Why does parallelStream use a ForkJoinPool, not a normal thread pool?

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Refering to Java's Fork/Join vs ExecutorService - when to use which?, a traditional thread pool is usually used to process many independent requests; and a ForkJoinPool is used to process coherent/recursive tasks, where a task may spawn another subtask and join on it later.

So, why does Java-8's parallelStream use ForkJoinPool by default but not a traditional executor?

In many cases, we use forEach() after a stream() or parallelStream() and then submit a functional interface as an argument. From my point of view, these tasks are independent, aren't they?

forkjoinpool java-stream parallel-processing concurrency threadpool
1 Answers

One important thing is that a ForkJoinPool can execute "normal" tasks (e.g. Runnable, Callable) as well, so it's not just meant to be used with recursively-created tasks.

Another (important) thing is that ForkJoinPool has multiple queues, one for each worker thread, for the tasks, where a normal executor (e.g. ThreadPoolExecutor) has just one. This has much impact on what kind of tasks they should run.

The smaller and the more tasks a normal executor has to execute, the higher is the overhead of synchronization for distributing tasks to the workers. If most of the tasks are small, the workers will access the internal task queue often, which leads to synchronization overhead.

Here's where the ForkJoinPool shines with its multiple queues. Every worker just takes tasks from its own queue, which doesn't need to be synchronized by blocking most of the time, and if it's empty, it can steal a task from another worker, but from the other end of the queue, which also leads rarely to synchronization overhead as work-stealing is supposed to be rather rare.

Now what does that have to do with parallel streams? The streams-framework is designed to be easy to use. Parallel streams are supposed to be used when you want to split something up in many concurrent tasks easily, where all tasks are rather small and simple. Here's the point where the ForkJoinPool is the reasonable choice. It provides the better performance on huge numbers of smaller tasks and it can handle longer tasks as well, if it has to.

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