I'm typing this all from memory so I might be wrong, but the short answer is that gen_tcp:recv/2 is a blocking call. There is a private API called prim_inet:asy

I'm typing this all from memory so I might be wrong, but the short answer is that gen_tcp:recv/2 is a blocking call. There is a private API called prim_inet:async_recv which is async, and ironically gen_tcp builds on it. There have been attempts writing experimental HTTP servers using prim_inet:async_recv which have much better performance,