Understanding std::hardware_destructive_interference_size and std::hardware_constructive_interference_size

Question

C++17 added std::hardware_destructive_interference_size and std::hardware_constructive_interference_size. First, I thought it is just a portable way to get the size of a L1

Answer 1

I would almost always expect these values to be the same.

Regarding above, I would like to make a minor contribution to the accepted answer. A while ago, I saw a very good use-case where these two should be defined separately in the folly library. Please see the caveat about Intel Sandy Bridge processor.

https://github.com/facebook/folly/blob/3af92dbe6849c4892a1fe1f9366306a2f5cbe6a0/folly/lang/Align.h

//  Memory locations within the same cache line are subject to destructive
//  interference, also known as false sharing, which is when concurrent
//  accesses to these different memory locations from different cores, where at
//  least one of the concurrent accesses is or involves a store operation,
//  induce contention and harm performance.
//
//  Microbenchmarks indicate that pairs of cache lines also see destructive
//  interference under heavy use of atomic operations, as observed for atomic
//  increment on Sandy Bridge.
//
//  We assume a cache line size of 64, so we use a cache line pair size of 128
//  to avoid destructive interference.
//
//  mimic: std::hardware_destructive_interference_size, C++17
constexpr std::size_t hardware_destructive_interference_size =
    kIsArchArm ? 64 : 128;
static_assert(hardware_destructive_interference_size >= max_align_v, "math?");

//  Memory locations within the same cache line are subject to constructive
//  interference, also known as true sharing, which is when accesses to some
//  memory locations induce all memory locations within the same cache line to
//  be cached, benefiting subsequent accesses to different memory locations
//  within the same cache line and heping performance.
//
//  mimic: std::hardware_constructive_interference_size, C++17
constexpr std::size_t hardware_constructive_interference_size = 64;
static_assert(hardware_constructive_interference_size >= max_align_v, "math?");