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
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
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.
来源:https://stackoverflow.com/questions/47591733/multithreading-using-kotlin-coroutines