How to set the number of threads to use for par

Question

I know that you can set the number of threads to use for all .par operations like so: collection.parallel.ForkJoinTasks.defaultForkJoinPool.setParallelism(parlevel: Int)

But is it possible to set the number of threads to use for just one .par call?

Answer 1

In Scala 2.11 you should use parallel collection task support, like this:

parallelCollection.tasksupport = new ForkJoinTaskSupport(
    new java.util.concurrent.ForkJoinPool(parlevel))

parallelCollection.map( ... )

See task support documentation

Answer 2

You could create a block that sets the parallelism level and then execute specific methods within that block:

def withParallelism[A](n : Int)(block : => A) : A = {
  import collection.parallel.ForkJoinTasks.defaultForkJoinPool._
  val defaultParLevel = getParallelism
  setParallelism(n)
  val ret = block
  setParallelism(defaultParLevel)
  ret
}

And then call it as such:

withParallelism(2) {
  (1 to 100).par.map(_ * 2)
}