Why Kotlin coroutines run in the same thread sequentially?

Question

I thought that calling a "suspend" function from coroutine context using launch makes the call asynchronous. But in the example below I see that 2 invocations of placeOrder method are not running in the same thread one after another. What is my mistake?

import kotlinx.coroutines.launch
import kotlinx.coroutines.runBlocking
import java.io.File

fun main() = runBlocking {
    t("1")
    launch {
        t("2")
        placeOrder("C:\\Users")
        t("3")
    }
    launch {
        t("12")
        placeOrder("C:\\Program Files")
        t("13")
    }
    t("4")
}


fun t(s: String) {
    val currentThread = Thread.currentThread()
    println(s + ": " + currentThread.name + " " +     currentThread.id)
}

suspend fun placeOrder(d:String): String {
    t("placeOrder $d")
    val user = createUser(d) // asynchronous call to user service
    val order = createOrder(user) // asynchronous call to order service
    t("placeOrder $d finished")
    return order
}

suspend fun createUser(d:String): String {
    t("createUser $d")
    val toString = File(d).walk().map {
        it.length()
    }.sum().toString()
    t("createUser $d finished")
    return toString
}

suspend fun createOrder(user: String): String {
    t("createOrder $user")
    val toString = File("C:\\User").walk().map {
        it.length()
    }.sum().toString()
    t("createOrder $user finished")
    return toString
}

Output:

1: main 1
4: main 1
2: main 1
placeOrder C:\Users: main 1
createUser C:\Users: main 1
createUser C:\Users finished: main 1
createOrder 1094020270277: main 1
createOrder 1094020270277 finished: main 1
placeOrder C:\Users finished: main 1
3: main 1
12: main 1
placeOrder C:\Program Files: main 1
createUser C:\Program Files: main 1
createUser C:\Program Files finished: main 1
createOrder 5651227104: main 1
createOrder 5651227104 finished: main 1
placeOrder C:\Program Files finished: main 1
13: main 1

Answer 1

Instead of writing suspendable IO, you wrote blocking IO:

File(d).walk().map {
    it.length()
}

Your functions never actually suspend and instead they block the single thread associated with their runBlocking dispatcher.

You gave your coroutines no opportunity to execute concurrently.

If you applied withContext(IO) { ... } around the above code, you'd get concurrency, but of the plain-old Java type, several threads being blocked in IO operations together.