Difference between thread and coroutine in Kotlin

What means that coroutine is like light-weight thread?

Coroutine, like a thread, represents a sequence of actions that are executed concurrently with other coroutines (threads).

What is the difference?

A thread is directly linked to the native thread in the corresponding OS (operating system) and consumes a considerable amount of resources. In particular, it consumes a lot of memory for its stack. That is why you cannot just create 100k threads. You are likely to run out of memory. Switching between threads involves OS kernel dispatcher and it is a pretty expensive operation in terms of CPU cycles consumed.

A coroutine, on the other hand, is purely a user-level language abstraction. It does not tie any native resources and, in the simplest case, uses just one relatively small object in the JVM heap. That is why it is easy to create 100k coroutines. Switching between coroutines does not involve OS kernel at all. It can be as cheap as invoking a regular function.

Are kotlin's coroutines actually running in parallely / concurrently? Even in multi-core system, there is only one coroutine running at any given time (is it right?)

A coroutine can be either running or suspended. A suspended coroutine is not associated to any particular thread, but a running coroutine runs on some thread (using a thread is the only way to execute anything inside an OS process). Whether different coroutines all run on the same thread (a thus may use only a single CPU in a multicore system) or in different threads (and thus may use multiple CPUs) is purely in the hands of a programmer who is using coroutines.

In Kotlin, dispatching of coroutines is controlled via coroutine context. You can read more about then in the Guide to kotlinx.coroutines

Here I'm starting 100000 coroutines, what happens behind this code?

Assuming that you are using launch function and CommonPool context from the kotlinx.coroutines project (which is open source) you can examine their source code here:

• launch is defined here https://github.com/Kotlin/kotlinx.coroutines/blob/master/core/kotlinx-coroutines-core/src/main/kotlin/kotlinx/coroutines/experimental/Builders.kt
• CommonPool is defined here https://github.com/Kotlin/kotlinx.coroutines/blob/master/core/kotlinx-coroutines-core/src/main/kotlin/kotlinx/coroutines/experimental/CommonPool.kt

The launch just creates new coroutine, while CommonPool dispatches coroutines to a ForkJoinPool.commonPool() which does use multiple threads and thus executes on multiple CPUs in this example.

The code that follows launch invocation in {...} is called a suspending lambda. What is it and how are suspending lambdas and functions implemented (compiled) as well as standard library functions and classes like startCoroutines, suspendCoroutine and CoroutineContext is explained in the corresponding Kotlin coroutines design document.

Since I used coroutines only on JVM, I will talk about JVM backend, there are also Kotlin Native and Kotlin JavaScript but these backends for Kotlin are out of my scope.

So let's start with comparing Kotlin coroutines to other languages coroutines. Basically, you should know that there are two types of Coroutines: stackless and stackful. Kotlin implements stackless coroutines - it means that coroutine doesn't have its own stack, and that limiting a little bit what coroutine can do. You can read a good explanation here.

Examples:

• Stackless: C#, Scala, Kotlin
• Stackful: Quasar, Javaflow

What it means that coroutine is like light-weight thread?

It means that coroutine in Kotlin doesn't have its own stack, it doesn't map on a native thread, it doesn't require context switching on a processor.

What is the difference?

Thread - preemptively multitasking. (usually). Coroutine - cooperatively multitasking.

Thread - managed by OS (usually). Coroutine - managed by a user.

Are kotlin's coroutines actually running in parallel / concurrently?

It depends, you can run each coroutine in its own thread, or you can run all coroutines in one thread or some fixed thread pool.

More about how coroutines execute here.

Even in a multi-core system, there is only one coroutine running at any given time (is it right?)

No, see the previous answer.

Here I'm starting 100000 coroutines, what happens behind this code?

Actually, it depends. But assume that you write the following code:

fun main(args: Array<String>) {
    for (i in 0..100000) {
        async(CommonPool) {
            delay(1000)
        }
    }
}

This code executes instantly.

Because we need to wait for results from async call.

So let's fix this:

fun main(args: Array<String>) = runBlocking {
    for (i in 0..100000) {
        val job = async(CommonPool) {
            delay(1)
            println(i)
        }

        job.join()
    }
}

When you run this program kotlin will create 2 * 100000 instances of Continuation, which will take a few dozen Mb of RAM, and in console, you will see numbers from 1 to 100000.

So lets rewrite this code in this way:

fun main(args: Array<String>) = runBlocking {

    val job = async(CommonPool) {
        for (i in 0..100000) {
            delay(1)
            println(i)
        }
    }

    job.join()
}

What we achieve now? Now we create only 100001 instances of Continuation, and this is much better.

Each created Continuation will be dispatched and executed on CommonPool (which is a static instance of ForkJoinPool).