False sharing of guarded member variables?

Question

Consider:

class Vector
{
  double x, y, z;
  // …
};

class Object
{
  Vector Vec1, Vec2;
  std::mutex Mtx1, Mtx2;

  void ModifyVec1() { std::lock_guard Lock(Mtx1); /* … */ }
  void ModifyVec2() { std::lock_guard Lock(Mtx2); /* … */ }
};

If either the mutexes or the guarded variables are stored contiguously and they share a cache line when cached, can this cause a sort of “cross-locking”?

If so, is it a good practice to declare the mutexes right after (or before) the variable they guard?

将类与std::hardware_destructive_interference_size( P0154 )对齐可能会避免这种影响。对象的过度对齐是否值得潜在的好处？

Answer 1

您的变量在您的问题中似乎无关，所以而不是hardware_destructive_interference_size您可能想要的hardware_constructive_interference_size：

struct keep_together {
    std::mutex m;
    Vector v;
};

alignas(std::hardware_constructive_interference_size) keep_together k1;
alignas(std::hardware_constructive_interference_size) keep_together k2;

destructive您想要用于无锁队列等情况，线程正在读取两个不同的atomics 并且您想要确保它们都实际被加载。如果这是一个问题，您需要解释为什么您要避免错误共享。

在它所保护的变量之后（或之前）声明互斥锁以增加它们位于同一缓存行的机会是一个好习惯吗？

这就是constructive干扰。