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!

delay

suspends while it waits, which allows the other

launch

block to run on the same single thread that is available

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

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

runBlocking

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

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.

main will then terminate before the coroutines finish @svenjacobs

Right, in this case best work with

async

and

await

Or use runBlocking 😁

I though problem by OP was that

launch

inside

runBlocking

do not run concurrently? 🤔