c# - 为什么 Parallel.ForEach 不运行多个线程？

Question

今天我尝试对 foreach 语句进行一些优化，它适用于 XDocument。

优化前:

foreach (XElement elem in xDoc.Descendants("APSEvent").ToList())
{
    //some operations
}

优化后:

Parallel.ForEach(xDoc.Descendants("APSEvent").ToList(), elem =>
{
    //same operations
});

我看到 Parallel.ForEach(...) 中的 .NET 只打开了一个线程!因此 Parallel 的时间跨度比标准的 foreach 大。

为什么你认为.NET只开了1个线程？因为文件锁定？谢谢

Answer 1

按照设计，Parallel.ForEach 可能会使用比请求更少的线程来实现更好的性能。根据 MSDN [link] :

By default, the Parallel.ForEach and Parallel.For methods can use a variable number of tasks. That's why, for example, the ParallelOptions class has a MaxDegreeOfParallelism property instead of a "MinDegreeOfParallelism" property. The idea is that the system can use fewer threads than requested to process a loop.

The .NET thread pool adapts dynamically to changing workloads by allowing the number of worker threads for parallel tasks to change over time. At run time, the system observes whether increasing the number of threads improves or degrades overall throughput and adjusts the number of worker threads accordingly.