performance penalty of message passing as opposed to shared data

Question

There is a lot of buzz these days about not using locks and using Message passing approaches like Erlang. Or about using immutable datastructures like in Functional programming

Answer 1

1. Erlang provides supervisors and gen_server callbacks for synchronous calls, so you will know about it if a message isn't delivered: either the gen_server call returns a timeout, or your whole node will be brought down and up if the supervisor is triggered.
2. usually if the processes are on the same node, message-passing languages optimise away the data copying, so it's almost like shared memory, except if the object is changed used by both afterward, which can not be done using shared memory either anyways
3. There is some state which is kept by processes by passing it around to themselves in the recursive tail-calls, also some state can be of course passed through messages. I don't use mnesia much, but it is a transactional database, so once you have passed the operation to mnesia (and it has returned) you are pretty much guaranteed it will go through..
4. Which is why it is easy to tie such applications into erlang with the use of ports or drivers. The easiest are the ports, it's much like a unix pipe, though I think performance isn't that great...and as said, message-passing usually ends up just being pointer passing anyways as the VM/compiler optimise the memory copy out.