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DOC: Update Dispatchers (Scala) Chapter. See #1471

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Patrik Nordwall 2011-12-12 21:20:32 +01:00
parent 69ea6db3f1
commit 03e731e098
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5
akka-actor/src/main/resources/reference.conf
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@ -103,7 +103,7 @@ akka {
type = "Dispatcher" # Must be one of the following
# Dispatcher, (BalancingDispatcher, only valid when all actors using it are of the same type),
# A FQCN to a class inheriting MessageDispatcherConfigurator with a no-arg visible constructor
name = "DefaultDispatcher" # Optional, will be a generated UUID if omitted
name = "DefaultDispatcher" # Name used in log messages and thread names.
keep-alive-time = 60s # Keep alive time for threads
core-pool-size-min = 8 # minimum number of threads to cap factor-based core number to
core-pool-size-factor = 8.0 # No of core threads ... ceil(available processors * factor)
@ -115,7 +115,8 @@ akka {
task-queue-size = -1 # Specifies the bounded capacity of the task queue (< 1 == unbounded)
task-queue-type = "linked" # Specifies which type of task queue will be used, can be "array" or "linked" (default)
allow-core-timeout = on # Allow core threads to time out
throughput = 5 # Throughput for Dispatcher, set to 1 for complete fairness
throughput = 5 # Throughput defines the number of messages that are processed in a batch before the
# thread is returned to the pool. Set to 1 for complete fairness.
throughput-deadline-time = 0ms # Throughput deadline for Dispatcher, set to 0 or negative for no deadline
mailbox-capacity = -1 # If negative (or zero) then an unbounded mailbox is used (default)
# If positive then a bounded mailbox is used and the capacity is set using the property

14
akka-actor/src/main/scala/akka/dispatch/Dispatchers.scala
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@ -14,6 +14,7 @@ import akka.actor.Scheduler
import akka.actor.ActorSystem.Settings
import com.typesafe.config.Config
import com.typesafe.config.ConfigFactory
import akka.config.ConfigurationException
trait DispatcherPrerequisites {
def eventStream: EventStream
@ -27,6 +28,10 @@ case class DefaultDispatcherPrerequisites(
val scheduler: Scheduler) extends DispatcherPrerequisites
/**
* It is recommended to define the dispatcher in configuration to allow for tuning
* for different environments. Use the `newFromConfig` method to create a dispatcher
* as specified in configuration.
*
* Scala API. Dispatcher factory.
* <p/>
* Example usage:
@ -62,9 +67,10 @@ class Dispatchers(val settings: ActorSystem.Settings, val prerequisites: Dispatc
val defaultDispatcherConfig = settings.config.getConfig("akka.actor.default-dispatcher")
// TODO PN Shouldn't we fail hard if default-dispatcher is wrong?
lazy val defaultGlobalDispatcher =
from(defaultDispatcherConfig) getOrElse newDispatcher("AkkaDefaultGlobalDispatcher", 1, MailboxType).build
lazy val defaultGlobalDispatcher: MessageDispatcher =
from(defaultDispatcherConfig) getOrElse {
throw new ConfigurationException("Wrong configuration [akka.actor.default-dispatcher]")
}
/**
* Creates an thread based dispatcher serving a single actor through the same single thread.
@ -183,7 +189,7 @@ class Dispatchers(val settings: ActorSystem.Settings, val prerequisites: Dispatc
case true ⇒
val conf = cfg.getConfig(key)
val confWithName = conf.withFallback(ConfigFactory.parseMap(Map("name" -> simpleName).asJava))
from(confWithName).getOrElse(default)
from(confWithName).getOrElse(throw new ConfigurationException("Wrong configuration [%s]".format(key)))
}
}

128
akka-docs/scala/code/DispatcherDocSpec.scala Normal file
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@ -0,0 +1,128 @@
package akka.docs.dispatcher
import org.scalatest.{ BeforeAndAfterAll, WordSpec }
import org.scalatest.matchers.MustMatchers
import akka.testkit.AkkaSpec
import akka.dispatch.PriorityGenerator
import akka.actor.Props
import akka.actor.Actor
import akka.dispatch.UnboundedPriorityMailbox
import akka.event.Logging
import akka.event.LoggingAdapter
import akka.util.duration._
import akka.actor.PoisonPill
object DispatcherDocSpec {
val config = """
akka.logLevel=INFO
//#my-dispatcher-config
my-dispatcher {
type = Dispatcher # Dispatcher is the name of the event-based dispatcher
core-pool-size-min = 2 # minimum number of threads to cap factor-based core number to
core-pool-size-factor = 2.0 # No of core threads ... ceil(available processors * factor)
core-pool-size-max = 10 # maximum number of threads to cap factor-based number to
throughput = 100 # Throughput defines the number of messages that are processed in a batch before the
# thread is returned to the pool. Set to 1 for complete fairness.
}
//#my-dispatcher-config
//#my-pinned-config
my-pinned-dispatcher {
type = Dispatcher
core-pool-size-min = 1
core-pool-size-max = 1
}
//#my-pinned-config
//#my-bounded-config
my-dispatcher-bounded-queue {
type = Dispatcher
core-pool-size-factor = 8.0
max-pool-size-factor = 16.0
task-queue-size = 100 # Specifies the bounded capacity of the task queue
task-queue-type = "array" # Specifies which type of task queue will be used, can be "array" or "linked" (default)
throughput = 3
}
//#my-bounded-config
//#my-balancing-config
my-balancing-dispatcher {
type = BalancingDispatcher
}
//#my-balancing-config
"""
class MyActor extends Actor {
def receive = {
case x ⇒
}
}
}
class DispatcherDocSpec extends AkkaSpec(DispatcherDocSpec.config) {
import DispatcherDocSpec.MyActor
"defining dispatcher" in {
//#defining-dispatcher
import akka.actor.Props
val dispatcher = system.dispatcherFactory.newFromConfig("my-dispatcher")
val myActor1 = system.actorOf(Props[MyActor].withDispatcher(dispatcher), name = "myactor1")
val myActor2 = system.actorOf(Props[MyActor].withDispatcher(dispatcher), name = "myactor2")
//#defining-dispatcher
}
"defining dispatcher with bounded queue" in {
val dispatcher = system.dispatcherFactory.newFromConfig("my-dispatcher-bounded-queue")
}
"defining priority dispatcher" in {
//#prio-dispatcher
val gen = PriorityGenerator { // Create a new PriorityGenerator, lower prio means more important
case 'highpriority ⇒ 0 // 'highpriority messages should be treated first if possible
case 'lowpriority ⇒ 100 // 'lowpriority messages should be treated last if possible
case PoisonPill ⇒ 1000 // PoisonPill when no other left
case otherwise ⇒ 50 // We default to 50
}
// We create a new Priority dispatcher and seed it with the priority generator
val dispatcher = system.dispatcherFactory.newDispatcher("foo", 5, UnboundedPriorityMailbox(gen)).build
val a = system.actorOf( // We create a new Actor that just prints out what it processes
Props(new Actor {
val log: LoggingAdapter = Logging(context.system, this)
self ! 'lowpriority
self ! 'lowpriority
self ! 'highpriority
self ! 'pigdog
self ! 'pigdog2
self ! 'pigdog3
self ! 'highpriority
self ! PoisonPill
def receive = {
case x ⇒ log.info(x.toString)
}
}).withDispatcher(dispatcher))
/*
Logs:
'highpriority
'highpriority
'pigdog
'pigdog2
'pigdog3
'lowpriority
'lowpriority
*/
//#prio-dispatcher
awaitCond(a.isTerminated, 5 seconds)
}
"defining balancing dispatcher" in {
val dispatcher = system.dispatcherFactory.newFromConfig("my-balancing-dispatcher")
}
}

194
akka-docs/scala/dispatchers.rst
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@ -18,66 +18,75 @@ The event-based Actors currently consume ~600 bytes per Actor which means that y
Default dispatcher
------------------
For most scenarios the default settings are the best. Here we have one single event-based dispatcher for all Actors created. The dispatcher used is this one:
For most scenarios the default settings are the best. Here we have one single event-based dispatcher for all Actors created.
The default dispatcher is available from the ``ActorSystem.dispatcher`` and can be configured in the ``akka.actor.default-dispatcher``
section of the :ref:`configuration`.
.. code-block:: scala
Dispatchers.globalDispatcher
But if you feel that you are starting to contend on the single dispatcher (the 'Executor' and its queue) or want to group a specific set of Actors for a dedicated dispatcher for better flexibility and configurability then you can override the defaults and define your own dispatcher. See below for details on which ones are available and how they can be configured.
If you are starting to get contention on the single dispatcher (the ``Executor`` and its queue) or want to group a specific set of Actors
for a dedicated dispatcher for better flexibility and configurability then you can override the defaults and define your own dispatcher.
See below for details on which ones are available and how they can be configured.
Setting the dispatcher
----------------------
Normally you set the dispatcher from within the Actor itself. The dispatcher is defined by the 'dispatcher: MessageDispatcher' member field in 'ActorRef'.
You specify the dispatcher to use when creating an actor.
.. code-block:: scala
class MyActor extends Actor {
self.dispatcher = ... // set the dispatcher
...
}
You can also set the dispatcher for an Actor **before** it has been started:
.. code-block:: scala
actorRef.dispatcher = dispatcher
.. includecode:: code/DispatcherDocSpec.scala
:include: imports,defining-dispatcher
Types of dispatchers
--------------------
There are six different types of message dispatchers:
There are 4 different types of message dispatchers:
* Thread-based
* Thread-based (Pinned)
* Event-based
* Priority event-based
* Work-stealing
* Work-stealing (Balancing)
Factory methods for all of these, including global versions of some of them, are in the 'akka.dispatch.Dispatchers' object.
It is recommended to define the dispatcher in :ref:`configuration` to allow for tuning for different environments.
Example of a custom event-based dispatcher, which can be created with ``system.dispatcherFactory.newFromConfig("my-dispatcher")``
as in the example above:
.. includecode:: code/DispatcherDocSpec.scala#my-dispatcher-config
Default values are taken from ``default-dispatcher``, i.e. all options doesn't need to be defined.
.. warning::
Factory methods for creating dispatchers programmatically are available in ``akka.dispatch.Dispatchers``, i.e.
``dispatcherFactory`` of the ``ActorSystem``. These methods will probably be changed or removed before
2.0 final release, because dispatchers need to be defined by configuration to work in a clustered setup.
Let's now walk through the different dispatchers in more detail.
Thread-based
^^^^^^^^^^^^
The 'PinnedDispatcher' binds a dedicated OS thread to each specific Actor. The messages are posted to a 'LinkedBlockingQueue' which feeds the messages to the dispatcher one by one. A 'PinnedDispatcher' cannot be shared between actors. This dispatcher has worse performance and scalability than the event-based dispatcher but works great for creating "daemon" Actors that consumes a low frequency of messages and are allowed to go off and do their own thing for a longer period of time. Another advantage with this dispatcher is that Actors do not block threads for each other.
The ``PinnedDispatcher`` binds a dedicated OS thread to each specific Actor. The messages are posted to a ``LinkedBlockingQueue``
which feeds the messages to the dispatcher one by one. A ``PinnedDispatcher`` cannot be shared between actors. This dispatcher
has worse performance and scalability than the event-based dispatcher but works great for creating "daemon" Actors that consumes
a low frequency of messages and are allowed to go off and do their own thing for a longer period of time. Another advantage with
this dispatcher is that Actors do not block threads for each other.
It would normally by used from within the actor like this:
FIXME PN: Is this the way to configure a PinnedDispatcher, and then why "A ``PinnedDispatcher`` cannot be shared between actors."
.. code-block:: scala
The ``PinnedDispatcher`` is configured as a event-based dispatcher with with core pool size of 1.
class MyActor extends Actor {
self.dispatcher = Dispatchers.newPinnedDispatcher(self)
...
}
.. includecode:: code/DispatcherDocSpec.scala#my-pinned-config
Event-based
^^^^^^^^^^^
The 'Dispatcher' binds a set of Actors to a thread pool backed up by a 'BlockingQueue'. This dispatcher is highly configurable and supports a fluent configuration API to configure the 'BlockingQueue' (type of queue, max items etc.) as well as the thread pool.
The event-based ``Dispatcher`` binds a set of Actors to a thread pool backed up by a ``BlockingQueue``. This dispatcher is highly configurable
and supports a fluent configuration API to configure the ``BlockingQueue`` (type of queue, max items etc.) as well as the thread pool.
The event-driven dispatchers **must be shared** between multiple Actors. One best practice is to let each top-level Actor, e.g. the Actors you define in the declarative supervisor config, to get their own dispatcher but reuse the dispatcher for each new Actor that the top-level Actor creates. But you can also share dispatcher between multiple top-level Actors. This is very use-case specific and needs to be tried out on a case by case basis. The important thing is that Akka tries to provide you with the freedom you need to design and implement your system in the most efficient way in regards to performance, throughput and latency.
The event-driven dispatchers **must be shared** between multiple Actors. One best practice is to let each top-level Actor, e.g.
the Actors you create from ``system.actorOf`` to get their own dispatcher but reuse the dispatcher for each new Actor
that the top-level Actor creates. But you can also share dispatcher between multiple top-level Actors. This is very use-case specific
and needs to be tried out on a case by case basis. The important thing is that Akka tries to provide you with the freedom you need to
design and implement your system in the most efficient way in regards to performance, throughput and latency.
It comes with many different predefined BlockingQueue configurations:
@ -87,38 +96,24 @@ It comes with many different predefined BlockingQueue configurations:
* Unbounded ArrayBlockingQueue
* SynchronousQueue
You can also set the rejection policy that should be used, e.g. what should be done if the dispatcher (e.g. the Actor) can't keep up and the mailbox is growing up to the limit defined. You can choose between four different rejection policies:
You can also set the rejection policy that should be used, e.g. what should be done if the dispatcher (e.g. the Actor) can't keep up
and the mailbox is growing up to the limit defined. You can choose between four different rejection policies:
* java.util.concurrent.ThreadPoolExecutor.CallerRuns - will run the message processing in the caller's thread as a way to slow him down and balance producer/consumer
* java.util.concurrent.ThreadPoolExecutor.AbortPolicy - rejected messages by throwing a 'RejectedExecutionException'
* java.util.concurrent.ThreadPoolExecutor.AbortPolicy - rejected messages by throwing a ``RejectedExecutionException``
* java.util.concurrent.ThreadPoolExecutor.DiscardPolicy - discards the message (throws it away)
* java.util.concurrent.ThreadPoolExecutor.DiscardOldestPolicy - discards the oldest message in the mailbox (throws it away)
You can read more about these policies `here <http://java.sun.com/javase/6/docs/api/index.html?java/util/concurrent/RejectedExecutionHandler.html>`_.
Here is an example:
Here is an example of a bounded mailbox:
.. code-block:: scala
import akka.actor.Actor
import akka.dispatch.Dispatchers
import java.util.concurrent.ThreadPoolExecutor.CallerRunsPolicy
class MyActor extends Actor {
self.dispatcher = Dispatchers.newDispatcher(name, throughput = 15)
.withNewThreadPoolWithLinkedBlockingQueueWithCapacity(100)
.setCorePoolSize(16)
.setMaxPoolSize(128)
.setKeepAliveTimeInMillis(60000)
.setRejectionPolicy(new CallerRunsPolicy)
.build
...
}
.. includecode:: code/DispatcherDocSpec.scala#my-bounded-config
The standard :class:`Dispatcher` allows you to define the ``throughput`` it
should have, as shown above. This defines the number of messages for a specific
Actor the dispatcher should process in one single sweep; in other words, the
dispatcher will bunch up to ``throughput`` message invocations together when
dispatcher will bunch up to ``throughput`` messages together when
having elected an actor to run. Setting this to a higher number will increase
throughput but lower fairness, and vice versa. If you don't specify it explicitly
then it uses the value (5) defined for ``default-dispatcher`` in the :ref:`configuration`.
@ -129,62 +124,22 @@ Priority event-based
^^^^^^^^^^^^^^^^^^^^
Sometimes it's useful to be able to specify priority order of messages, that is done by using Dispatcher and supply
an UnboundedPriorityMailbox or BoundedPriorityMailbox with a java.util.Comparator[MessageInvocation] or use a akka.dispatch.PriorityGenerator (recommended):
an UnboundedPriorityMailbox or BoundedPriorityMailbox with a ``java.util.Comparator[Envelope]`` or use a
``akka.dispatch.PriorityGenerator`` (recommended):
Creating a Dispatcher using PriorityGenerator:
.. code-block:: scala
.. includecode:: code/DispatcherDocSpec.scala#prio-dispatcher
import akka.dispatch._
import akka.actor._
val gen = PriorityGenerator { // Create a new PriorityGenerator, lower prio means more important
case 'highpriority => 0 // 'highpriority messages should be treated first if possible
case 'lowpriority => 100 // 'lowpriority messages should be treated last if possible
case otherwise => 50 // We default to 50
}
val a = Actor.actorOf( // We create a new Actor that just prints out what it processes
Props(new Actor {
self ! 'lowpriority
self ! 'lowpriority
self ! 'highpriority
self ! 'pigdog
self ! 'pigdog2
self ! 'pigdog3
self ! 'highpriority
def receive = {
case x => println(x)
}
}).withDispatcher(new Dispatcher("foo", 5, UnboundedPriorityMailbox(gen)))) // We create a new Priority dispatcher and seed it with the priority generator
Prints:
'highpriority
'highpriority
'pigdog
'pigdog2
'pigdog3
'lowpriority
'lowpriority
Work-stealing event-based
^^^^^^^^^^^^^^^^^^^^^^^^^
The 'BalancingDispatcher' is a variation of the 'Dispatcher' in which Actors of the same type can be set up to share this dispatcher and during execution time the different actors will steal messages from other actors if they have less messages to process. This can be a great way to improve throughput at the cost of a little higher latency.
The ``BalancingDispatcher`` is a variation of the ``Dispatcher`` in which Actors of the same type can be set up to
share this dispatcher and during execution time the different actors will steal messages from other actors if they
have less messages to process. This can be a great way to improve throughput at the cost of a little higher latency.
Normally the way you use it is to create an Actor companion object to hold the dispatcher and then set in in the Actor explicitly.
.. code-block:: scala
object MyActor {
val dispatcher = Dispatchers.newBalancingDispatcher(name).build
}
class MyActor extends Actor {
self.dispatcher = MyActor.dispatcher
...
}
.. includecode:: code/DispatcherDocSpec.scala#my-balancing-config
Here is an article with some more information: `Load Balancing Actors with Work Stealing Techniques <http://janvanbesien.blogspot.com/2010/03/load-balancing-actors-with-work.html>`_
Here is another article discussing this particular dispatcher: `Flexible load balancing with Akka in Scala <http://vasilrem.com/blog/software-development/flexible-load-balancing-with-akka-in-scala/>`_
@ -195,14 +150,18 @@ Making the Actor mailbox bounded
Global configuration
^^^^^^^^^^^^^^^^^^^^
You can make the Actor mailbox bounded by a capacity in two ways. Either you define it in the configuration file under 'default-dispatcher'. This will set it globally.
You can make the Actor mailbox bounded by a capacity in two ways. Either you define it in the :ref:`configuration` file under
``default-dispatcher``. This will set it globally as default for the DefaultDispatcher and for other configured dispatchers,
if not specified otherwise.
.. code-block:: ruby
actor {
default-dispatcher {
mailbox-capacity = -1 # If negative (or zero) then an unbounded mailbox is used (default)
# If positive then a bounded mailbox is used and the capacity is set to the number specified
akka {
actor {
default-dispatcher {
task-queue-size = 1000 # If negative (or zero) then an unbounded mailbox is used (default)
# If positive then a bounded mailbox is used and the capacity is set to the number specified
}
}
}
@ -211,26 +170,11 @@ Per-instance based configuration
You can also do it on a specific dispatcher instance.
For the 'Dispatcher' and the 'ExecutorBasedWorkStealingDispatcher' you can do it through their constructor
.. code-block:: scala
class MyActor extends Actor {
val mailboxCapacity = BoundedMailbox(capacity = 100)
self.dispatcher = Dispatchers.newDispatcher(name, throughput, mailboxCapacity).build
...
}
For the 'PinnedDispatcher', it is non-shareable between actors, and associates a dedicated Thread with the actor.
Making it bounded (by specifying a capacity) is optional, but if you do, you need to provide a pushTimeout (default is 10 seconds). When trying to send a message to the Actor it will throw a MessageQueueAppendFailedException("BlockingMessageTransferQueue transfer timed out") if the message cannot be added to the mailbox within the time specified by the pushTimeout.
.. code-block:: scala
class MyActor extends Actor {
import akka.util.duration._
self.dispatcher = Dispatchers.newPinnedDispatcher(self, mailboxCapacity = 100,
pushTimeOut = 10 seconds)
...
}
.. includecode:: code/DispatcherDocSpec.scala#my-bounded-config
For the ``PinnedDispatcher``, it is non-shareable between actors, and associates a dedicated Thread with the actor.
Making it bounded (by specifying a capacity) is optional, but if you do, you need to provide a pushTimeout (default is 10 seconds).
When trying to send a message to the Actor it will throw a MessageQueueAppendFailedException("BlockingMessageTransferQueue transfer timed out")
if the message cannot be added to the mailbox within the time specified by the pushTimeout.