NIFs are executed by regular scheduled threads of the VM. See [the Erlang docs] (erl_nif — OTP 29.0.2 (erts 17.0.2)) for the big fat warning on long-running work, special care should be taken when a call takes more time than about 1ms. I usually just scheduled NIFs on an async scheduler (dirty CPU mostly) when the NIF was supposed to work for some longer time.
Note that Rustler doesn’t implement all of erl_nif, for example creating threads in the VM is not implemented as rust threads can be spawned if necessary.
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