KotlinCoroutineDispatchers.kt

import kotlinx.coroutines.Dispatchers
import kotlinx.coroutines.launch
import kotlinx.coroutines.runBlocking
import kotlinx.coroutines.withContext
import org.junit.jupiter.api.Test
import kotlin.time.ExperimentalTime
import kotlin.time.measureTime

class DispatcherTest {


    @Test
    @ExperimentalTime
    fun testDefaultDispatcher() = runBlocking {

        val duration = measureTime {
            withContext(Dispatchers.Default) {
                repeat(times) {
                    launch {
                        blockingIOOperation()
                    }
                }
            }
        }
        println("Time taken by Dispatchers.Default: $duration")
    }

    @Test
    @ExperimentalTime
    fun testIODispatcher() = runBlocking {
        val duration = measureTime {
            withContext(Dispatchers.IO) {
                repeat(times) {
                    launch {
                        blockingIOOperation()
                    }
                }
            }
        }
        println("Time taken by Dispatchers.IO: $duration")
    }

    /**
     * This represents non-Kotlin blocking code.
     */
    private fun blockingIOOperation() {
        Thread.sleep(sleep)
    }

    companion object {
        const val times = 128
        const val sleep = 1000L
    }
}

On a 8-core hyperthreaded CPU, the Dispatchers.Default has 16 threads.
Dispatchers.IO has 64 threads.

For times value > 64 and of the order of 2, Default dispatcher takes 4 times (64/16) the duration of IO dispatcher to execute.