Scala stateful actor, recursive calling faster than using vars? - performance

Question

Sample code below. I'm a little curious why MyActor is faster than MyActor2. MyActor recursively calls process/react and keeps state in the function parameters whereas MyActor2 keeps state in vars. MyActor even has the extra overhead of tupling the state but still runs faster. I'm wondering if there is a good explanation for this or if maybe I'm doing something "wrong".
I realize the performance difference is not significant but the fact that it is there and consistent makes me curious what's going on here.
Ignoring the first two runs as warmup, I get:
MyActor:
559
511
544
529
vs.
MyActor2:
647
613
654
610
import scala.actors._
object Const {
val NUM = 100000
val NM1 = NUM - 1
}
trait Send[MessageType] {
def send(msg: MessageType)
}
// Test 1 using recursive calls to maintain state
abstract class StatefulTypedActor[MessageType, StateType](val initialState: StateType) extends Actor with Send[MessageType] {
def process(state: StateType, message: MessageType): StateType
def act = proc(initialState)
def send(message: MessageType) = {
this ! message
}
private def proc(state: StateType) {
react {
case msg: MessageType => proc(process(state, msg))
}
}
}
object MyActor extends StatefulTypedActor[Int, (Int, Long)]((0, 0)) {
override def process(state: (Int, Long), input: Int) = input match {
case 0 =>
(1, System.currentTimeMillis())
case input: Int =>
state match {
case (Const.NM1, start) =>
println((System.currentTimeMillis() - start))
(Const.NUM, start)
case (s, start) =>
(s + 1, start)
}
}
}
// Test 2 using vars to maintain state
object MyActor2 extends Actor with Send[Int] {
private var state = 0
private var strt = 0: Long
def send(message: Int) = {
this ! message
}
def act =
loop {
react {
case 0 =>
state = 1
strt = System.currentTimeMillis()
case input: Int =>
state match {
case Const.NM1 =>
println((System.currentTimeMillis() - strt))
state += 1
case s =>
state += 1
}
}
}
}
// main: Run testing
object TestActors {
def main(args: Array[String]): Unit = {
val a = MyActor
// val a = MyActor2
a.start()
testIt(a)
}
def testIt(a: Send[Int]) {
for (_ <- 0 to 5) {
for (i <- 0 to Const.NUM) {
a send i
}
}
}
}
EDIT: Based on Vasil's response, I removed the loop and tried it again. And then MyActor2 based on vars leapfrogged and now might be around 10% or so faster. So... lesson is: if you are confident that you won't end up with a stack overflowing backlog of messages, and you care to squeeze every little performance out... don't use loop and just call the act() method recursively.
Change for MyActor2:
override def act() =
react {
case 0 =>
state = 1
strt = System.currentTimeMillis()
act()
case input: Int =>
state match {
case Const.NM1 =>
println((System.currentTimeMillis() - strt))
state += 1
case s =>
state += 1
}
act()
}

Answer 1

Such results are caused with the specifics of your benchmark (a lot of small messages that fill the actor's mailbox quicker than it can handle them).
Generally, the workflow of react is following:
Actor scans the mailbox;
If it finds a message, it schedules the execution;
When the scheduling completes, or, when there're no messages in the mailbox, actor suspends (Actor.suspendException is thrown);
In the first case, when the handler finishes to process the message, execution proceeds straight to react method, and, as long as there're lots of messages in the mailbox, actor immediately schedules the next message to execute, and only after that suspends.
In the second case, loop schedules the execution of react in order to prevent a stack overflow (which might be your case with Actor #1, because tail recursion in process is not optimized), and thus, execution doesn't proceed to react immediately, as in the first case. That's where the millis are lost.
UPDATE (taken from here):
Using loop instead of recursive react
effectively doubles the number of
tasks that the thread pool has to
execute in order to accomplish the
same amount of work, which in turn
makes it so any overhead in the
scheduler is far more pronounced when
using loop.

Answer 2

Just a wild stab in the dark. It might be due to the exception thrown by react in order to evacuate the loop. Exception creation is quite heavy. However I don't know how often it do that, but that should be possible to check with a catch and a counter.

Answer 3

The overhead on your test depends heavily on the number of threads that are present (try using only one thread with scala -Dactors.corePoolSize=1!). I'm finding it difficult to figure out exactly where the difference arises; the only real difference is that in one case you use loop and in the other you do not. Loop does do fair bit of work, since it repeatedly creates function objects using "andThen" rather than iterating. I'm not sure whether this is enough to explain the difference, especially in light of the heavy usage by scala.actors.Scheduler$.impl and ExceptionBlob.