# Actors and Concurrency

**URL:** <https://www.ruby-forum.com/t/actors-and-concurrency/128828>\
**Category:** Ruby\
**Created:** [February 12, 2008, 12:19am UTC](https://www.ruby-forum.com/t/actors-and-concurrency/128828 "2008-02-12T00:19:05Z")\
**Posts on this page:** 9\
**Page:** 1

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**Author:** ![fedzor](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/bbe5ce/32.png) [@fedzor](https://www.ruby-forum.com/u/fedzor)\
**Post date:** [February 12, 2008, 12:19am UTC](https://www.ruby-forum.com/t/actors-and-concurrency/128828/1 "2008-02-12T00:19:05Z")

</div>

So I was messing with actors today, and I found that I couldn’t get  
an actor to run concurrently with the main thread. Am I just wishing  
fibers are something they’re not?

I had:

Actor.spawn  
sleep 1  
puts 5  
end

puts 6

and it went:  
5  
6

So what do I have to do to get Actors to go concurrent on my ass? I  
was hoping for:  
6  
5

Thanks,  
Ari  
--------------------------------------------|  
If you’re not living on the edge,  
then you’re just wasting space.

---

<div class="post-metadata">

**Author:** ![fedzor](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/bbe5ce/32.png) [@fedzor](https://www.ruby-forum.com/u/fedzor)\
**Post date:** [February 12, 2008, 1:00am UTC](https://www.ruby-forum.com/t/actors-and-concurrency/128828/2 "2008-02-12T01:00:43Z")

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On Tue, 12 Feb 2008 08:18:03 +0900, fedzor [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> So I was messing with actors today, and I found that I couldn’t get  
> an actor to run concurrently with the main thread. Am I just wishing  
> fibers are something they’re not?

To a point. However, note that if you want an actor to sleep for one  
second  
and permit other actors to proceed, you can just receive with a timeout  
of  
one second, rather than using sleep. Receive is how revactor actors  
transfer  
control to one another.

Of course, if the main thread exits (as it would in your code), then the  
spawned actor won’t have a chance to finish.

-mental

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<div class="post-metadata">

**Author:** ![fedzor](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/bbe5ce/32.png) [@fedzor](https://www.ruby-forum.com/u/fedzor)\
**Post date:** [February 12, 2008, 1:10am UTC](https://www.ruby-forum.com/t/actors-and-concurrency/128828/3 "2008-02-12T01:10:41Z")

</div>

On Tue, 12 Feb 2008 09:00:38 +0900, “Jason R.”  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Second, Actors aren’t really threads or Fibers. There’s a single  
> “dispatcher” thread/process/whatever that handles sending out events  
> to the pool of actors. So while Actors may look like threads and  
> concurrency, they aren’t.

Not necessarily true. Different actor implementations may have  
different  
scheduling behavior. For example, if you’re using actors from the  
Omnibus  
library, you get a thread per actor, each of which handles its own event  
processing. On JRuby, that means you get a native thread per actor.

On the other hand, a native thread per actor isn’t a good way to achieve  
performance.

-mental

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<div class="post-metadata">

**Author:** ![fedzor](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/bbe5ce/32.png) [@fedzor](https://www.ruby-forum.com/u/fedzor)\
**Post date:** [February 12, 2008, 3:13am UTC](https://www.ruby-forum.com/t/actors-and-concurrency/128828/4 "2008-02-12T03:13:44Z")

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On Feb 11, 2008, at 6:59 PM, MenTaLguY wrote:

> actors transfer  
> control to one another.
> 
> Of course, if the main thread exits (as it would in your code),  
> then the  
> spawned actor won’t have a chance to finish.

Can you give me some example code of working with receive? I kind of  
understand, but feel like some code would really nail it in.

Thanks,  
Ari  
-------------------------------------------|  
Nietzsche is my copilot

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<div class="post-metadata">

**Author:** ![fedzor](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/bbe5ce/32.png) [@fedzor](https://www.ruby-forum.com/u/fedzor)\
**Post date:** [February 12, 2008, 1:02am UTC](https://www.ruby-forum.com/t/actors-and-concurrency/128828/5 "2008-02-12T01:02:28Z")

</div>

(others can get more specific here, but here’s the general idea)

First of all, Ruby does not have concurrency. Not even 1.9 allows  
concurrently running threads. The Ruby VM is a preemptive kernel.  
Eventually Ruby’s threads will be system threads _and_ fully  
concurrent, but that’s a long time from now.

Second, Actors aren’t really threads or Fibers. There’s a single  
“dispatcher” thread/process/whatever that handles sending out events  
to the pool of actors. So while Actors may look like threads and  
concurrency, they aren’t.

So put the two together, and you’re going to get sequential code  
execution.

Jason

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<div class="post-metadata">

**Author:** ![fedzor](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/bbe5ce/32.png) [@fedzor](https://www.ruby-forum.com/u/fedzor)\
**Post date:** [February 12, 2008, 3:14am UTC](https://www.ruby-forum.com/t/actors-and-concurrency/128828/6 "2008-02-12T03:14:54Z")

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On Feb 11, 2008, at 7:00 PM, Jason R. wrote:

> concurrency, they aren’t.
> 
> So put the two together, and you’re going to get sequential code  
> execution.

I understand that there isn’t really _true_ concurrency, but is there  
any possible way to get, for example, my previous program to print “6  
\n5” instead of “5\n6”? I was hoping for some sort of Thread-like  
concurrency, but apparently am just wishing.

~ Ari  
English is like a pseudo-random number generator - there are a  
bajillion rules to it, but nobody cares.

---

<div class="post-metadata">

**Author:** ![fedzor](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/bbe5ce/32.png) [@fedzor](https://www.ruby-forum.com/u/fedzor)\
**Post date:** [February 13, 2008, 8:29pm UTC](https://www.ruby-forum.com/t/actors-and-concurrency/128828/7 "2008-02-13T20:29:14Z")

</div>

I’m a newbie, so please set me straight. I’ve seen several people make  
the  
statement that ‘Ruby does not have concurrency’. If this is so, what is  
the  
Threads class all about? And what is happening in this piece of  
code…Is  
this somehow the appearance of concurrency without being ‘real’  
concurrency?  
What are the limitations here?

Thanks!

Leonard

#!/usr/bin/env ruby

```
# Fetch three urls simultaneously

require 'net/http'

pages = %w( www.rubycentral.com
            www.google.com
            www.pragprog.com
        )

threads = []

for page in pages
threads << Thread.new(page) { |myPage|

    h = Net::HTTP.new(myPage, 80)
    puts "Fetching: #{myPage}"
    resp, data = h.get('/', nil )
    puts "Got #{myPage}: #{resp.message}"
}
end

threads.each { |aThread| aThread.join }

```

Fetching: [www.rubycentral.com](http://www.rubycentral.com)  
Fetching: [www.google.com](http://www.google.com)  
Fetching: [www.pragprog.com](http://www.pragprog.com)  
Got [www.google.com](http://www.google.com): OK  
Got [www.pragprog.com](http://www.pragprog.com): OK  
Got [www.rubycentral.com](http://www.rubycentral.com): OK

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<div class="post-metadata">

**Author:** ![fedzor](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/bbe5ce/32.png) [@fedzor](https://www.ruby-forum.com/u/fedzor)\
**Post date:** [February 13, 2008, 8:41pm UTC](https://www.ruby-forum.com/t/actors-and-concurrency/128828/8 "2008-02-13T20:41:53Z")

</div>

Yeah, sorry, it can be confusing.

Ruby has threads, and they do run separate execution paths. However,  
these threads do not run _at the same time_. The Ruby 1.8 VM is a  
preemptive scheduler, in that it gives Thread 1 x milliseconds of  
execution time, then Thread 2, then Thread 3, looping back to Thread

1. So in the end it looks as if they are running simultaneously, but  
they in fact are not.

This is actually the same in Ruby 1.9, except instead of Green threads  
(built, managed, and controlled exclusively by Ruby), it uses System  
threads (pthreads), but it still controls the thread execution to be  
the same as 1.8 Green threads.

I hope I explained that clearly enough. Please say if you’re still  
confused.

Jason

---

<div class="post-metadata">

**Author:** ![fedzor](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/bbe5ce/32.png) [@fedzor](https://www.ruby-forum.com/u/fedzor)\
**Post date:** [February 13, 2008, 8:43pm UTC](https://www.ruby-forum.com/t/actors-and-concurrency/128828/9 "2008-02-13T20:43:40Z")

</div>

On Thu, 14 Feb 2008 04:28:33 +0900, Leonard C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> I’m a newbie, so please set me straight. I’ve seen several people make the  
> statement that ‘Ruby does not have concurrency’. If this is so, what is  
> the Threads class all about? And what is happening in this piece of code…Is  
> this somehow the appearance of concurrency without being ‘real’  
> concurrency?

Unfortunately, people are confusing concurrency with simultaneity. Most  
Ruby implementations (JRuby is an exception) do not permit Ruby threads  
to run truly simultaneously (e.g. on multiple processors); instead,  
their execution is simply interleaved in a non-deterministic fashion.

However, this is still concurrency in the technical sense of the term.

-mental