.net - ThreadPool.QueueUserWorkItem与Parallel.For

Question

我试图了解Parralel.For和ThreadPool.QueueUserWorkItem之间的区别。

硬件和软件:

英特尔i5(四核)

Windows 7 64位教授

DotNet 4.5

案例1代码:ThreadPool

for (int index = 0; index < 5; index++)
{
  ThreadPool.QueueUserWorkItem((indexParam) =>
  {
    int threadID = Thread.CurrentThread.ManagedThreadId;
    Thread.Sleep(1000);
    BeginInvoke((Action)delegate { listBox1.Items.Add("Completed " + indexParam.ToString() + " using thread " + threadID.ToString() + "  (" + DateTime.Now.Second.ToString() + "." + DateTime.Now.Millisecond.ToString("000") + ")"); });
  }, index);
}

输出:

Completed 0 using thread 10 (45.871)
Completed 1 using thread 11 (45.875)
Completed 2 using thread 12 (45.875)
Completed 3 using thread 13 (45.875)
Completed 4 using thread 10 (46.869)

情况2代码:Parallel.For

  ParallelLoopResult result = Parallel.For(0, 5, (int index, ParallelLoopState loopState) =>
  {
    int threadID = Thread.CurrentThread.ManagedThreadId;
    Thread.Sleep(1000);
    BeginInvoke((Action)delegate { listBox1.Items.Add("Completed " + index.ToString() + " using thread " + threadID.ToString() + "  (" + DateTime.Now.Second.ToString() + "." + DateTime.Now.Millisecond.ToString("000") + ")"); });
  });

输出:

Completed 0 using thread 10 (16.923)
Completed 1 using thread 11 (16.925)
Completed 2 using thread 12 (16.925)
Completed 3 using thread 13 (16.926)
Completed 4 using thread 14 (16.926)

问题:

从案例1的结果来看，似乎只有四个线程处于 Activity 状态，然后使用第一个空闲线程来完成最终任务。在情况2中，似乎有五个线程被立即专用并“同时”执行。
为什么QueueUserWorkItem的线程处理不像并行类那样不使用第五个线程？
(ThreadPool.GetAvailableThreads(...)确认1023个工作线程可用)。

Answer 1

我相信“ Activity ”线程和“可用”线程之间是有区别的。线程池将重用线程，如果它决定需要更多线程，它将开始将可用线程移到 Activity 线程。如果您想一次启动很多东西，这可能会令人沮丧，因为每个可用线程可能需要大约2秒钟才能启动。

MSDN Managed Threadpool

As part of its thread management strategy, the thread pool delays before creating threads. Therefore, when a number of tasks are queued in a short period of time, there can be a significant delay before all the tasks are started.

如果要重复运行这些任务或添加任务，则应该看到其他线程处于 Activity 状态。