scala actors vs threads and blocking IO

This is not a correctness problem because the actor library will spawn more threads as necessary (is that right?)

So far as I understand, that is not right. The actor is blocked, and sending another message to it causes that message to sit in the actors mailbox until that actor can receive it or react to the message.

In Programming in Scala (1), it explicitly states that actors should not block. If an actor needs to do something long running it should pass the work to a second actor, so that the main actor can free itself up and go read more messages from its mailbox. Once the worker has completed the work, it can signal that fact back to the main actor, which can finish doing whatever it has to do.

Since workers too have mailboxes, you will end up with several workers busily working their way through the work. If you don't have enough CPU to handle that, their queues will just get bigger and bigger. Eventually you can scale out by using remote actors. Akka might be more useful in such cases.

(1) Chapter 32.5 of Programming in Scala (Odersky, Second edition, 2010)

EDIT: I found this:

The scheduler method of the Actor trait can be overridden to return a ResizableThreadPoolScheduler, which resizes its thread pool to avoid starvation caused by actors that invoke arbitrary blocking methods.

Found it at: http://www.scala-lang.org/api/current/scala/actors/Actor.html

So, that means depending on the scheduler impl you set, perhaps the pool used to run the actors will be increased. I was wrong when I said you were wrong :-) The rest of the answer still holds true.

How thing work in erlang is that all blocking operation should be don by send message because when your actor is blocked waiting for a message it's yielding the thread to other actor.

So if you want to do some blocking operation like reading from a file you should do a FileReader actor that use Non-bloking api to read and write from file. And have your other actor use this actor (send and receive message to it) as an api to read and write to file.