[erlang-questions] moving process state - copy on write - heap memory increase

Roland <>
Tue Sep 11 23:45:18 CEST 2012


Hello!

I want to move the state of my process to another process. Unfortunately 
this is not possible without a huge increase in memory consumption as by 
copying the state the "copy on write semantics" are lost.

This behaviour is demonstrated in my attached simplified t_process module.

-> {ok, Pid} = t_process:start_link().
-> t_process:bloat(Pid). --> memory consumption of the process is at 
6657256 after "bloat" is finished
-> t_process:reload_bag(P1). --> memory consumption is at 26295176 (4 
times as much)

No data has not been modified in any way, the data stays the same.

My question is: Despite what i want to do with this state transfer, is 
there any way to copy the state of a process without losing the "copy on 
write semantics", so that the target process has the same memory 
footprint as the original one?

Can someone point me to any direction/documentation on how i can get 
arround this issue?

Thank you!

-module(t_process).

-behaviour(gen_server).

-export([start_link/0, init/1, handle_call/3, handle_cast/2, 
handle_info/2, terminate/2, code_change/3, bloat/1, reload_bag/1]).

-record(process, {bag = []}).

start_link() ->
	gen_server:start_link(?MODULE, [], []).

bloat(Pid) ->
     gen_server:cast(Pid, bloat).

reload_bag(Pid) ->
     gen_server:call(Pid, reload_bag).

init([]) ->
	{ok, #process{}}.

handle_call(reload_bag, _From, State) ->
     BinaryBag = erlang:term_to_binary(State#process.bag), % simulate 
state transfer / loss of semantics
     io:format("Bag size is ~w.", [erlang:size(BinaryBag)]),
     {reply, ok, State#process{bag = erlang:binary_to_term(BinaryBag)}};

handle_call(_Call, _From, State) ->
     {reply, ok, State}.

handle_cast(bloat, State) ->
     NewBag = lists:concat([State#process.bag, lists:foldl(fun(_C, List) 
-> List ++ [{"ABCDEFGHIJKLMNOPQRSTUVWXYZ"}] end, [], lists:seq(1, 50000))]),
     io:format("Bloating finished."),
     {noreply, State#process{bag = NewBag}};

handle_cast(_Cast, State) ->
     {noreply, State}.

handle_info(_Info, State) ->
     {noreply, State}.

terminate(_Reason, _State) ->
     ok.

code_change(_OldVsn, State, _Extra) ->
     {ok, State}.




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