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

[Michaël COQUARD]

Hi Peti,

Thank you for your quick response, this is exactly what I was looking for. It was the garbage collector activity.

Michael

Le 19/09/2018 à 17:32, Peti Gömöri a écrit :
> Hi Michael,
>
> With the module `msacc` you can get more info about what the dirty scheduler is busy with. For example it can do garbage collection too.
>
> On Wed, Sep 19, 2018 at 5:19 PM, Michaël COQUARD <michael.coquard@REDACTED <mailto:michael.coquard@REDACTED>> wrote:
>
>     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
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>

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