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• Part I — Writing the cache

Let’s try out our lit­tle cache. First I want to write a syn­chro­nous ver­sion of it as a base­line.

    Private Shared Sub TestSync(ByVal sites() As String, ByVal sitesToDownload As Integer, ByVal howLong As Integer)
        Dim syncCache As New Dictionary(Of String, String)
        Dim count = sites.Count()
        Dim url1 = "http://moneycentral.msn.com/investor/invsub/results/statemnt.aspx?Symbol="
        For i = 0 To sitesToDownload - 1
            Dim html As String = ""
            Dim url = url1 & sites(i Mod count)
            If Not syncCache.TryGetValue(url, html) Then
                html = LoadWebPage(url)
                syncCache(url) = html
            End If
            DoWork(html, howLong)
        Next
    End Sub

This is a loop that loads web­pages in the cache if they are not al­ready there. sites is a list of tick­ers used to com­pose the urls; sitesToDown­load is the to­tal num­ber of sites to down­load, so that a sin­gle url can be loaded mul­ti­ple times; how­Long rep­re­sents the work to be done on each loaded page.

In this ver­sion the cache is sim­ply a Dictionary and there is no par­al­lelism. The two bold lines is where the cache is man­aged.

DoWork is this.

    Public Shared Sub DoWork(ByVal html As String, ByVal howLong As Integer)
        Thread.Sleep(howLong)
    End Sub

Let’s take a look at the asyn­chro­nous ver­sion.

    Private Shared Sub TestAsync(ByVal sites() As String, ByVal sitesToDownload As Integer, ByVal howLong As Integer)
        Dim htmlCache As New HtmlCache
        Dim count = sites.Count()
        Dim url = "http://moneycentral.msn.com/investor/invsub/results/statemnt.aspx?Symbol="
        Using ce = New CountdownEvent(sitesToDownload)
            For i = 1 To sitesToDownload
                htmlCache.GetHtmlAsync(
                    url & sites(i Mod count),
                    Sub(s)
                        DoWork(s, howLong)
                        ce.Signal()
                    End Sub)
            Next
            ce.Wait()
        End Using

There are sev­eral points worth mak­ing on this:

• The lambda used as sec­ond pa­ra­me­ter for GetHtmlAsync is in­voked on a dif­fer­ent thread when­ever the html has been re­trieved (which could be im­me­di­ately if the cache has down­loaded the url be­fore)
• CountDownEvent al­lows a thread to wait for a cer­tain num­ber of sig­nals to be sent. The wait­ing hap­pens on the main thread in the ce.Wait() in­struc­tion. The trig­ger­ing of the event hap­pens in the lambda de­scribed in the point above (the ce.Sig­nal() in­struc­tion)

This is the dri­ver for the over­all test­ing.

    Private Shared Sub TestPerf(ByVal s As String, ByVal a As Action, ByVal iterations As Integer)
        Dim clock As New Stopwatch
        clock.Start()
        For i = 1 To iterations
            a()
        Next
        clock.Stop()
        Dim ts = clock.Elapsed
        Dim elapsedTime = String.Format(s & ": {0:00}:{1:00}:{2:00}.{3:00}", ts.Hours, ts.Minutes, ts.Seconds, ts.Milliseconds / 10)
        Console.WriteLine(elapsedTime, "RunTime")
    End Sub

There is not much to say about it. Start the clock, per­form a bunch of it­er­a­tions of the passed lambda, stop the clock, print out per­for­mance.

And fi­nally the main method. Note that all the ad­justable pa­ra­me­ters are fac­tored out be­fore the calls to TestPerf.

    Public Shared Sub Main()
        Dim tickers = New String() {"mmm", "aos", "shlm", "cas", "abt", "anf", "abm", "akr", "acet", "afl", "agl", "adc", "apd",
                                           "ayr", "alsk", "ain", "axb", "are", "ale", "ab", "all"}
        Dim sitesToDownload = 50
        Dim workToDoOnEachUrlInMilliSec = 20
        Dim perfIterations = 5
        TestPerf("Async", Sub() TestAsync(tickers, sitesToDownload, workToDoOnEachUrlInMilliSec), perfIterations)
        TestPerf("Sync", Sub() TestSync(tickers, sitesToDownload, workToDoOnEachUrlInMilliSec), perfIterations)
    End Sub

Feel free to change (tick­ers, sitesToDown­load, work­ToDoOnEachUrlIn­Mil­liSec, per­fIt­er­a­tions). Depending on the ra­tios be­tween these pa­ra­me­ters and the num­ber of cores on your ma­chine, you’re go­ing to see dif­fer­ent re­sults. Which high­lights the fact that par­al­leliz­ing your al­go­rithms can yield per­for­mance gains or not de­pend­ing on both soft­ware and hard­ware con­sid­er­a­tions. I get ~3X im­prove­ment on my box. I at­tached the full source file for your amuse­ment.

AsyncCache.vb