erts_milli_sleep is only used in testing and on operating systems without a monotonic time source.
What is used to sleep is either futex or WaitForSingleObject with some spinning done around it.
This is the relevant code for unix: otp/erts/lib_src/pthread/ethr_event.c at master · erlang/otp · GitHub.
When sleeping in poll, timerfd_create(timerfd_create(2) - Linux manual page) is used to increase the resolution of the timer when triggered.
