dan-cooke
Hi all,
I’m just learning elixir for fun , and I would love to understand a little bit more about the memory allocations that are made during the following code snippet taken from the Mix OTP tutorial:
defp loop_acceptor(socket) do
{:ok, client} = :gen_tcp.accept(socket)
{:ok, pid} = Task.Supervisor.start_child(KVServer.TaskSupervisor, fn -> serve(client) end)
:ok = :gen_tcp.controlling_process(client, pid)
loop_acceptor(socket)
end
Heres what I am thinking so far:
loop_acceptorfunction is called andsocketis placed onto the stack for this function call:gen_tcp.accept(socket)is called which creates a socket resource calledclient, this is currently owned by the process that calledloop_acceptorTask.Supervisor.start_childis called, the anon function creates a closure which captures theservefunction and theclient.
Thats where I get stuck.
- Does this closure which runs in a new process result in the variables it captures being “copied” into the new process heap/stack?
- If so.. how is it possible to “copy” a resource like a socket without creating multiple OS sockets for the same connection?
- As the serve function belongs to the surrounding
KVServermodule does this mean that the supervised task gets a copy of the KVServer module too?
If anyone has any good documentation to learn more about BEAM memory, or perhaps some tips on how I can debug these allocations myself.
I’m very green with BEAM
Any help is greatly appreciated, thanks!
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derek-zhou
The authoritative source of BEAM internal is the Beam book:
It really depends on how deep you want to go. From the 10,000 foot above, you can consider all values are allocated on the per-process heap and are immutable. So there will not be cyclic references, GC is straight forward, and closure is easy to implement.
Of cause there are exceptions, but the native code hide the ugly parts and maintain the happy illusion.
al2o3cr
#1: yes, the closure captures its environment. Rebinding
clientafter the call toTask.Supervisor.start_childwon’t change the value the child process sees.#2: the thing that’s returned from
:gen_tcp.acceptis usually a “port”; short short version it’s a way to name the thing that’s actually holding the OS socket. Callingcontrolling_processtells the port that it should send messages etc to the newly-started task instead of the process that originally called:gen_tcp.accept.#3: there’s no state attached to the module
KVServer, so asking whether there’s a second “copy” of it isn’t particularly meaningful. There’s some very tricky corner-cases around hot-code reloading that make callingserve(client)different from callingKVServer.serve(client)(even from inside a function defined inKVServer!) but most people will never encounter them.