21 Language support library [language.support]

21.6 Dynamic memory management [support.dynamic]

21.6.5 Hardware interference size [hardware.interference]

inline constexpr size_t hardware_destructive_interference_size = implementation-defined;

1
#

This number is the minimum recommended offset between two concurrently-accessed objects to avoid additional performance degradation due to contention introduced by the implementation. It shall be at least alignof(max_­align_­t).

[ Example: 

struct keep_apart {
  alignas(hardware_destructive_interference_size) atomic<int> cat;
  alignas(hardware_destructive_interference_size) atomic<int> dog;
};

 — end example ]

inline constexpr size_t hardware_constructive_interference_size = implementation-defined;

2
#

This number is the maximum recommended size of contiguous memory occupied by two objects accessed with temporal locality by concurrent threads. It shall be at least alignof(max_­align_­t).

[ Example: 

struct together {
  atomic<int> dog;
  int puppy;
};
struct kennel {
  // Other data members...
  alignas(sizeof(together)) together pack;
  // Other data members...
};
static_assert(sizeof(together) <= hardware_constructive_interference_size);

 — end example ]