How, if at all, do Erlang Processes map to Kernel Threads?

Question

Erlang is known for being able to support MANY lightweight processes; it can do this because these are not processes in the traditional sense, or even threads like in P-thre

Answer 1

I would like to add some input to what was described in the accepted answer.

Erlang Scheduler is the essential part of the Erlang Runtime System and provides its own abstraction and implementation of the conception of lightweight processes atop the OS threads.

Each Scheduler runs within a single OS thread. Normally, there are as many schedulers as CPU (cores) are on he hardware (it is configurable though and naturally does not bring much value when number of schedulers exceeds those of hardware cores). The system might also be configured that scheduler will not jump between OS threads.

Now, when the Erlang process is being created it is entirely the responsibility of the ERTS and Scheduler to manage life cycle and resources consumption as well as its memory footprint etc.

One of the core implementation details is that each process has a time budget of 2000 reductions available when the Scheduler picks up that process from the run queue. Each progress in the system (even I/O) is guaranteed to have a reductions budget. That is what actually makes ERTS a system with preemptive multitasking.

I would recommend a great blog post on that topic by Jesper Louis Andersen http://jlouisramblings.blogspot.com/2013/01/how-erlang-does-scheduling.html

As the short answer: Erlang processes are not OS threads and do not map to them directly. Erlang Schedulers are what runs on the OS threads and provide smart implementation of more finely grained Erlang processes hiding those details behind programmer's eyes.