OpenMP的鍛鍊omp_bug2.c

Question

這是OpenMP的網站上練習： https://computing.llnl.gov/tutorials/openMP/exercise.html

#include "stdafx.h" 
#include <omp.h> 
#include <stdio.h> 
#include <stdlib.h> 

int _tmain(int argc, _TCHAR* argv[]) 
{ 
int nthreads, i, tid; 
float total; 

/*** Spawn parallel region ***/ 
#pragma omp parallel private(i, tid) // i changed this line 
    { 
    /* Obtain thread number */ 
    tid = omp_get_thread_num(); 
    /* Only master thread does this */ 
    if (tid == 0) { 
    nthreads = omp_get_num_threads(); 
    printf("Number of threads = %d\n", nthreads); 
    } 
    printf("Thread %d is starting...\n",tid); 

    #pragma omp barrier 

    /* do some work */ 
    total = 0.0; 
    #pragma omp for schedule(dynamic,10) 
    for (i=0; i<1000000; i++) 
    total = total + i*1.0; 

    printf ("Thread %d is done! Total= %e\n",tid,total); 

    } 
} 

的輸出，這是

Number of threads = 4 
Thread 0 is starting... 
Thread 3 is starting... 
Thread 2 is starting... 
Thread 1 is starting... 
Thread 0 is done! Total= 0.000000e+000 
Thread 3 is done! Total= 0.000000e+000 
Thread 2 is done! Total= 0.000000e+000 
Thread 1 is done! Total= 0.000000e+000 

這意味着我們有一個問題可變「總」

這是對網站的幫助

這是我的解決方案：你認爲這是正確的方法嗎？

#include "stdafx.h" 
#include <omp.h> 
#include <stdio.h> 
#include <stdlib.h> 

int _tmain(int argc, _TCHAR* argv[]) 
{ 
int nthreads, i, tid; 
float total; 

/*** Spawn parallel region ***/ 
#pragma omp parallel private(total,tid) 
    { 
    /* Obtain thread number */ 
    tid = omp_get_thread_num(); 
    total= 0.0; 
    /* Only master thread does this */ 
    if (tid == 0) { 
    nthreads = omp_get_num_threads(); 
    printf("Number of threads = %d\n", nthreads); 
    } 
    printf("Thread %d is starting...\n",tid); 



    #pragma omp parallel for schedule(static,10)\ 
    private(i)\ 
    reduction(+:total) 
    for (i=0; i<1000000; i++) 
    total = total + i*1.0; 

    printf ("Thread %d is done! Total= %e\n",tid,total); 

    } /*** End of parallel region ***/ 
} 

這是我的新的輸出：

Number of threads = 4 
Thread 0 is starting... 
Thread 1 is starting... 
Thread 0 is done! Total= 4.999404e+011 
Thread 2 is starting... 
Thread 1 is done! Total= 4.999404e+011 
Thread 2 is done! Total= 4.999404e+011 
Thread 3 is starting... 
Thread 3 is done! Total= 4.999404e+011 

Answer 1

是你一定要total是一個線程私有變量。有一件事你可能會在一個真實的例子做的就是減少線程專用totals在結束一個單一的全球總（和只讓一個線程打印出結果，然後）。一種方法是簡單的

#pragma omp atomic 
global_total += total 

在最後（有更好的方法，雖然使用減少）。

PS：循環計數器omp for默認都是私有的，所以你其實並不需要明確指定。