Scala list.par stdout dissapears

Asked Dec 17 '19 at 13:24

Active Dec 20 '19 at 15:34

Viewed 101 times

I'm trying to map elements of a list and print some logs about the progress. But when I parallelise list, my stdout logs just stop appearing.

Why are they gone? Shouldn't the stdout of all threads go to the same place?

> List(1,2,3,4).map(println(_))
1
2
3
4
res1: List[Unit] = List((), (), (), ())

> List(1,2,3,4).par.map(println(_))
res2: scala.collection.parallel.immutable.ParSeq[Unit] = ParVector((), (), (), ())

Looks weird. Am I missing something here?
I'm doing it in Databricks, if that makes any difference.

edited Dec 20 '19 at 15:34

asked Dec 17 '19 at 13:24

1

Done `List(1,2,3,4).par.foreach(print)` works ? – amer Dec 17 '19 at 13:30
nope. `List(1,2,3,4).par.foreach(print)` does not print any results for me. However, `List(1,2,3,4).foreach(print)` works as expected and gives "1234". – Viacheslav Shalamov Dec 17 '19 at 13:35
Both cases are just hanging in my REPL (Scala 2.12.8, JDK 1.8.0_222) – Andriy Plokhotnyuk Dec 17 '19 at 13:46
Where and how are you running this? – Luis Miguel Mejía Suárez Dec 17 '19 at 14:32
I'm running it in a Databricks notebook. – Viacheslav Shalamov Dec 17 '19 at 14:51
1

@ViacheslavShalamov probably such environment is not suited for parallel processing, at least not for printing, probably each thread lost connection with the notebook shell, so even if they printed out to the std that wasn't passed to the notebook. – Luis Miguel Mejía Suárez Dec 17 '19 at 18:58
1

it's a REPL issue: [details about the bug](https://github.com/scala/bug/issues/8119), [explanation of the root cause](https://stackoverflow.com/questions/15176199/scala-parallel-collection-in-object-initializer-causes-a-program-to-hang/15176433#15176433), [lazy val initialization SIP](http://docs.scala-lang.org/sips/pending/improved-lazy-val-initialization.html) – Sergey Romanovsky Dec 17 '19 at 23:18

0 Answers0