> They are far apart on the chip, and sometimes even on different chips with some link between. They aren't always. On a NUMA machine some are closer than othe

> They are far apart on the chip, and sometimes even on different chips with some link between. They aren't always. On a NUMA machine some are closer than others; M-series is an example. The cores are in clusters where some of the cache is shared, so atomics are cheap as long as it doesn't leave the cluster.