如果不在主线程中调用，则返回一个默认的并行队列。
If the call is made from any other thread, this function returns the default concurrent queue.
典型的消息代理系统经常将以异步方式处理来自相同流或并行队列的多个请求。
A typical message broker system will often be processing multiple requests from the same stream or queue in a parallel, asynchronous manner.
下面的类是一种常见的并行实用程序模型：任务队列。
The following class is a model of a common concurrent utility: a work queue.
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