在Scala循环中创建ForkJoinTask用于多线程时的奇怪行为

Question

The job is to convert the values from the src array and write the new values to the dst array.

When I create ForkJoinTasks in a while loop

val height: Int = 4;
val numTasks: Int = 2;

var tasks = scala.collection.mutable.ListBuffer.empty[ForkJoinTask[Unit]]

val jump: Int = src.height / numTasks
var from: Int = 0;

while (from < height) {
  val end: Int = height.min(from + jump);
  val t: ForkJoinTask[Unit] = task {
    run(src, dst, from, end) // (2,2), (2,1), what is happening?
  }
  from = end
}

for (t <- tasks.toList) {
  t.join()
}

Then strangely, the run function takes the (from, end) arguments value as (2, 2). But If I split it manually into two tasks, then it works normally, as (0,1) and (1,2).

val t1 = task {
  run(src, dst, 0, height / 2); // (0, 1)
}
val t2 = task {
  run(src, dst, height / 2, height); // (1, 2)
}

t1.join()
t2.join()

I am having hard time to figure out what is going on. This is my very first Scala program, so I might be missing something very trivial. Any comment would be appreciated.

Thank you.

Answer 1

Removing references to src or dst your code seems ok as returns ranges

0 2
2 4

import shapeless.PolyDefns.~>
import shapeless.{ HList, HNil }
import io.netty.util.internal.chmv8.ForkJoinTask

object main extends App {

  val height: Int = 4;
  val numTasks: Int = 2;

  var tasks = scala.collection.mutable.ListBuffer.empty[ForkJoinTask[Unit]]

  val jump: Int = height / numTasks
  var from: Int = 0;

  while (from < height) {
    val end: Int = height.min(from + jump);
    val t: ForkJoinTask[Unit] =  {
      run( from, end) // (2,2), (2,1), what is happening?
    }
    from = end
  }

  for (t <- tasks.toList) {
    t.join()
  }


  def run (from: Int, end:Int) ={
    println(s"$from $end")
    null
  }

}