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    <p>It is really hard to figure out what you are actually asking here, but here are some pointers:</p> <ul> <li><p>If you are running on a modern JVM, there is typically a one-to-one relationship between Java threads and OS threads. (IIRC, Solaris allows you to do this differently ... but that's the exception.)</p></li> <li><p>The amount of real parallelism you will get using threads, or anything built on top of threads is limited by the number of processors / cores that are available to the application. Beyond that, you will find that not all threads are actually executing at any given instant.</p></li> <li><p>If you have 1000 Actors all trying to access the database "at the same time", then most of them will actually be waiting on the database itself, or on the thread scheduler. Whether this amounts to making 1000 sequential requests (i.e. strict serialization) will depend on the database and the queries / updates that the actors are doing.</p></li> </ul> <p>The bottom line is that a computer system has hard limits on the resources available for doing stuff; e.g. number of processors, speed of processors, memory bandwidth, disc access times, network bandwidth, etc. You can design an application to be smart about the way it uses available resources, but you can't get it to use more resources than there actually are.</p> <hr> <p>On reading the text that you quoted, it seems to me that it is talking about <em>two different kinds of actors</em>:</p> <p>Thread-based actors have a 1 to 1 relationship with threads. There's no way you could have millions of this kind of actor in 4Gb memory.</p> <p>Event-based actors work differently. Instead of having a thread at all times, they would mostly be sitting in a queue waiting for an event to happen. When that happened, an event processing thread would grab the actor from the queue and execute the "action" associated with the event. When the action finished, the thread moves onto another actor / event pair.</p> <p>The quoted text is saying that the memory overhead of an <em>event-based</em> actor is ~600 bytes. They don't include the event thread ... because the event thread is shared by multiple actors.</p> <hr> <p>Now I'm not an expert on Scala / Actors, but it is pretty obvious that there are certain things that you should avoid when using event-based actors. For instance, you should probably avoid talking directly to an external database because that is liable to block the event processing thread.</p>
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    1. COIf this is true: ...there is typically a one-to-one relationship between Java threads and OS threads... then what is the point of this: ...you can create more than 6.5 million Actors on 4 G RAM...
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    2. COOK so I've probably been thinking about it wrong then. I just hear 'actors' and 'great for concurrency' as interwoven, but I shouldn't consider all actors to be acting concurrently at all times, which is where my mind was heading.
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    3. COIt means that "event-based actors" are not threads. They only use threads while handling the events.
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