如何告诉openmp不要同步一个数组 [英] How to tell openmp not to synchronize an array

问题描述

  #pragma omp parallel for 
 for（i = 0; i  {
 ..... 
 index = get_index（...）; 
 array [index] = ...; 
 ..... 
} 
  

现在， index 对于每个线程来说都是唯一的（它不会为不同的线程重叠），但是当然OpenMP不能对此进行猜测，我想使用同步对象来访问



如何让openmp不要为 array 并依靠我， index 值对于不同的线程是唯一的。我尝试在私人列表中放入 array ，但是出现了分段错误。


解决方案

结构在结尾有一个隐含的障碍。

但是openmp不会通过访问为你同步内存。相反，它为每个线程提供共享和私有内存块的机制。

在你的情况下， index 需要是私人的。否则，当你访问 array [index] 时，每个线程写入相同的内存位置，并且你将遇到竞争状态。

为了演示，我明确设置了行为，虽然默认情况下 i 是私有的， array 默认情况下是共享的。

  #pragma omp parallel for private（i，index）shared（array）
 for（i = 0; i < N; i ++）
 {
 ..... 
 index = get_index（...）; 
 array [index] = ...; 
 ..... 
} 
  

I have a code that has the following structure.

#pragma omp parallel for
for( i = 0; i < N; i++ )
{
    .....
    index = get_index(...);
    array[index] = ...;
    .....
}

Now the value of index is unique for each thread (it never gets overlapped for different threads), but of course OpenMP can't make a guess for this and I suppose is using synchronization objects to access array.

How can I ask openmp not to use synchronization objects for array and rely on me that index value is unique for different threads. I try to put array in private list, but got segmentation fault for that.

解决方案

Openmp does synchronize at barriers or implicit barriers. For example, there is an implicit barrier at the end of a for construct unless a nowait clause is specified.

But openmp does not synchronize memory by access for you. Instead, it provides mechanisms for shared and private memory blocks for each thread.

In your case, index needs to be private. Otherwise each thread writes to the same memory location and you will have a race condition, when you access array[index].

For demonstration, I set the behavior explicitly, although i is private by default and array shared by default.

#pragma omp parallel for private(i, index) shared(array)
for( i = 0; i < N; i++ )
{
    .....
    index = get_index(...);
    array[index] = ...;
    .....
}

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