here - Stefan-Marr.de

3. Which Concepts for Concurrent and Parallel Progr. does a VM need to Support?
1 object fairness {
2 class FairActor () extends Actor {
3 // ...
4 def act () { loop {
5 react {
6 case (v : int ) => { data = v }
7 case ( wait ) => {
8 // busy - waiting section
9 if ( data > 0) println ( data )
10 else self ! wait
11 }
12 case ( start ) => {
13 calc ! (add , 4, 5)
14 self ! wait
15 }
16 }}}}}
Listing 3.2: Example for missing Scheduling Guarantees: Without the guarantee of
fair scheduling, busy-waiting can starve other actors forever. [Karmani et al., 2009,
Fig. 3]
system from making progress. The example they give is shown in Lst. 3.2.
Here the actor busy-waits by sending itself a message to await the arrival of
the computation result. While the example is contrived, livelocks like this can
be effectively avoided by fair scheduling.
An ad hoc solution to these problems is the use of a lightweight task representation
for actors, which is used to schedule these tasks on top of the threading
mechanism provided by the VM to guarantee the desired fairness property.
To this end, the Java standard library provides the ExecutorServices 20
and the CLI offers a TaskScheduler. 21
Ad hoc solutions to enforce scheduling policies have limitations and introduce
tradeoffs. Since these solutions build on top of the VM’s thread
abstraction, the provided guarantees are restricted. Computationally expensive
operations and blocking primitives can bind the underlying thread. This
restricts the ability of the scheduler to enforce the desired guarantees, because
as Karmani et al. note, the actor or task scheduler is prevented from
running. It is possible to compensate to a certain degree for such effects by
20 http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ExecutorServic
e.html
21 http://msdn.microsoft.com/en-us/library/system.threading.tasks.taskscheduler
.aspx
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