使用Kotlin协程进行多线程 [英] Multithreading using Kotlin Coroutines

问题描述

我正在尝试科林式协同程序，并具有以下代码:

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() }
}

这是我的输出:

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

根据Roman Elizarov的回答另一个与协程相关的问题:

"该发布会创建新的协程，而CommonPool会派遣 关联到确实使用多个的ForkJoinPool.commonPool() 线程，因此在此示例中可以在多个CPU上执行."

根据Java 8 文档:

"对于需要单独或自定义池的应用程序， 可以使用给定的目标并行度级别构造ForkJoinPool； 默认情况下，等于可用处理器的数量."

为什么只使用3个工作线程?即使将异步任务的数量增加到1000+，也有3个工作线程.

我的配置: 具有双核CPU的Mac/High Sierra(具有超线程，因此有4个可见内核)，Kotlin 1.2，kotlinx-coroutines-core:0.19.3和JVM 1.8

解决方案

如果您查看此答案

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

解决方案

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

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