Question

下面是我能够开发的用于计算嵌套在VB.NET（Visual Studio 2010，.NET Framework 4）中的Parallel.for循环内的循环内的运行总和的最佳表示。请注意，当在屏幕上显示“sum”的结果时，两个总和之间存在细微差别，因此并行化变体中的信息丢失。那么信息是如何丢失的，以及发生了什么？任何人都可以提供一些关于在这种背景下保持运行总和的方法的“显微手术”吗？（注意Parallel.for的新用户：我通常不使用从零开始的方法，因此在Parallel.for语句中，I1循环最多为101，因为代码使用101-1作为上限。这是因为假设基于零的计数器，MS开发了并行代码：

    Dim sum As Double = 0
    Dim lock As New Object
    Dim clock As New Stopwatch
    Dim i, j As Integer
    clock.Start()
    sum = 0
    For i = 1 To 100
        For j = 1 To 100
            sum += Math.Log(0.9999)
        Next j
    Next i
    clock.Stop()
    MsgBox(sum & "  " & clock.ElapsedMilliseconds)
    sum = 0
    clock.Reset()
    clock.Start()
    Parallel.For(1, 101, Sub(i1)
                             Dim temp As Double = 0
                             For j1 As Integer = 1 To 100
                                 temp += Math.Log(0.9999)
                             Next
                             SyncLock lock
                                 sum += temp
                             End SyncLock
                         End Sub)
    clock.Stop()
    MsgBox(sum & "  " & clock.ElapsedMilliseconds)

Answer 1

你正在使用双打和双精简是不准确。在非并行循环中，所有错误都直接存储在sum中。在并行循环中，您有一个额外的tmp，稍后会添加到sum中。在非并行循环中使用相同的tmp（在内循环运行后添加到sum），最终结果将等于。

 Dim sum As Double = 0
    Dim lock As New Object
    Dim clock As New Stopwatch
    Dim i, j As Integer
    clock.Start()
    sum = 0
    For i = 1 To 100
        For j = 1 To 100
            sum += Math.Log(0.9999)
        Next j
    Next i
    clock.Stop()
    Console.WriteLine(sum & "  " & clock.ElapsedMilliseconds)
    sum = 0
    clock.Reset()

    clock.Start()
    sum = 0
    For i = 1 To 100
        Dim tmp As Double = 0
        For j = 1 To 100
            tmp += Math.Log(0.9999)
        Next
        sum += tmp
    Next i
    clock.Stop()
    Console.WriteLine(sum & "  " & clock.ElapsedMilliseconds)
    sum = 0
    clock.Reset()

    clock.Start()
    Parallel.For(1, 101, Sub(i1)
                             Dim temp As Double = 0
                             For j1 As Integer = 1 To 100
                                 temp += Math.Log(0.9999)
                             Next
                             SyncLock lock
                                 sum += temp
                             End SyncLock
                         End Sub)
    clock.Stop()
    Console.WriteLine(sum & "  " & clock.ElapsedMilliseconds)

End Sub

输出：

-1,00005000333357  0
-1,00005000333347  0
-1,00005000333347  26

结论：如果你使用double，那么（a + b）+ c不是（总是）等于a +（b + c）

<强>更新

一个更简单的例子：

    Dim sum As Double
    For i = 1 To 100
        sum += 0.1
    Next
    Console.WriteLine(sum)

    sum = 0
    For i = 1 To 2
        Dim tmp As Double = 0
        For j = 1 To 50
            tmp += 0.1
        Next
        sum += tmp
    Next
    Console.WriteLine(sum)

现在输出

9,99999999999998
10

VB.NET在Parallel.for Synclock内部的嵌套循环中运行总和丢失信息

1 个答案: