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Erlang / Erlang Advanced Interview questions

What is a dirty scheduler and when should a NIF use one?

A regular NIF runs inline on a normal BEAM scheduler thread, which is fine for short operations but dangerous for anything that blocks or takes more than roughly a millisecond, since it stalls that entire scheduler and every process waiting behind it. Dirty schedulers are a separate pool of OS threads reserved for long-running native work, so it doesn't compete with the normal schedulers running lightweight Erlang processes.

There are two dirty scheduler pools:

  • Dirty CPU schedulers — for CPU-intensive native computation (heavy math, compression).
  • Dirty I/O schedulers — for blocking I/O operations (file access, slow syscalls).

A NIF opts in by declaring itself with ERL_NIF_DIRTY_JOB_CPU_BOUND or ERL_NIF_DIRTY_JOB_IO_BOUND in its registration table, and the runtime routes calls to that function onto the appropriate dirty pool instead of a regular scheduler.

Why can a long-running NIF on a regular scheduler be dangerous?
What is the difference between the dirty CPU and dirty I/O scheduler pools?

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