How to control the number of parallel processes with .par in Scala?

Question

I am using the par.map expression to execute processes in parallel in Scala (SBT).

Consider list.par.map(function(_)) (I am preparing an MWE). This means that function(_) should be applied to all the elements of the list in a parallel fashion. In my example, list has 3 elements. But Scala executes only function(list(1)) and function(list(2)) in parallel, and only afterwards function(list(3)).

Is there a reason for this behaviour? Is there a relation with the fact that the programme is executed on a two-core processor? Or how could you impose to execute all three things in parallel?

http://www.scala-lang.org/api/current/scala/collection/parallel/immutable/ParRange.html#tasksupport_=(ts:scala.collection.parallel.TaskSupport):Unit — 余杰水, Nov 11 '16 at 01:45
Possible duplicate of [scala parallel collections degree of parallelism](http://stackoverflow.com/questions/5424496/scala-parallel-collections-degree-of-parallelism) — Andrey, Nov 11 '16 at 02:01

score 5 · Accepted Answer · edited May 23 '17 at 11:53

This question has been asked before:

and is well documented:

http://docs.scala-lang.org/overviews/parallel-collections/configuration.html

what you want is something like:

var parallelList = list.par
parallelList.tasksupport = new ForkJoinTaskSupport(
     new scala.concurrent.forkjoin.ForkJoinPool(parlevel))
parallelList.map(function(_))

That said if your running on a 2 core processor you only have two threads (unless the cores are hyper threaded of course) meaning you can't have more than 2 parallel operations at once.

@Karlo are you satisfied with this answer? – Danny Nov 16 '16 at 03:07 — Danny, Nov 16 '16 at 03:07

How to control the number of parallel processes with .par in Scala?

1 Answers1