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?

I suppose you could create a separate pool for that, but I don't think that's supported at the moment. — Daniel C. Sobral, Feb 06 '12 at 14:23
possible duplicate of [scala parallel collections degree of parallelism](http://stackoverflow.com/questions/5424496/scala-parallel-collections-degree-of-parallelism) — om-nom-nom, Nov 12 '13 at 14:27

score 20 · Answer 1 · edited Apr 22 '21 at 08:30

20

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

edited Apr 22 '21 at 08:30

samthebest

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answered Feb 20 '15 at 02:34

caiiiycuk

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score 17 · Accepted Answer · answered Feb 06 '12 at 12:31

17

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)
}

answered Feb 06 '12 at 12:31

Submonoid

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I do not like the fact that it changes the global state – Michel Lemay Nov 17 '16 at 15:57
1

What version of Scala & Java is this, I get compile error `not found: value setParallelism` – samthebest Apr 22 '21 at 08:25

How to set the number of threads to use for par

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