[erlang-questions] Dirty CPU scheduler and pure Erlang code

[Michaël COQUARD]

Hi,

I try to understand how the processes are scheduled on the dirty schedulers.

As I understand, the dirty schedulers are meant to execute uninterruptible native code to avoid blocking the standard schedulers (and other Erlang 
processes) for a long time.

I noticed the majority of all memory allocations are scheduled on the dirty schedulers: mmap(), munmap() and mremap() calls from the mseg_alloc and 
sys_alloc

Now, consider the following code:
-module(foo).
-compile([export_all,nowarn_export_all]).

loop_reverse_erl() ->
     L = lists:seq(1,1000000),
     loop_reverse_erl(L).

loop_reverse_erl(L) ->
     my_reverse(L,[]),
     loop_reverse_erl(L).

% vanilla reverse written in pure Erlang from efficiency_guide.erl
my_reverse([H|T], Acc) -> my_reverse(T, [H|Acc]);
my_reverse([], Acc) -> Acc.

When I launch the loop (foo:loop_reverse_erl().) from the shell, I see an activity on one of a dirty scheduler thread (with top -H -p <pid of beam.smp>)

The dirty scheduler doesn't do anything on a simple infinite loop like this
loop_reverse_erl(L) ->
     loop_reverse_erl(L).

I was expecting only native functions (BIFs / NIFs) are executed on the dirty schedulers.

Is it the expected behavior ? If yes, which conditions trigger the choice of the "dirty" or "normal" scheduler ?

OS: Linux RHEL 7
Erlang version: OTP/21 erts-10.0 (same on OTP/20)

Michael