使用 Kotlin 协程的多线程

Question

我正在试验 Kotlin Coroutines 并有以下代码：

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 对另一个协程相关问题的：

"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."

根据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."

为什么只使用了 3 个工作线程？即使我将异步任务的数量增加到 1000+，也有相同的 3 个工作线程。

我的配置： Mac/High 双核 Sierra cpu（Hyper-threading，因此有 4 个可见内核），Kotlin 1.2，kotlinx-coroutines-core:0.19.3 和 JVM 1.8

Answer 1

如果您查看 CommonPool 的实现，您会注意到它正在 java.util.concurrent.ForkJoinPool 或具有以下大小的线程池上工作：

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

使用 4 个可用处理器，这将导致 3 这回答了为什么您看到 3 个工作线程。

ForkJoinPool-size 可以确定如下（将相同）：

ForkJoinPool.commonPool().parallelism

如果您使用协程版本>=1.0

，请参阅

Answer 2

从 Coroutines 1.0 开始，代码看起来会略有不同，因为 CommonPool 现在将替换为 Dispatchers.Default：

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() }
}

此外，您现在将获得以下内容：

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.

使用 Kotlin 协程的多线程

Multithreading using Kotlin Coroutines

multithreading

jvm

kotlin

kotlinx.coroutines