Playing with ForkJoinPool of ExecutionContext.Implicits.global and inspecting its behavior when "blocking {}" is not used when advised to.

FutureExperiment.scala

import java.util.concurrent.Executors

import scala.concurrent._
import scala.concurrent.duration._
import ExecutionContext.Implicits.global

object FutureExperiment extends App {

  val cores = Runtime.getRuntime.availableProcessors
  val start = System.currentTimeMillis()
  val sleepTime = 1001
  // ------------------------------- select the test to run through this variable
  val testToRun = test07 
  // -------------------------------

  private def blockOp(workerIdx: Int*) = {
    Thread.sleep(sleepTime)
    val waitTime = (System.currentTimeMillis() - start) / 1000
    println(s"Worker #${workerIdx.mkString(",")} finished after ~${waitTime} secs.")
    waitTime
  }

  def test01 = {
    (1 to (cores + 1)).map { i =>
      Future {
        blockOp(i)
      }
    }
  }

  def test02 = {
    (1 to (cores + 1)).map { i =>
      Future {
        blocking {
          blockOp(i)
        }
      }
    }
  }

  def test03(implicit executionContext: ExecutionContext) = {
    (1 to cores).map { i =>
      Future {
        blocking {
          blockOp(1, i)
        }
      } (executionContext)
    } ++
    (1 to cores).map { i =>
      Future {
        blockOp(2, i)
      } (executionContext)
    }
  }

  def test04() = {
    implicit val threadPool = ExecutionContext.fromExecutor(Executors.newFixedThreadPool(cores * 2))
    test03
  }

  def test05() = { // all {core * 5} tasks will be executed in parallel
    (1 to cores * 2).map { i =>
      Future {
        blocking {
          blockOp(1, i)
        }
      }
    } ++
    (1 to cores * 3).map { i =>
      Future {
        blocking {
          blockOp(2, i)
        }
      }
    }
  }

  def test06 = { // only 12 tasks will be executed in parallel
    (1 to cores).flatMap { i =>
      Future {
        blocking {
          blockOp(1, i)
        }
      } ::
      Future {
        blockOp(2, i)
      } ::
      Nil
    } ++
    (1 to cores).map { i =>
      Future {
        blocking {
          blockOp(1, i)
        }
      }
    }
  }


  def test07 = { // #{cores + 1 + 1} tasks will be executed in parallel
    (1 to (cores + 1)).map { i =>
      Future {
        blocking {
          blockOp(1, i)
        }
      }
    } ++
    (1 to cores * 3).map { i =>
      Future {
        blockOp(2, i)
      }
    }
  }

  val allTasks = Await.result(Future.sequence(testToRun), (sleepTime * 10) millis)
  println(s"""
******************************************************************
Total Cores:  ${cores}
Tasks:  ${allTasks.length}
Grouped Execution Time [N tasks finished after ~M secs]:  ${
    allTasks
      .groupBy(x => x)
      .toList
      .sortBy(_._1)
      .map(i => s"[${i._2.length}, ~${i._1} sec]")
      .mkString(" | ")
}
Maximum Execution Time:  ${allTasks.max}
******************************************************************
""")

}

test01-result

Worker #12 finished after ~1 secs.
Worker #6 finished after ~1 secs.
Worker #5 finished after ~1 secs.
Worker #1 finished after ~1 secs.
Worker #8 finished after ~1 secs.
Worker #2 finished after ~1 secs.
Worker #10 finished after ~1 secs.
Worker #3 finished after ~1 secs.
Worker #4 finished after ~1 secs.
Worker #7 finished after ~1 secs.
Worker #9 finished after ~1 secs.
Worker #11 finished after ~1 secs.
Worker #13 finished after ~2 secs.

******************************************************************
Total Cores:  12
Tasks:  13
Grouped Execution Time [N tasks finished after ~M secs]:  [12, ~1 sec] | [1, ~2 sec]
Maximum Execution Time:  2
******************************************************************

test02-result

Worker #10 finished after ~1 secs.
Worker #8 finished after ~1 secs.
Worker #13 finished after ~1 secs.
Worker #5 finished after ~1 secs.
Worker #3 finished after ~1 secs.
Worker #7 finished after ~1 secs.
Worker #2 finished after ~1 secs.
Worker #1 finished after ~1 secs.
Worker #6 finished after ~1 secs.
Worker #9 finished after ~1 secs.
Worker #4 finished after ~1 secs.
Worker #12 finished after ~1 secs.
Worker #11 finished after ~1 secs.

******************************************************************
Total Cores:  12
Tasks:  13
Grouped Execution Time [N tasks finished after ~M secs]:  [13, ~1 sec]
Maximum Execution Time:  1
******************************************************************

test03-result

Worker #1,7 finished after ~1 secs.
Worker #1,8 finished after ~1 secs.
Worker #1,11 finished after ~1 secs.
Worker #1,10 finished after ~1 secs.
Worker #1,6 finished after ~1 secs.
Worker #1,3 finished after ~1 secs.
Worker #1,4 finished after ~1 secs.
Worker #1,9 finished after ~1 secs.
Worker #1,5 finished after ~1 secs.
Worker #1,2 finished after ~1 secs.
Worker #2,1 finished after ~1 secs.
Worker #1,1 finished after ~1 secs.
Worker #1,12 finished after ~1 secs.
Worker #2,2 finished after ~2 secs.
Worker #2,3 finished after ~2 secs.
Worker #2,4 finished after ~2 secs.
Worker #2,7 finished after ~2 secs.
Worker #2,6 finished after ~2 secs.
Worker #2,9 finished after ~2 secs.
Worker #2,5 finished after ~2 secs.
Worker #2,10 finished after ~2 secs.
Worker #2,8 finished after ~2 secs.
Worker #2,12 finished after ~2 secs.
Worker #2,11 finished after ~2 secs.

******************************************************************
Total Cores:  12
Tasks:  24
Grouped Execution Time [N tasks finished after ~M secs]:  [13, ~1 sec] | [11, ~2 sec]
Maximum Execution Time:  2
******************************************************************

test04-result

Worker #1,11 finished after ~1 secs.
Worker #2,2 finished after ~1 secs.
Worker #2,3 finished after ~1 secs.
Worker #1,9 finished after ~1 secs.
Worker #1,7 finished after ~1 secs.
Worker #1,10 finished after ~1 secs.
Worker #1,12 finished after ~1 secs.
Worker #2,5 finished after ~1 secs.
Worker #2,7 finished after ~1 secs.
Worker #1,4 finished after ~1 secs.
Worker #2,10 finished after ~1 secs.
Worker #1,6 finished after ~1 secs.
Worker #1,1 finished after ~1 secs.
Worker #1,8 finished after ~1 secs.
Worker #1,3 finished after ~1 secs.
Worker #1,2 finished after ~1 secs.
Worker #2,6 finished after ~1 secs.
Worker #1,5 finished after ~1 secs.
Worker #2,1 finished after ~1 secs.
Worker #2,12 finished after ~1 secs.
Worker #2,11 finished after ~1 secs.
Worker #2,9 finished after ~1 secs.
Worker #2,8 finished after ~1 secs.
Worker #2,4 finished after ~1 secs.

******************************************************************
Total Cores:  12
Tasks:  24
Grouped Execution Time [N tasks finished after ~M secs]:  [24, ~1 sec]
Maximum Execution Time:  1
******************************************************************

test05-result

Worker #2,16 finished after ~1 secs.
Worker #1,15 finished after ~1 secs.
Worker #1,4 finished after ~1 secs.
Worker #1,2 finished after ~1 secs.
Worker #2,30 finished after ~1 secs.
Worker #1,24 finished after ~1 secs.
Worker #1,17 finished after ~1 secs.
Worker #2,13 finished after ~1 secs.
Worker #2,20 finished after ~1 secs.
Worker #1,16 finished after ~1 secs.
Worker #1,3 finished after ~1 secs.
Worker #2,3 finished after ~1 secs.
Worker #2,21 finished after ~1 secs.
Worker #2,23 finished after ~1 secs.
Worker #1,14 finished after ~1 secs.
Worker #2,26 finished after ~1 secs.
Worker #2,31 finished after ~1 secs.
Worker #2,22 finished after ~1 secs.
Worker #1,13 finished after ~1 secs.
Worker #1,19 finished after ~1 secs.
Worker #2,14 finished after ~1 secs.
Worker #2,29 finished after ~1 secs.
Worker #2,25 finished after ~1 secs.
Worker #1,23 finished after ~1 secs.
Worker #2,10 finished after ~1 secs.
Worker #2,2 finished after ~1 secs.
Worker #2,19 finished after ~1 secs.
Worker #1,8 finished after ~1 secs.
Worker #2,33 finished after ~1 secs.
Worker #1,10 finished after ~1 secs.
Worker #1,11 finished after ~1 secs.
Worker #1,5 finished after ~1 secs.
Worker #2,17 finished after ~1 secs.
Worker #2,12 finished after ~1 secs.
Worker #1,12 finished after ~1 secs.
Worker #1,9 finished after ~1 secs.
Worker #2,9 finished after ~1 secs.
Worker #1,20 finished after ~1 secs.
Worker #2,24 finished after ~1 secs.
Worker #2,7 finished after ~1 secs.
Worker #2,4 finished after ~1 secs.
Worker #1,6 finished after ~1 secs.
Worker #2,18 finished after ~1 secs.
Worker #2,32 finished after ~1 secs.
Worker #1,21 finished after ~1 secs.
Worker #2,35 finished after ~1 secs.
Worker #1,1 finished after ~1 secs.
Worker #2,36 finished after ~1 secs.
Worker #2,5 finished after ~1 secs.
Worker #1,18 finished after ~1 secs.
Worker #2,6 finished after ~1 secs.
Worker #1,22 finished after ~1 secs.
Worker #2,11 finished after ~1 secs.
Worker #2,28 finished after ~1 secs.
Worker #2,34 finished after ~1 secs.
Worker #2,15 finished after ~1 secs.
Worker #2,27 finished after ~1 secs.
Worker #1,7 finished after ~1 secs.
Worker #2,8 finished after ~1 secs.
Worker #2,1 finished after ~1 secs.

******************************************************************
Total Cores:  12
Tasks:  60
Grouped Execution Time [N tasks finished after ~M secs]:  [60, ~1 sec]
Maximum Execution Time:  1
******************************************************************

test06-result

Worker #1,2 finished after ~1 secs.
Worker #1,3 finished after ~1 secs.
Worker #1,5 finished after ~1 secs.
Worker #2,1 finished after ~1 secs.
Worker #2,2 finished after ~1 secs.
Worker #1,6 finished after ~1 secs.
Worker #1,1 finished after ~1 secs.
Worker #2,6 finished after ~1 secs.
Worker #1,4 finished after ~1 secs.
Worker #2,4 finished after ~1 secs.
Worker #2,5 finished after ~1 secs.
Worker #2,3 finished after ~1 secs.
Worker #2,7 finished after ~2 secs.
Worker #1,8 finished after ~2 secs.
Worker #1,7 finished after ~2 secs.
Worker #2,8 finished after ~2 secs.
Worker #2,9 finished after ~2 secs.
Worker #1,9 finished after ~2 secs.
Worker #1,10 finished after ~2 secs.
Worker #2,10 finished after ~2 secs.
Worker #1,11 finished after ~2 secs.
Worker #1,12 finished after ~2 secs.
Worker #2,11 finished after ~2 secs.
Worker #2,12 finished after ~2 secs.
Worker #1,2 finished after ~3 secs.
Worker #1,6 finished after ~3 secs.
Worker #1,8 finished after ~3 secs.
Worker #1,5 finished after ~3 secs.
Worker #1,1 finished after ~3 secs.
Worker #1,11 finished after ~3 secs.
Worker #1,7 finished after ~3 secs.
Worker #1,12 finished after ~3 secs.
Worker #1,10 finished after ~3 secs.
Worker #1,3 finished after ~3 secs.
Worker #1,9 finished after ~3 secs.
Worker #1,4 finished after ~3 secs.

******************************************************************
Total Cores:  12
Tasks:  36
Grouped Execution Time [N tasks finished after ~M secs]:  [12, ~1 sec] | [12, ~2 sec] | [12, ~3 sec]
Maximum Execution Time:  3
******************************************************************

test07-result

Worker #1,11 finished after ~1 secs.
Worker #1,1 finished after ~1 secs.
Worker #1,2 finished after ~1 secs.
Worker #1,5 finished after ~1 secs.
Worker #1,8 finished after ~1 secs.
Worker #1,13 finished after ~1 secs.
Worker #1,4 finished after ~1 secs.
Worker #1,9 finished after ~1 secs.
Worker #1,7 finished after ~1 secs.
Worker #1,3 finished after ~1 secs.
Worker #2,1 finished after ~1 secs.
Worker #1,12 finished after ~1 secs.
Worker #1,6 finished after ~1 secs.
Worker #1,10 finished after ~1 secs.
Worker #2,3 finished after ~2 secs.
Worker #2,4 finished after ~2 secs.
Worker #2,2 finished after ~2 secs.
Worker #2,5 finished after ~2 secs.
Worker #2,6 finished after ~2 secs.
Worker #2,7 finished after ~2 secs.
Worker #2,10 finished after ~2 secs.
Worker #2,9 finished after ~2 secs.
Worker #2,8 finished after ~2 secs.
Worker #2,12 finished after ~2 secs.
Worker #2,11 finished after ~2 secs.
Worker #2,14 finished after ~2 secs.
Worker #2,13 finished after ~2 secs.
Worker #2,15 finished after ~2 secs.
Worker #2,16 finished after ~3 secs.
Worker #2,17 finished after ~3 secs.
Worker #2,18 finished after ~3 secs.
Worker #2,19 finished after ~3 secs.
Worker #2,21 finished after ~3 secs.
Worker #2,22 finished after ~3 secs.
Worker #2,20 finished after ~3 secs.
Worker #2,24 finished after ~3 secs.
Worker #2,23 finished after ~3 secs.
Worker #2,27 finished after ~3 secs.
Worker #2,26 finished after ~3 secs.
Worker #2,25 finished after ~3 secs.
Worker #2,28 finished after ~3 secs.
Worker #2,29 finished after ~3 secs.
Worker #2,30 finished after ~4 secs.
Worker #2,31 finished after ~4 secs.
Worker #2,32 finished after ~4 secs.
Worker #2,33 finished after ~4 secs.
Worker #2,34 finished after ~4 secs.
Worker #2,35 finished after ~4 secs.
Worker #2,36 finished after ~4 secs.

******************************************************************
Total Cores:  12
Tasks:  49
Grouped Execution Time [N tasks finished after ~M secs]:  [14, ~1 sec] | [14, ~2 sec] | [14, ~3 sec] | [7, ~4 sec]
Maximum Execution Time:  4
******************************************************************