also Erlang "threads" are called processes but are not OS threads nor processes - there is generally one scheduler per core and the processes are swapped in and out by the scheduler.
They're more like coroutines, but they can be safely pre-empted because Erlang has very little mutable data.
Erlang's scheduler also takes dataflow (e.g. which processes have new messages to process?) into account, so some kinds of overhead are O(active processes) rather than O(processes).
