pixel color (24 bit = 3 bytes) */
                if (Iteration == IterationMax)
                {
                    // Point within the set. Mark it as black
                    color[0] = 0;
                    color[1] = 0;
                    color[2] = 0;
                }
                else 
                {
                    // Point outside the set. Mark it as white
                    double c = 3*log((double)Iteration)/log((double)(IterationMax) - 1.0);
                    if (c < 1)
                    {
                        color[0] = 0;
                        color[1] = 0;
                        color[2] = 255*c;
                    }
                    else if (c < 2)
                    {
                        color[0] = 0;
                        color[1] = 255*(c-1);
                        color[2] = 255;
                    }
                    else
                    {
                        color[0] = 255*(c-2);
                        color[1] = 255;
                        color[2] = 255;
                    }
                }

                finalResultBuffer[(iY*iXmaxHalf)+iX][0] = color[0];
                finalResultBuffer[(iY*iXmaxHalf)+iX][1] = color[1];
                finalResultBuffer[(iY*iXmaxHalf)+iX][2] = color[2];


            }
        }

        result_offset = 1;
        for(i=1; i<processors; i++){

            MPI_Recv(finalResultBuffer, 1, recvMagicBlock, i, 0, MPI_COMM_WORLD, &stat);
            result_offset += 1;
        }



    } else if ((my_rank % 2 == 0) && (my_rank != 0)) {

        // MPI_Recv(&receivedAlert, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, &stat);
        // printf("rank; %d\n", my_rank);
        resultBuffer = malloc(linePerProcess * iXmaxHalf * sizeof(color));

        for(iY = processMinY; iY < processMaxY; iY++) {
            Cy = CyMin + (iY * PixelHeight);
            if (fabs(Cy) < (PixelHeight / 2))
            {
                Cy = 0.0; /* Main antenna */
            }
            for(iX = 0; iX < iXmaxHalf; iX++)
            {

                Cx = CxMin + (iX * PixelWidth);
                /* initial value of orbit = critical point Z= 0 */
                Zx = 0.0;
                Zy = 0.0;
                Zx2 = Zx * Zx;
                Zy2 = Zy * Zy;

            /* */
                for(Iteration = 0; Iteration < IterationMax && ((Zx2 + Zy2) < ER2); Iteration++)
                {
                    Zy = (2 * Zx * Zy) + Cy;
                    Zx = Zx2 - Zy2 + Cx;
                    Zx2 = Zx * Zx;
                    Zy2 = Zy * Zy;
                };

            /* compute  pixel color (24 bit = 3 bytes) */
                if (Iteration == IterationMax)
                {
                    // Point within the set. Mark it as black
                    color[0] = 0;
                    color[1] = 0;
                    color[2] = 0;
                }
                else 
                {
                    // Point outside the set. Mark it as white
                    double c = 3*log((double)Iteration)/log((double)(IterationMax) - 1.0);
                    if (c < 1)
                    {
                        color[0] = 0;
                        color[1] = 0;
                        color[2] = 255*c;
                    }
                    else if (c < 2)
                    {
                        color[0] = 0;
                        color[1] = 255*(c-1);
                        color[2] = 255;
                    }
                    else
                    {
                        color[0] = 255*(c-2);
                        color[1] = 255;
                        color[2] = 255;
                    }
                }

                resultBuffer[((iY-processMinY)*iXmaxHalf)+iX][0] = color[0];
                resultBuffer[((iY-processMinY)*iXmaxHalf)+iX][1] = color[1];
                resultBuffer[((iY-processMinY)*iXmaxHalf)+iX][2] = color[2];

            }
        }
        MPI_Send(resultBuffer, linePerProcess * iXmaxHalf, MPI_UNSIGNED_CHAR, 0, 0, MPI_COMM_WORLD);

        free(resultBuffer);

    } else {

        // MPI_Recv(&receivedAlert, 1, MPI_INT, 0, 0, MPI_COMM_WORLD, &stat);
        // printf("rank; %d\n", my_rank);
        resultBufferTwo = malloc(linePerProcess * iXmaxHalf * sizeof(color));

        for(iY = processMinY; iY < processMaxY; iY++) {
            Cy = CyMin + (iY * PixelHeight);
            if (fabs(Cy) < (PixelHeight / 2))
            {
                Cy = 0.0; /* Main antenna */
            }
            for(iX = iXmaxHalf; iX < iXmax; iX++)
            {

                Cx = CxMin + (iX * PixelWidth);
                /* initial value of orbit = critical point Z= 0 */
                Zx = 0.0;
                Zy = 0.0;
                Zx2 = Zx * Zx;
                Zy2 = Zy * Zy;

            /* */
                for(Iteration = 0; Iteration < IterationMax && ((Zx2 + Zy2) < ER2); Iteration++)
                {
                    Zy = (2 * Zx * Zy) + Cy;
                    Zx = Zx2 - Zy2 + Cx;
                    Zx2 = Zx * Zx;
                    Zy2 = Zy * Zy;
                };

            /* compute  pixel color (24 bit = 3 bytes) */
                if (Iteration == IterationMax)
                {
                    // Point within the set. Mark it as black
                    color[0] = 0;
                    color[1] = 0;
                    color[2] = 0;
                }
                else 
                {
                    // Point outside the set. Mark it as white
                    double c = 3*log((double)Iteration)/log((double)(IterationMax) - 1.0);
                    if (c < 1)
                    {
                        color[0] = 0;
                        color[1] = 0;
                        color[2] = 255*c;
                    }
                    else if (c < 2)
                    {
                        color[0] = 0;
                        color[1] = 255*(c-1);
                        color[2] = 255;
                    }
                    else
                    {
                        color[0] = 255*(c-2);
                        color[1] = 255;
                        color[2] = 255;
                    }
                }


                resultBufferTwo[((iY-processMinY)*iXmaxHalf)+(iX - iXmaxHalf)][0] = color[0];
                resultBufferTwo[((iY-processMinY)*iXmaxHalf)+(iX - iXmaxHalf)][1] = color[1];
                resultBufferTwo[((iY-processMinY)*iXmaxHalf)+(iX - iXmaxHalf)][2] = color[2];
                // printf("rank: %d - value: %u%u%u\n", my_rank,resultBufferTwo[((iY-processMinY)*iXmax)+iX][0],resultBufferTwo[((iY-processMinY)*iXmax)+iX][1],resultBufferTwo[((iY-processMinY)*iXmax)+iX][2]);

            }
        }
        MPI_Send(resultBufferTwo, iXmaxHalf * linePerProcess, MPI_UNSIGNED_CHAR, 0, 0, MPI_COMM_WORLD);

        free(resultBufferTwo);

    }

    if (my_rank == 0) {

        endTime = MPI_Wtime();
        printf("Process time (s): %lf\n", endTime - startTime);
        /*create new file,give it a name and open it in binary mode  */
        fp = fopen(filename, "wb"); /* b -  binary mode */

        /*write ASCII header to the file (PPM file format)*/
        fprintf(fp,"P6\n %s\n %d\n %d\n %d\n", comment, iXmax, iYmax, MaxColorComponentValue);
        for(iY = 0; iY < iYmax; iY++)
        {
            for(iX = 0; iX < iXmax; iX++)
                {
                    fwrite(finalResultBuffer[(iY*iXmax)+iX], 1, 3, fp);
                }
        }

        fclose(fp);

        free(finalResultBuffer);
    }

    MPI_Finalize();
    return 0;
 }

如果有人能帮助我理解我在这里做错了什么，我将不胜感激。

Answer 1

问：我在这里做错了什么？

• 代码不计算复平面上的分形，只复制常量color[]
• my_rank == 0 处理收集其他人生成的结果（分形生成器是迭代的，因此在复平面中每个 [X,iY]-point 具有不同的运行时间）并且它本身在定向时存储来自未初始化/未修改的相同常数值的常数值的color[] == [0,0,0] 到整个finalResultBuffer[][]
• 您没有像上面声明的那样在 P1-P6 之间拆分工作，但是您的代码将代码执行路径拆分为三种情况 - 1) my_rank == 0（“master”，即收集结果（它本身不计算一点 cmplex 迭代器，而是存储黑点，指示的地方）并写出一个文件），2) my_rank % 2 == 0（所有非零偶数排名，没有一个执行一个步骤来实际计算复杂的分形迭代器并且所有存储黑点无处不在），3） my_rank % 2 == 1（所有奇数排名，其中没有一个步骤可以实际计算复杂的分形迭代器，但所有商店到处都是黑点）