Multithreading using Kotlin Coroutines

Question

I'm experimenting with Kotlin Coroutines and have following code:

fun main(args: Array<String>) = runBlocking {
    val cores = Runtime.getRuntime().availableProcessors()
    println("number of cores: $cores")

    val jobs = List(10) {
        async(CommonPool) {
            delay(100)
            println("async #$it on thread ${Thread.currentThread().name}")
        }
    }
    jobs.forEach { it.join() }
}

This is my output:

number of cores: 4
async number:0 on thread ForkJoinPool.commonPool-worker-2
async number:2 on thread ForkJoinPool.commonPool-worker-3
async number:3 on thread ForkJoinPool.commonPool-worker-3
async number:4 on thread ForkJoinPool.commonPool-worker-3
async number:5 on thread ForkJoinPool.commonPool-worker-3
async number:1 on thread ForkJoinPool.commonPool-worker-1
async number:7 on thread ForkJoinPool.commonPool-worker-3
async number:6 on thread ForkJoinPool.commonPool-worker-2
async number:9 on thread ForkJoinPool.commonPool-worker-3
async number:8 on thread ForkJoinPool.commonPool-worker-1

According to Roman Elizarov's answer to another coroutines related question:

"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."

According to Java 8 documentation:

"For applications that require separate or custom pools, a ForkJoinPool may be constructed with a given target parallelism level; by default, equal to the number of available processors."

Why there are only 3 worker threads being used? There are the same 3 worker threads even when I increase number of async tasks to 1000+.

My configuration: Mac/High Sierra with dual core cpu (with Hyper-threading, thus 4 visible cores), Kotlin 1.2, kotlinx-coroutines-core:0.19.3 and JVM 1.8

s1m0nw1 · Accepted Answer · 2018-11-08 09:56:19Z

6

If you look at the implementation of CommonPool, you'll notice that it's working on java.util.concurrent.ForkJoinPool or a thread pool with the following size:

(Runtime.getRuntime().availableProcessors() - 1).coerceAtLeast(1)

With 4 available processors, this will result in 3 which answers why you do see 3 worker threads.

The ForkJoinPool-size can be determined as follows (will be the same):

ForkJoinPool.commonPool().parallelism

Please see this answer if you work with coroutines version >= 1.0

edited Nov 8, 2018 at 9:56

answered Dec 1, 2017 at 11:34

s1m0nw1

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s1m0nw1 · Accepted Answer · 2018-11-05 16:02:50Z

4

As of Coroutines 1.0, the code will look slightly different since CommonPool will be replaced with Dispatchers.Default now:

fun main(args: Array<String>) = runBlocking {
    val cores = Runtime.getRuntime().availableProcessors()
    println("number of cores: $cores")

    val jobs = List(10) {
        async(Dispatchers.Default) {
            delay(100)
            println("async #$it on thread ${Thread.currentThread().name}")
        }
    }
    jobs.forEach { it.join() }
}

Also, you will now get the following:

It is backed by a shared pool of threads on JVM. By default, the maximal number of threads used by this dispatcher is equal to the number CPU cores, but is at least two.

answered Nov 5, 2018 at 16:02

s1m0nw1

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