cpp

One atomic increment is correct and does not scale, because every thread fights for the same cache line. The ordering that is sufficient and why the default is stronger than needed, the padding that removes contention entirely, and the arithmetic showing when sharding is worth it.

The standard forces one heap allocation per entry and a pointer dereference per lookup, and that is a specification requirement rather than a library weakness. The measured cost on this machine, the open-addressing layout that removes it, and the two cases where the standard container is still the right pick.