memory_order_relaxed有哪些用例

Question

C++内存模型具有宽松的原子性,它不会对内存操作进行任何排序保证.除了我在这里找到的C中的邮箱示例:

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

基于本文中的激励示例:

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

我很好奇这种同步机制的其他用例.

Answer 1

我在工作中经常看到的一个简单例子是统计计数器.如果要计算事件发生的次数,但除了使增量安全之外不需要跨线程的任何类型的同步,使用 memory_order_relaxed是有意义的.

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.
  [...]
}

在这两种情况下,都不需要使用更强的内存顺序.在某些平台上,这样做会对性能产生负面影响.