[erlang-questions] gen_server state is not released for gc

Jon Watte <>
Wed Jan 18 18:53:44 CET 2012


Again, the question is whether a smart enough compiler or VM couldn't just
get rid of this reference? Specifically, because nothing is called without
the module specifier, the VM could inline it, and in turn drop the
reference to the binary, right?
Whereas if the call to nothing was made with module name qualification,
then that inline replacement could not take place, and the reference would
have to be retained?

Sincerely,

jw


--
Americans might object: there is no way we would sacrifice our living
standards for the benefit of people in the rest of the world. Nevertheless,
whether we get there willingly or not, we shall soon have lower consumption
rates, because our present rates are unsustainable.



2012/1/16 Björn-Egil Dahlberg <>

>
>
> Den 16 januari 2012 18:24 skrev Jon Watte <>:
>
> Is this an artifact of the current implementation?
>> Semantically, I don't see why a sufficiently smart VM wouldn't be able to
>> collect the binary at that point.
>>
>
> Hmm, yes, I did not answer correctly to the example. (Bad teacher) "A"
> isn't used in the above example so it will be garbed. Let me show you by
> example what I mean.
>
> -module(test).
>
> -compile([export_all]).
>
> go1() ->
>     {ok, A} = file:read_file("test.erl"),
>     go1(A).
>
> go1(A) ->
>     ok = nothing(A),
>     erlang:garbage_collect(),
>     io:format("~p~n", [erlang:process_info(self(), binary)]),
>     ok.
>
> go2() ->
>     {ok, A} = file:read_file("test.erl"),
>     go2(A).
>
> go2(A) ->
>     erlang:garbage_collect(),
>     ok = nothing(A),
>     io:format("~p~n", [erlang:process_info(self(), binary)]),
>     ok.
>
> nothing(_) -> ok.
>
>
> What will the two functions go1 and go2 produce?
>
> Eshell V5.9.1  (abort with ^G)
> 1> test:go1().
> {binary,[]}
> ok
> 2> test:go2().
> {binary,[{11960880,575,2}]}
> ok
>
>
> // Björn-Egil
>
>
>> Sincerely,
>>
>> jw
>>
>>
>> --
>> Americans might object: there is no way we would sacrifice our living
>> standards for the benefit of people in the rest of the world. Nevertheless,
>> whether we get there willingly or not, we shall soon have lower consumption
>> rates, because our present rates are unsustainable.
>>
>>
>>
>> 2012/1/13 Björn-Egil Dahlberg <>
>>
>>> Den 13 januari 2012 23:11 skrev Max Lapshin <>:
>>>
>>> I thought, that calling gc will be a problem:
>>>>
>>>>
>>>> A = crypto:rand_bytes(1024*1024*1024),
>>>> erlang:garbagecollect(),
>>>> {reply, ok, State}
>>>>
>>>>
>>>> Will gc free A? I think that no, because A is still used in the place,
>>>> where gc is called. Am I right?
>>>>
>>>
>>> Correct.
>>>
>>> To be a little technical (and don't take this as an absolute truth in
>>> the details),
>>> What we see here is "A is referencing a large binary", which is true.
>>> What happens internally is that Eterm A is a boxed pointer from the stack
>>> or a register which points to a ProcBin on the heap. The ProcBin in turn
>>> holds an increment of a Reference counted binary off heap. Both the stack
>>> and the registers will become part of the root set during the garbage
>>> collection hence letting the binary be reachable and therefore "live".  I
>>> believe that you need leave the function (release the stack) in order to
>>> free your binary.
>>>
>>> A gc without the A in the stack will not let the ProcBin survive during
>>> the copy phase of the gc. When the ProcBin is removed the RefcBinary will
>>> decrement and if it reaches zero, i.e. the last ProcBin has died, that too
>>> will be removed.
>>>
>>> // Björn-Egil
>>>
>>> _______________________________________________
>>> erlang-questions mailing list
>>> 
>>> http://erlang.org/mailman/listinfo/erlang-questions
>>>
>>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://erlang.org/pipermail/erlang-questions/attachments/20120118/72fda9a0/attachment.html>


More information about the erlang-questions mailing list