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POWhy cancelling 6 WebRequest using Task.Start takes longer that Thread.Start
 

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  1. POWhy cancelling 6 WebRequest using Task.Start takes longer that Thread.Start
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    <p>I have custom asynchronous WebRequest class that I am testing to find out how fast the request will be cancelled and found some odd results, that starting 6 WebRequests with TPL and cancelling them right away takes 25 sec, but when I used just regular background threads it took only 5 sec.</p> <p><strong>Update</strong>: Running them without cancelling takes with Task.Start 9 sec and Thread.Start 3 sec accordingly. </p> <pre><code>Imports System.Net Imports System.Threading Imports System.IO Imports System.Threading.Tasks Module Module2 Dim closeEvent As New ManualResetEvent(False) Dim sw As System.Diagnostics.Stopwatch ' first domain is invalid Dim urls() As String = { "http://www.jbihgcgfxfdxdwewloevedfhvcdfb.com", "http://www.hanselman.com/blog/", "http://www.stackoverflow.com/questions/", "http://www.finderscope.net", "http://msdn.microsoft.com/en-us/library/az24scfc.aspx", "http://www.developerfusion.com/tools/convert/csharp-to-vb/" } Sub Test1() sw = System.Diagnostics.Stopwatch.StartNew Dim action As Action(Of Object) = Sub(url As String) Dim wReq As WebRequest = WebRequest.Create(url) RunWebStreamAsync(wReq, closeEvent) Console.WriteLine("done in {0} ms : {1}", sw.ElapsedMilliseconds, Left(url, 50)) End Sub Try '' Cosntruct a started task Dim t(5) As task For i As Integer = 0 to 5 t(i) = New Task(action, urls(i)) Next For Each tsk In t tsk.Start() Next closeEvent.Set() Task.WaitAll(t) Catch ex As Exception Console.WriteLine(ex.Message) End Try Console.WriteLine("total in {0} ms", sw.ElapsedMilliseconds) End Sub Dim WaitCount As Integer = 6 Sub Test2() sw = System.Diagnostics.Stopwatch.StartNew For i As Integer = 0 to 5 StartThread(urls(i)) Next closeEvent.Set() Do While WaitCount &gt; 0 Thread.Sleep(1000) Loop Console.WriteLine("total in {0} ms", sw.ElapsedMilliseconds) End Sub Private Sub StartThread(ByVal url As String) Dim trd As Thread = New Thread(AddressOf ThreadTask) trd.IsBackground = True trd.Start(url) End Sub Private Sub ThreadTask(ByVal arg As Object) Dim url As String = arg Try Dim wReq As WebRequest = WebRequest.Create(url) RunWebStreamAsync(wReq, closeEvent) Catch End Try Console.WriteLine("done in {0} ms : {1}", sw.ElapsedMilliseconds, Left(url, 50)) Interlocked.Decrement(WaitCount) End Sub Public Sub RunWebStreamAsync(ByVal wr As WebRequest, ByVal CloseTask As ManualResetEvent) Dim hwra As New MyWebRequest hwra.LoadAsync(wr) Do While hwra.AsyncOperationInProgress If CloseTask.WaitOne(1000) = True Then If hwra.CancellationPending = False Then hwra.CancellationPending = True wr.Abort() End If Else Thread.Sleep(100) End If Loop End Sub Class MyWebRequest Public Property CancellationPending As Boolean Public Property AsyncOperationInProgress As Boolean Public Sub LoadAsync(ByVal req As WebRequest) AsyncOperationInProgress = True ' Invoke BeginGetResponse on a threadpool thread, as it has ' unpredictable latency Dim bgrd = (New WaitCallback(AddressOf Me.BeginGetResponseDelegate)) bgrd.BeginInvoke(req, Nothing, Nothing) End Sub ' Solely for calling BeginGetResponse itself asynchronously. Private Sub BeginGetResponseDelegate(ByVal arg As Object) If CancellationPending Then PostCompleted(Nothing, True) Else Dim req As WebRequest = DirectCast(arg, WebRequest) req.BeginGetResponse(New AsyncCallback(AddressOf GetResponseCallback), req) End If End Sub Private Sub PostCompleted(ByVal p1 As Object, ByVal p2 As Boolean) AsyncOperationInProgress = False ' do something End Sub Private Sub GetResponseCallback(ByVal result As IAsyncResult) If CancellationPending Then PostCompleted(Nothing, True) Else Try ' Continue on with asynchronous reading. PostCompleted(Nothing, True) Catch ex As Exception ' Since this is on a non-UI thread, we catch any exceptions and ' pass them back as data to the UI-thread. PostCompleted(ex, False) End Try End If End Sub End Class End Module </code></pre>
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    1. USwalter
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    1. COYou should call `Thread.Join` instead of `Sleep` and `WaitCount`.
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      1. POWhy cancelling 6 WebRequest using Task.Start takes longer that Thread.Start
      1. USSLaks
    2. COwith Thread.Join i will be waiting first one to complete than second, i wanted to be independent
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      1. POWhy cancelling 6 WebRequest using Task.Start takes longer that Thread.Start
      1. USwalter
    3. COWhat difference does it make? Either way, you're waiting for them all to complete. Calling `Join()` does not affect the target threads at all; they will still run in parallel.
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      1. POWhy cancelling 6 WebRequest using Task.Start takes longer that Thread.Start
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