cache_aligned_allocator¶
[memory_allocation.cache_aligned_allocator]
A cache_aligned_allocator is a class template that models the allocator requirements from the [allocator.requirements] ISO C++ section.
The cache_aligned_allocator allocates memory on cache line boundaries, in order to avoid false sharing and potentially improve performance.
False sharing is a situation when logically distinct items occupy the same cache line,
which can hurt performance if multiple threads attempt to access the different items simultaneously.
Even though the items are logically separate, the processor hardware may have to transfer the cache line between the processors
as if they were sharing a location. The net result can be much more memory traffic than if the logically distinct items were on different cache lines.
However, this class is sometimes an inappropriate replacement for default allocator, because the benefit of allocating on a cache line comes at the price
that cache_aligned_allocator implicitly adds pad memory. Therefore allocating many small objects with cache_aligned_allocator may increase memory usage.
// Defined in header <tbb/cache_aligned_allocator.h>
namespace tbb {
template<typename T> class cache_aligned_allocator {
public:
using value_type = T;
using size_type = std::size_t;
using propagate_on_container_move_assignment = std::true_type;
using is_always_equal = std::true_type;
cache_aligned_allocator() = default;
template<typename U>
cache_aligned_allocator(const cache_aligned_allocator<U>&) noexcept;
T* allocate(size_type);
void deallocate(T*, size_type);
size_type max_size() const noexcept;
};
}
Member Functions¶
-
T *
allocate(size_type n)¶ Returns a pointer to the allocated
n * sizeof(T)bytes of memory, aligned on a cache-line boundary. The allocation may include extra hidden padding.
Non-member Functions¶
These functions provide comparison operations between two cache_aligned_allocator instances.
template<typename T, typename U>
bool operator==(const cache_aligned_allocator<T>&,
const cache_aligned_allocator<U>&) noexcept;
template<typename T, typename U>
bool operator!=(const cache_aligned_allocator<T>&,
const cache_aligned_allocator<U>&) noexcept;
The namespace where these functions are defined is unspecified, as long as they may be used in respective binary operation expressions on cache_aligned_allocator objects.
For example, an implementation may define the classes and functions in the same unspecified internal namespace,
and define tbb::cache_aligned_allocator as a type alias for which the non-member functions are reachable only via argument-dependent lookup.