[phoe-krk]

Correct. Erlang (and therefore Elixir) stdio is implemented by means of sending messages to a process that does low-level writing to a console. When a single message is served, the other are queued up.

Therefore, when two processes (let's name them A and B) want to output something at the same time, the result is going to be AAAABBBB or BBBBAAAA (depending on the order in which the messages arrive in the mailbox), but never BBAABBAA or anything similar.

[gmfawcett]

...where "AAAA" and "BBBB" are two distinct messages, and not two sets of four messages each ("A", "A", "A", "A").

When I first read your example, it sounded like you were saying that the IO process would exhaust all messages from one source process before processing any messages from the other, no matter what order they arrived in.

[phoe-krk]

Yes, correct. I wasn't clear enough.