Comparing implementations of

the Actor ModelJim Roepcke<jr@uvic.ca>

Concurrency Summit 2008

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Actor models?

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This kind?3

No, not that kind...

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The kind in Joe Armstrong’s Erlang,

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The kind in Joe Armstrong’s Erlang,

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Evan Phoenix’s Rubinius,

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Martin Odersky’s Scala,

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and Steve Dekorte’sIo.

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What is theActor Model?

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It’s a trendy model for

concurrent programming

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Massively parallel future

• The future has begun!

• We had lots of time to figure this out

• Programmers are still using threads and mutexes in “high level” code

• Bad programmer!

• ABSTRACTION FTW!

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What’s old is new again

• Actors defined by Hewitt in 1973

• More work by his PhD students in the 80s, Clinger, Agha

• Adopted by Erlang in the late 80s for the telecom industry

• Picking up steam as concurrency becomes a hot issue again

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Actors win because they are simple

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Actors communicate15

Actors spawn16

Actors run in parallel17

Actors listen18

Actors avoid deadlock19

Actors perform great!20

Now: up close21

ComparingActors to Actors

• Implemented a program in Erlang, Rubinius, Scala and Io

• My experiences with each

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OM NOM NOM!

• Implemented the “burger problem” from CSC 464 Assignment 1 in all four languages

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supervisor

spawn!

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Student

Cook

Student

Student

Order Line

CookCook

Burger Line

Student

StudentStudent

Burger Burger

Burger

hungry

burger!

enjoy!

ok

mmm

supervisor 25

Actors communicate26

Communication

• Asynchronous Message Passing

• Encourages no memory sharing

• Each actor has a “mailbox” which queues messages sent

• “send” mechanism to deliver a message to an actor

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Sending messages

Erlang

pid ! message

Rubinius

actor << message

Scala

actor ! message

Io

obj @@method(args)

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Actors and burgers29

Actors spawn30

Creating actors

• To create concurrency, actors create more actors and send them messages

• Actors are independently executing entities

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Creating actors

Erlang

pid = spawn(fun)

Rubinius

actor = Actor.spawn ...

Scala

sa = new SomeActor()sa.start()

Io

sa := SomeActor clone

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Erlang Actors

• Actors are called processes

• Process creation and context switching is very fast and lightweight

• Processes cannot share memory

• Typically run a tail-recursive function

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Rubinius Actors

• Actor class comes with Rubinius VM

• Green Ruby thread per actor

• VM assigns a Mailbox per thread

• Remote Actors new, not perfect yet

• “Orders of magnitude” worse than Erlang: Rubinius developer on IRC

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Scala Actors

• scala.actors package comes with Scala

• Hybrid execution model, actors run in their own thread or via events

• Claim massive scalability using events and thread pools

• Remote Actors supported

• act() loops to stay alive

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Io Actors

• Io uses coroutines for concurrency

• Scheduling is FIFO, no prioritization

• One kernel thread per VM, async I/O

• @@ runs the method on a dedicated per-object coroutine for messages

• Remote actors not supported

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Actors run in parallel37

Parallelism: Erlang

• SMP Erlang automatically puts processes on different cores/CPUs

• Messaging actors in remote VMs is seamless

• spawn(Node, fun) to spawn an actor on a different VM which may be remote

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Parallelism: Rubinius

• One kernel thread per Rubinius VM

• Processor affinity for multi-core?

• Use VMActor to create an actor on a remote VM

• Messaging actors in remote VMs is seamless thanks to duck typing

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Parallelism: Scala

• When using event-based actors, Scala creates multple threads to balance load across cores

• Use RemoteActor class to find an actor on a different VM

• Messaging actors in remote VMs is seamless

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Parallelism: Io

•Nope :-(

•One kernel thread per VM

•No support for remote actors built-in

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Actors listen42

Receiving messages

• Take a message off the mailbox queue

• Decide if you want to deal with it

• Messages are data

• Timeouts

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Receiving messagesErlang

loop(State) -> % a tail-recursive function receive {hello, From} -> greet(From), loop(State) ping -> doPingThing(), loop(State) {goodbye, From} -> From ! {kthxbye, self(), State} % no loop after 100 loop(State+1) % inc # of timeouts end.

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Receiving messagesRubinius

def actor_process() looping = true while looping Actor.receive do |filter| filter.when Hello do |message| greet(message.from) end filter.when Goodbye do |message| message.from << KThxBye[self] looping = false end endend

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Receiving messagesScala

def act() loop { receive { case Hello(from) => greet(from) case Ping() => doPingThing() case Goodbye(from) => { from ! KThxBye(self) exit() } } }}

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Receiving messagesIo

MyActor := Object cloneMyActor hello := method(from, greet(from))MyActor ping := method(doPingThing)MyActor goodbye := method(from, from @@kthxbye(self))

anActor := MyActor clone // create actor instance anActor @@hello(self)anActor @@pinganActor @@goodbye(self) // look ma, no loops!

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Actors avoid deadlock48

No locks, no dead

• Agha(1985) says the semantics of the Actor model mean that deadlock doesn’t exist in a syntactic sense

• Semantic deadlock can be detected and removed easily using wait-for graphs

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Actors perform great!50

Fine grained, hot CPU

• Lightweight context switching

• Actors don’t suffer from locking granularity issues because there are no locks

• Actors can run “full out” on as many CPUs/machines as the underlying language can make available

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Conclusion

• Light, fast processes needed to see huge scalability

• Erlang’s the fastest and most mature (see OTP)

• You can’t sneeze in the JVM without declaring it’s chemical composition

• Io is a beautiful little language

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“erlang is nice, because

everything just works”

Paul JenkinsApril 11, 2008

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What next?

• Iolang... Io with:

• Support for multiple cores

• Easy way: one VM per core?

• Selective receive

• Support for remote actors

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No cows were hurt...55

Picture creditsBillie Joe: http://flickr.com/photos/15034493@N00/112357008/sizes/l/

Joe Armstrong: http://flickr.com/photos/mbiddulph/2037845171/sizes/l/Evan Phoenix: http://flickr.com/photos/scoop/1403258349/sizes/o/Steve Dekorte: http://flickr.com/photos/x180/276960722/sizes/o/

Martin Odersky: http://picasaweb.google.com/JavaPolis.com/JavaPolis2007/photo#5144254997958003122Bruno: http://www.smh.com.au/ffximage/2006/10/31/bruno_wideweb__470x329,0.jpg

Coady: http://flickr.com/photos/sebastian_bergmann/116486823/sizes/o/Ships: http://flickr.com/photos/lesec/361956524/sizes/o/

Jim: http://flickr.com/photos/kteague/278136366/sizes/o/LOLcode: http://flickr.com/photos/i-marco/534996407/sizes/o/

Triplets: http://flickr.com/photos/origamijoel/217238954/sizes/l/Spawn: http://flickr.com/photos/austenhaines/399622840/

Parallel runners: http://flickr.com/photos/moodeous/205711924/sizes/o/Microscopic: http://flickr.com/photos/braid44/2232461763/sizes/o/

Emoburger: http://flickr.com/photos/gx9/269025682/Cow: http://flickr.com/photos/66164549@N00/542696674/sizes/o/

icanhascheezburger: http://icanhascheezburger.com/2007/01/11/i-can-has-cheezburger/

Thanks for listening

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QuestionsMake it quick, Paul’s next

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Dedicated toCheryl, Cyan, Xavier, and

Justin

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