Since serde_rustler is a NIF that doesn’t yield, it should be running on dirty schedulers - after a quick glance at the code, I think it doesn’t do that right now. In production environment, NIFs that don’t yield can wreck havoc on the runtime. BEAM relies on the fact that scheduler threads checkpoint frequently with some VM services. Arranging NIF code so that in can yield properly usually has noticeable runtime cost - that’s probably the main source of speed-up over jiffy for your code. Running on dirty schedulers, on the other hand, has the context switch overhead, but more importantly has the caveats of running on a thread pool - if you exhaust the thread pool, the operations become blocking in practice.
What I meant about the parallel option is that the variations in the results are going to be much higher - that’s expected and has various sources - the VM overhead, other processes in the OS, etc.
