hi everyone. total n00b question here. all help gr...

# coroutines

[Paul Martin]

hi everyone. total n00b question here. all help gratefully received. 🙏

i've got this code, which is based on one of the examples from the kotlinlang.org docs:

fun main() = runBlocking {
    launch {
        println("block 1: start")
        delay(2000)
        println("block 1: end")
    }
    launch {
        println("block 2: start")
        delay(1000)
        println("block 2: end")
    }

    Unit
}

as expected, the output is:

block 1: start
block 2: start
block 2: end
block 1: end

so the two launch blocks are running concurrently.

next i try and replace the delay calls with some functions of my own:

fun functionThatTakesALongTime() {
    for (j in 1..5) {
        for (i in 1..1_000_000_000) {
            i * i * i
        }
    }
}

fun functionThatTakesAShortTime() {
    1 + 1
}

fun main() = runBlocking {
    launch {
        println("block 1: start")
        functionThatTakesALongTime()
        println("block 1: end")
    }
    launch {
        println("block 2: start")
        functionThatTakesAShortTime()
        println("block 2: end")
    }

    Unit
}

this time the output is this:

block 1: start
block 1: end
block 2: start
block 2: end

in other words it ran the whole of the first launch block before starting on the second one. want i want is for them to run concurrently as in the first example.

i guess there's something fundamental that i'm missing! what is it about delay which enables the launch blocks to run concurrently?

thanks!

[octylFractal]

delay suspends while it waits, which allows the other launch block to run on the same single thread that is available

[octylFractal]

this might help if you're not aware of suspending vs. blocking https://medium.com/@elye.project/understanding-suspend-function-of-coroutines-de26b070c5ed

[Paul Martin]

thank you - i understand now why this works like this.

[Paul Martin]

i think i'm getting my head around it now. apparently what i actually want is for my two functions to run in parallel, not concurrently. i didn't appreciate that they mean two different things.

[Paul Martin]

i think the docs are not very clear in this respect. the examples make it seem like two functions with a delay in them can be running in parallel, but they're actually running concurrently in a single thread

[octylFractal]

yes, though I think the issue is that requires understanding more about suspension and runBlocking

[octylFractal]

it's part of the issue where you don't want the documentation to be too much at once, but you also need to touch on important concepts

[octylFractal]

I think the docs could do better at emphasizing what suspending really means though

[uli]

runBlocking defaults to using the blocked thread for it's dispatcher. So it is single threaded.

[uli]

You can pass Dispatcher.default as parameter to run blocking and your code works as expected again

[svenjacobs]

runBlocking should imho only be used in tests... when you replace runBlocking with with(GlobalScope) your code should run in parallel as expected. Note that the usage of GlobalScope is okay for your little test application but usually there are more granular scopes that should be used.

[uli]

main will then terminate before the coroutines finish @svenjacobs

[svenjacobs]

Right, in this case best work with async and await

[svenjacobs]

Or wait on the Job returned by launch with join

[uli]

Or use runBlocking 😁

[svenjacobs]

I though problem by OP was that launch inside runBlocking do not run concurrently? 🤔