MPI程序使用MPI_Wtime()计算总时间时出错，求技术支持

问题背景 #

尝试统计每个处理器的计算耗时及程序总耗时，但运行时触发MPI_ERR_RANK错误，此前相同计时逻辑在其他代码中可正常运行。

报错信息 #

[Sid-Laptop:4987] *** An error occurred in MPI_Recv
[Sid-Laptop:4987] *** reported by process [852688897,2]
[Sid-Laptop:4987] *** on communicator MPI_COMM_WORLD
[Sid-Laptop:4987] *** MPI_ERR_RANK: invalid rank
[Sid-Laptop:4987] *** MPI_ERRORS_ARE_FATAL (processes in this communicator will now abort,
[Sid-Laptop:4987] ***    and potentially your MPI job)
Enter a, b and n 
[Sid-Laptop:04980] 2 more processes have sent help message help-mpi-errors.txt / mpi_errors_are_fatal
[Sid-Laptop:04980] Set MCA parameter "orte_base_help_aggregate" to 0 to see all help / error messages

问题分析 #

• 未初始化的source变量：非0进程执行MPI_Recv时，source变量未赋值，导致MPI尝试从非法进程号接收数据，触发MPI_ERR_RANK。数据由进程0发送，需将source固定为0。
• Trap函数缺失返回值：Trap函数计算完积分后无返回语句，导致integral变量值未定义，后续通信和计算出现异常。
• local_b计算逻辑错误：原代码local_b = (local_a + local_n) * h逻辑错误，正确区间终点应为local_a + local_n * h。
• n无法被进程数整除的场景遗漏：当前local_n = n/size只处理了n能被进程数整除的情况，若无法整除会导致部分区间未被计算。

修复后的完整代码 #

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

float Trap(float local_a, float local_b, int local_n, float h);
float f(float x);

int main(int argc, char** argv){
    int my_rank;
    double time1, time2, duration, global;
    int size;
    float a ;
    float b ;
    int n ;
    float h;
    float local_a;
    float local_b;
    int local_n;
    float integral;
    float total;
    int source = 0; // 初始化source为进程0
    int dest = 0;
    int tag = 0;
    MPI_Status status;

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

    if (my_rank == 0){
        printf("Enter a, b and n\n");
        scanf("%f %f %d", &a, &b, &n);
        for ( dest = 1 ; dest < size; dest++){
            MPI_Send(&a, 1 , MPI_FLOAT, dest , 0, MPI_COMM_WORLD);
            MPI_Send(&b, 1 , MPI_FLOAT, dest , 1, MPI_COMM_WORLD);
            MPI_Send(&n, 1 , MPI_INT, dest , 2, MPI_COMM_WORLD);
        }
    }
    else{
        // 从进程0接收数据
        MPI_Recv(&a, 1, MPI_FLOAT, source, 0, MPI_COMM_WORLD, &status);
        MPI_Recv(&b, 1, MPI_FLOAT, source, 1, MPI_COMM_WORLD, &status);
        MPI_Recv(&n, 1, MPI_INT, source, 2, MPI_COMM_WORLD, &status);
    }

    MPI_Barrier(MPI_COMM_WORLD);
    time1 = MPI_Wtime();

    h = (b - a)/n;
    local_n = n / size;
    // 处理n无法被size整除的情况，最后一个进程承担剩余任务
    if(my_rank == size - 1){
        local_n += n % size;
    }
    local_a = a + my_rank * (n / size) * h;
    // 修正区间终点计算逻辑
    local_b = local_a + local_n * h;

    integral = Trap(local_a, local_b, local_n, h);

    if (my_rank == 0){
        total = integral;
        for (source = 1; source < size; source++){
            MPI_Recv(&integral, 1, MPI_FLOAT, source, tag, MPI_COMM_WORLD, &status);
            total += integral;
        }
    }
    else {
        MPI_Send(&integral, 1, MPI_FLOAT, dest, tag, MPI_COMM_WORLD); 
    }

    time2 = MPI_Wtime();
    duration = time2 - time1;
    MPI_Reduce(&duration, &global,1,MPI_DOUBLE,MPI_SUM,0,MPI_COMM_WORLD);      

    if (my_rank == 0){
        printf("With n = %d trapezoids, our estimate\n", n);
        printf("of the integral from %f to %f = %0.8f\n",a,b,total);
        printf("Global runtime is %f\n",global);
    }    
    printf("Runtime at %d is %f\n", my_rank,duration); 

    MPI_Finalize();
    return 0;
}

// 添加返回语句，返回计算好的积分值
float Trap(float local_a, float local_b, int local_n, float h){
    float integral;
    float x;

    integral = (f(local_a) + f(local_b))/2.0;
    x = local_a;
    for (int i = 1; i <= local_n-1; i++){
        x += h;
        integral += f(x);
    }
    integral *= h;
    return integral;
}

float f(float x){
    return x*x;
}

修复说明 #

1. 初始化source为0，确保非0进程从正确节点接收数据；
2. 给Trap函数添加返回语句，传递计算完成的积分值；
3. 修正local_b的计算逻辑，保证区间划分正确；
4. 补充n无法被进程数整除时的处理逻辑，避免遗漏计算区间；
5. 主函数末尾添加return 0;，符合C语言规范。

内容的提问来源于stack exchange，提问作者h3avyc0der