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kotlinlang

I hope `parTraverse` uses fibers underneath, how can I see that effect apart from measuring time (like by printing Fiber-id or something). I tried this example:

```@Test
    fun `parTraverse + blocking`() {
        val time = measureTimeMillis {
            runBlocking {
                (1..20).parTraverse {
                    blockingWork(it)
                }
            }
        }
        // 3s, multiple blocked bg threads
        println("Done in $time ms [${Thread.currentThread().name}]")
    }```

Hey <@UKN6KD936>,
Coroutines/Fibers/etc are all still being scheduled on `ExecutorService` like you'd manually do from Java.

The only offer higher level abstractions so you don't have to deal with them manually.
Afaik "@coroutine#1" is being printed if you use dispatchers from KotlinX, but those are still eventually scheduled on the `ForkJoinPool` like you see happening here.

We provide an `IOPool` (which you should definitely use when using `Thread.sleep`, since that blocks the thread). If you use that one you'll see `arrow-effects-worker-x` being printed instead.

Scheduling coroutine is done by `intercepted()` in combination with a dispatching `ContinuationInterceptor`.

Long story short, `suspend`/coroutines use the same technique as `IO` in Arrow  Fx or Cats effects in Scala, RxJava, Reactor etc. It hides the complexity of using `Executors` directly, which offers a much more powerful and safe way to work with highly concurrent programs.

Thanks Simon, I shall explore about IOPool

True green threads will only come to the JVM with Project Loom, which would allow you to use `VirtualThread` everywhere instead of executors/pools. This would allow you to treat every thread as a blocking-capable thread (`IOPool`) without paying any of the costs of doing so.

That open the door to some techniques that now would be considered too expensive, like looping on a thread to wait for a result. Or using `Thread.sleep` to get better precision over `ScheduledExecutorService`.