如果可能，我应该始终使用并行流吗? [英] Should I always use a parallel stream when possible?

问题描述

使用 Java 8 和 lambdas，可以轻松地将集合作为流进行迭代，并且与使用并行流一样容易.文档中的两个示例，第二个使用parallelStream:

With Java 8 and lambdas it's easy to iterate over collections as streams, and just as easy to use a parallel stream. Two examples from the docs, the second one using parallelStream:

myShapesCollection.stream()
    .filter(e -> e.getColor() == Color.RED)
    .forEach(e -> System.out.println(e.getName()));

myShapesCollection.parallelStream() // <-- This one uses parallel
    .filter(e -> e.getColor() == Color.RED)
    .forEach(e -> System.out.println(e.getName()));

只要不关心顺序，使用并行总是有益的吗?有人会认为将工作分配到更多内核上会更快.

As long as I don't care about the order, would it always be beneficial to use the parallel? One would think it is faster dividing the work on more cores.

还有其他考虑吗?什么时候应该使用并行流，什么时候应该使用非并行流?

Are there other considerations? When should parallel stream be used and when should the non-parallel be used?

^{(提出这个问题是为了引发关于如何以及何时使用并行流的讨论，不是因为我认为总是使用它们是个好主意.)}

推荐答案

与顺序流相比，并行流的开销要高得多.协调线程需要大量时间.默认情况下，我会使用顺序流，如果

A parallel stream has a much higher overhead compared to a sequential one. Coordinating the threads takes a significant amount of time. I would use sequential streams by default and only consider parallel ones if

• 我有大量的项目要处理(或者每个项目的处理都需要时间并且是可并行的)

• I have a massive amount of items to process (or the processing of each item takes time and is parallelizable)

我首先遇到了性能问题

我还没有在多线程环境中运行该进程(例如:在 Web 容器中，如果我已经有许多并行处理的请求，则可以在每个请求中添加额外的并行层负面影响多于正面影响)

I don't already run the process in a multi-thread environment (for example: in a web container, if I already have many requests to process in parallel, adding an additional layer of parallelism inside each request could have more negative than positive effects)

在您的示例中，性能无论如何都将由对 System.out.println() 的同步访问驱动，并且使此过程并行将没有任何影响，甚至是负面影响.

In your example, the performance will anyway be driven by the synchronized access to System.out.println(), and making this process parallel will have no effect, or even a negative one.

此外，请记住并行流并不能神奇地解决所有同步问题.如果进程中使用的谓词和函数使用共享资源，则必须确保一切都是线程安全的.尤其是，如果你并行，副作用是你真正需要担心的事情.

Moreover, remember that parallel streams don't magically solve all the synchronization problems. If a shared resource is used by the predicates and functions used in the process, you'll have to make sure that everything is thread-safe. In particular, side effects are things you really have to worry about if you go parallel.

无论如何，测量，不要猜测！只有测量才能告诉您并行性是否值得.

In any case, measure, don't guess! Only a measurement will tell you if the parallelism is worth it or not.

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