使用OpenMP计算圆周率值 [英] Using OpenMP to calculate the value of PI

问题描述

我想学习如何通过并行蒙特卡罗code，计算PI与迭代给定数量的值使用OpenMP的。在code的肉是这样的：

I'm trying to learn how to use OpenMP by parallelizing a monte carlo code that calculates the value of PI with a given number of iterations. The meat of the code is this:

  int chunk = CHUNKSIZE;                                                                                      

    count=0;                                                                                                  
#pragma omp parallel shared(chunk,count) private(i)                                                           
  {                                                                                                           


#pragma omp for schedule(dynamic,chunk)                                                                       
      for ( i=0; i<niter; i++) {                                                                              
        x = (double)rand()/RAND_MAX;                                                                          
        y = (double)rand()/RAND_MAX;                                                                          
        z = x*x+y*y;                                                                                          
        if (z<=1) count++;                                                                                    
      }                                                                                                       
  }                                                                                                           

  pi=(double)count/niter*4;                                                                                   
  printf("# of trials= %d , estimate of pi is %g \n",niter,pi);  

尽管这不是产生对给定的10,000次重复圆周率的适当值。如果所有的OpenMP的东西取出来，它工作正常。我要指出，我用蒙特卡罗code从这里： HTTP： //www.dartmouth.edu/~rc/classes/soft_dev/C_simple_ex.html

我只是用它来尝试学习OpenMP的。任何想法，为什么它的收敛于1.4ish？我不能增加多线程的变量？我猜问题是与变量计数。

I'm just using it to try to learn OpenMP. Any ideas why it's converging on 1.4ish? Can I not increment a variable with multiple threads? I'm guessing the problem is with the variable count.

谢谢！

推荐答案

好吧，我找到了答案。我需要使用的减少子句。因此，所有我不得不修改为：

Okay, I found the answer. I needed to use the REDUCTION clause. So all I had to modify was:

#pragma omp parallel shared(chunk,count) private(i)

到

#pragma omp parallel shared(chunk) private(i,x,y,z) reduction(+:count)

现在它汇聚在3.14 ...耶

Now it's converging at 3.14...yay

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