controllable stage parallelism

[conderls]

Is your feature request related to a problem? Please describe.
the stage parallelism is enabled by new Stage().parallel(true) and implemented by par collections

  private[this] def parallelFactories: Either[ParArray[Factory[_]], Array[Factory[_]]] = {
    if (_parallel) {
      Left(factories.par)
    } else {
      Right(factories)
    }
  }

to avoid Java Heap Space OOM, I think it is a good idea to set parallelism level.

Describe the solution you'd like
maybe it can be achieved with the thread pool executor:
with option new Stage().parallel(2) to control parallelism level = 2;
https://docs.scala-lang.org/overviews/parallel-collections/configuration.html

[qxzzxq]

Working on it :) It seems that the type signature of new ForkJoinTaskSupport() has been changed from scala 2.11 to 2.12, which prevents us from compiling with scala 2.11. I will try to bypass this issue by using reflection

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.