Hi. Trying to dig into the docs &amp; tutorials on <https://kotlinlang.org/docs/coroutines-basic-jvm...

# coroutines

[Fredrik Rødland]

Hi. Trying to dig into the docs & tutorials on
https://kotlinlang.org/docs/coroutines-basic-jvm.html and https://play.kotlinlang.org/hands-on/Introduction%20to%20Coroutines%20and%20Channels/01_Introduction for the n-th time to try to get the grip on them 😉
The doc says that the following code will return an arbitrary number and I agree with it; we don’t wait for the coroutines to finish.
However it does not for me - I’ve run it 20 times - each resulting in the sum being `500000500000`…
(java 15, kotlin 1.4.31, coroutines 1.4.3, macOS)
Any tips/insight as to why?

package samples

import kotlinx.coroutines.GlobalScope
import kotlinx.coroutines.launch
import java.util.concurrent.atomic.AtomicLong

fun main() {
    val c = AtomicLong()
        for (i in 1..1_000_000L)
        GlobalScope.launch {
            c.addAndGet(i)
        }
        println(c.get())
}

PS! if this is better suited as question in #getting-started please tell me and I’ll move it there.

[Zach Klippenstein (he/him) [MOD]]

Just because it’s arbitrary doesn’t mean it necessarily will actually change on every run - just that it could. It’s possible your machine just happens to be scheduling threads the same way way for each run, especially if the load from other processes isn’t changing much between executions.

[Zach Klippenstein (he/him) [MOD]]

You could try using differently-sized thread pools instead of global scope

[Fredrik Rødland]

I’ve tried GlobalScope.launch(<http://Dispatchers.IO|Dispatchers.IO>) - which gives the behaviour

[Fredrik Rødland]

I understand it’s not guaranteed to return the right sum, but I find it strange not being able to reproduce it on my machine

[Fredrik Rødland]

cc: @elizarov - do you have any thoughts on this?

[Zach Klippenstein (he/him) [MOD]]

So both Dispatchers.Default and <http://Dispatchers.IO|Dispatchers.IO> give the same behavior? I’m surprised too

[Fredrik Rødland]

yes, they do

[Fredrik Rødland]

Running with Dispatchers.IO takes longer time consistently tough:

[Fredrik Rødland]

fun main() {
    listOf(Dispatchers.Default, <http://Dispatchers.IO|Dispatchers.IO>).forEach { 
        println("time used: with $it: " + measureTimeMillis { test1(it) }) 
    }
}

private fun test1(dispatcher: CoroutineDispatcher) {
    val c = AtomicLong()
    for (i in 1..1_000_000L)
        GlobalScope.launch(dispatcher) {
            c.addAndGet(i)
        }
    println(c.get())
}

=>

500000500000
time used: with Dispatchers.Default: 2753
500000500000
time used: with <http://Dispatchers.IO|Dispatchers.IO>: 6783