Technically, why are processes in Erlang more efficient than OS threads?

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There are several contributing factors:

  1. Erlang processes are not OS processes. They are implemented by the Erlang VM using a lightweight cooperative threading model (preemptive at the Erlang level, but under the control of a cooperatively scheduled runtime). This means that it is much cheaper to switch context, because they only switch at known, controlled points and therefore don’t have to save the entire CPU state (normal, SSE and FPU registers, address space mapping, etc.).
  2. Erlang processes use dynamically allocated stacks, which start very small and grow as necessary. This permits the spawning of many thousands — even millions — of Erlang processes without sucking up all available RAM.
  3. Erlang used to be single-threaded, meaning that there was no requirement to ensure thread-safety between processes. It now supports SMP, but the interaction between Erlang processes on the same scheduler/core is still very lightweight (there are separate run queues per core).

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