什么是memory_order_relaxed的一些用例 [英] What are some use cases for memory_order_relaxed

问题描述

C ++内存模型具有放松的原子，不对存储器操作提供任何排序​​保证。除了我在这里找到的C中的邮箱示例：

The C++ memory model has relaxed atomics, which do not put any ordering guarantees on memory operations. Other than the mailbox example in C which I have found here:

http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1525.htm

基于本文中的激励示例：

Based on the motivating example in this paper:

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2153.pdf

我很好奇这种类型的同步机制的其他用例。

I was curious about other use cases for this type of synchronization mechanism.

推荐答案

我在我的工作中经常看到一个统计数据库。如果你
想计算一个事件发生的次数，但不需要任何类型的
同步跨线程，除了使增量安全，使用
memory_order_relaxed 有意义。

A simple example that I see in my work frequently is a stats counter. If you want to count the number of times an event happens but don't need any sort of synchronization across threads aside from making the increment safe, using memory_order_relaxed makes sense.

static std::atomic<size_t> g_event_count_;

void HandleEvent() {
  // Increment the global count. This operation is safe and correct even
  // if there are other threads concurrently running HandleEvent or
  // PrintStats.
  g_event_count_.fetch_add(1, std::memory_order_relaxed);

  [...]
}

void PrintStats() {
  // Snapshot the "current" value of the counter. "Current" is in scare
  // quotes because the value may change while this function is running.
  // But unlike a plain old size_t, reading from std::atomic<size_t> is
  // safe.
  const size_t event_count =
      g_event_count_.load(std::memory_order_relaxed);

  // Use event_count in a report.
  [...]
}

需要使用更强的内存顺序。在某些
平台上，这样做可能会对性能产生负面影响。

In both cases, there is no need to use a stronger memory order. On some platforms, doing so could have negative performance impact.

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