On: Shared_ptr<T>: the (not always) atomic reference counted smart pointer (2019)

[Tuna-Fish]

On modern CPUs atomic adds are now reasonably fast, but only when they are uncontended. If the cache line the value is on has to bounce between cpus, that is usually +100ns (not cycles) or so.

Writing performant parallel code always means absolutely minimizing communication between threads.

[loeg → Tuna-Fish]

Sure, but even the uncontended case is ~10x slower than regular ADD.