什么是memory_order_relaxed的一些用例

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:

基于本文中的激励示例：

Based on the motivating example in this paper:

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 youwant to count the number of times an event happens but don't need any sort ofsynchronization across threads aside from making the increment safe, usingmemory_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 someplatforms, doing so could have negative performance impact.

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