atomics

A sum reads every byte once and does one add per element, so the only question is whether you reach the bandwidth ceiling. The ceiling in milliseconds, four versions from an atomic per element to warp shuffles with sixteen loads in flight, and the bytes-in-flight math behind the last jump.

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.