.NET - 阻塞主线程，直到有任何可用线程

Question

我有一个进程，我的主线程正在读取文件并将其拆分为多个部分。然后这些部分需要进一步处理。我想利用任何可用的线程，以便下游处理使用尽可能多的 CPU（或尽可能多的内核）。我不想从主线程创建过多的积压，所以我需要主线程等待添加到队列中，直到有另一个可用线程。

我看到很多类似VB.NET 4.0: Looking to execute multiple threads, but wait until all threads are completed before resuming 的文章，但它们都在等待所有线程完成，而我只需要任何线程都可用

这是我可以使用Task Parallel Library 解决的问题，还是我应该手动创建线程并监控线程池？

Using Reader As New StreamReader(FileName)
    Do
        CurrentBlockSize = Reader.ReadBlock(CurrentBuffer, 0, BufferSize)

        RunningBuffer &= New String(CurrentBuffer)

        If RunningBuffer.Contains(RowDelimiter) Then
            LineParts = RunningBuffer.Split(RowDelimiter)

            For I As Integer = 0 To LineParts.Count - 1
                If I < LineParts.Count - 1 Then

                    'Make synchronous call that blocks until' 
                    'another thread is available to process the line'
                    AddLineToTheProcessingQueue(CurrentLine)

                Else
                    RunningBuffer = LineParts(I)
                End If
            Next
        End If

    Loop While CurrentBlockSize = BufferSize
End Using

Answer 1

将此代码粘贴到新的控制台应用程序中。

Imports System.Threading

Module Module1

    ' I just picked 6 randomly, not sure what is a good strategy for picking this number
    ' also, not sure what is the difference between a Worker Thread and a Completion thread
    Const MaxWorkerThreads As Integer = 6
    Const MaxCompletionPortThreads As Integer = 6

    Sub Main()

        ThreadPool.SetMaxThreads(MaxWorkerThreads, MaxCompletionPortThreads)

        Dim availableWorkerThreads As Integer
        Dim availableCompletionPortThreads As Integer

        For i As Integer = 0 To 100

            ' GetAvailableThreads returns results via output parameters
            ThreadPool.GetAvailableThreads(availableWorkerThreads, availableCompletionPortThreads)

            Dim tries As Integer = 0

            Do While (availableWorkerThreads = 0)
                ' this loop does not execute if there are available threads
                ' you may want to add a fail-safe to check "tries" in case the child threads get stuck
                tries += 1

                Console.WriteLine(String.Format("waiting to start item {0}, attempt {1}, available threads: {2}, {3}", i, tries, availableWorkerThreads, availableCompletionPortThreads))

                ' failure to call Sleep will make your program unresponsive
                Thread.Sleep(1000)

                ' call GetAvailableThreads again for the next test at the top of the loop
                ThreadPool.GetAvailableThreads(availableWorkerThreads, availableCompletionPortThreads)
            Loop

            ' this is how you pass parameters to a thread created through QueueUserWorkItem
            Dim parameters As Object() = {i}
            ThreadPool.QueueUserWorkItem(AddressOf DoWork, parameters)
            ' According to MSDN, you must Sleep after calling QueueUserWorkItem, or else the current thread will just exit
            Thread.Sleep(500)

        Next

    End Sub

    Sub DoWork(parameters As Object())
        Dim itemNumber = parameters(0)
        Dim sleepLength = itemNumber * 1000
        Console.WriteLine(String.Format("Item: {0} - sleeping for {1} miliseconds.", itemNumber, sleepLength))
        Thread.Sleep(sleepLength)
        Console.WriteLine(String.Format("Item: {0} - done sleeping.", itemNumber))
    End Sub

End Module

Answer 2

我不确定您究竟为什么要这样做，但您可以通过使用BlockingCollection 或设置了BoundedCapacity 的数据流块来实现非常相似的效果。

例如，如果您将容量设置为 1 并且您的消费者此时正忙，那么您将无法将第二个项目添加到队列中，直到其中一个消费者完成其当前工作并将该项目从队列。这两个版本都为您提供了一种等待，直到您可以将另一个项目添加到队列中。