multiple threaded program vb.net

Question

i have this code that is suppose to work with at starting the minimum number of threads, say 5. when it starts the first five threads, it automatically replaces any thread that is finished making sure that there is always five threads working.

the problem here is that, am finding it difficult to manage where am suppose perform locks

Private Sub StartProcess()
        intNextItemIndex = intMaxThreadCount
        ' start the first five threads
        For i = 0 To intMaxThreadCount
            If i < ListWorkItems.Count Then
                StartNextItem(i)
            End If
        Next
        ' update running threads
    End Sub

    Private Sub StartNextItem(ByVal index As Integer)
        Dim tt As New ThreadTask
        If index < ListWorkItems.Count Then

            ' start parsing thread
            Dim t As New Thread(AddressOf DoWork)
            With tt
                .Index = index
                .Task = "count"
                .Data = CStr(index * index)
            End With
            With t
                .IsBackground = True
                .Name = "Thread " & index
                .Start(tt)
            End With
            Thread.Sleep(100)
        End If
    End Sub

    Private Sub DoWork(ByVal data As Object)
        Dim tt As ThreadTask = CType(data, ThreadTask)
        Dim sTemp As String = Nothing
        Dim objLock As Object = New Object()
        Dim t As Double = 0
        myWatch = New Stopwatch
        Try
            Debug.Print("Incoming index: " & CInt(tt.Index))
            myWatch.Start()

                    SyncLock (objLock)
                        sTemp = PerformWork(ListWorkItems(CInt(tt.Index)).ToString)
                        'sTemp = CountTo()
                    End SyncLock

            myWatch.Stop()
            t = myWatch.ElapsedMilliseconds / 1000
            AddItem(tt.Index, t.ToString & " = " & sTemp)
            'AddItem(index, t.ToString)
        Catch ex As Exception
            ' catch error
            Debug.Print( ex.Message)
        End Try
 end sub

Public Sub AddItem(ByVal index As Integer, ByVal item As String)

    If Me.InvokeRequired Then
        Me.Invoke(New AddItemDelegate(AddressOf AddItem), index, item)
    Else
        Dim objLock As Object = New Object()
        Dim lvItem As New ListViewItem

        SyncLock (objLock)
            count += 1
            intNextItemIndex += 1
        End SyncLock

        With lvItem
            .Text = index.ToString
            .SubItems.Add(ListWorkItems(index).ToString)
            If item.Length < 100 Then
                .SubItems.Add(item.ToString)
            Else
                .SubItems.Add(item.Length.ToString)
            End If
        End With
        listView1.Items.Add(lvItem)


        If count = ListWorkItems.Count Then
            Button1.Enabled = True
            tsslStatus.Text = "Finished!!"
            Debug.Print("Finished: " & index.ToString)
        Else
            tsslStatus.Text = "Processing item (" & count + 1 & ")"
        End If

        StartNextItem(intNextItemIndex)
    End If

End Sub

can someone help me review it and tell me whats wrong with it?

Are the loks ok?

Is this way of doing it ok?

What possible improvements can i make

thanks all

Answer 1

Unfortunately, the whole approach is broken. You should make your parallel tasks explicit in the code so that you don’t need cross-thread communication any more. Secondly, you are writing your own thread scheduler here. – Don’t. Schedulers already exist, take advantage of that.

Have a look at the Task Parallel Library, in particular the Task and TaskScheduler classes.

Better yet, try to model your algorithm to benefit from data parallelism and eschew explicit threading completely in favour of the automatically parallelised computations.