16 Language support library [language.support]

16.6 Dynamic memory management [support.dynamic]

16.6.5 Hardware interference size [hardware.interference]

inline constexpr size_t hardware_destructive_interference_size = implementation-defined;
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;
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
]