[erlang-questions] About Garbage Collection

John Hughes <>
Wed Nov 10 07:53:44 CET 2010


Hi Volkan,

Message passing copies the message, but function arguments and results are not copied--they live on the local process heap. Hence memory cannot be deallocated on a return.

John 

Sent from my iPhone

On 10 Nov 2010, at 07:19, Ş. Volkan Erdoğan <> wrote:

> Hi,
> I have an idea about memory management in Erlang and although the idea
> is still raw I'd like to share with you.
> 
> As far as I know:
> - There are no global variables in Erlang. Memory is allocated in functions.
> - Function calls in processes and message passing operations to other
> processes have copying semantics.
> 
> So I believe that these properties guarantee that any memory location
> allocated by runtime is exclusive to the function which requested that
> memory and it is not possible for a reference of a memory location to
> escape from the function that created it. And as a result it is
> possible to safely deallocate all memory allocated by the function at
> function exit since those memory locations can only be accessed by
> that function. However I think it is possible to do better because
> Erlang has single assignment property and one can easily find the last
> location that a variable is used and can safely deallocate memory
> after that location. Also we can relax copying semantics for some type
> of function calls and pass a reference to the functions since compiler
> prevents variable modification (I am not sure if runtime is already
> doing this). To enable these operations following modifications are
> needed to be done in Erlang runtime and compiler:
> - Erlang virtual machine will need a new instruction for memory
> deallocation operations (This operation will be called DEL)
> - The compiler should pass references for parameters instead of
> copying them during function calls
> 
> After these modifications are done, the analysis algorithm described
> below can be executed for each function before that function is loaded
> to the virtual machine
> For each variable including the formal parameters of the function
> -Find last usage of the variable
> -If last usage of the parameter is in a function call
>    -do nothing (since it will be analyzed or already analyzed in that
> function as formal parameter)
> -Else
>    -Insert a DEL instruction for that variable just after the usage
> 
> To sum up I believe the properties of the language lets an analyzer to
> exactly find the life times of variables created in functions (btw ETS
> tables might need special care). This is much like working with heap
> memory in C, only this time an external program is inserting free
> operations.
> 
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