[RESOLVED] What is it with threads and performance?

[bergerkiller]

Hello everyone.

Not really asking for code, but I'm interested in knowing the basics of "thread performance".

For example, you try to copy a bitmap in a pixel by pixel operation. (Is slow, but it is a nice example)

        Dim IMG As Bitmap = Image.FromFile("c:\test.png")
        Dim COPY As New Bitmap(IMG.Width, IMG.Height)
        For X As Integer = 0 To IMG.Width - 1
            For Y As Integer = 0 To IMG.Height - 1
                COPY.SetPixel(X, Y, IMG.GetPixel(X, Y))
            Next
        Next

This litterally takes ages on a 1280 x 1024 bitmap, but what if I would use 1024 separate threads performing the copy of a single line?:

    Private IMG As Bitmap
    Private COPY As Bitmap
    Private started As Boolean
    Private line As Integer
    Private finishcount As Integer

    Private Sub CopyLine()
        Dim Y As Integer = line
        started = True
        For X As Integer = 0 To IMG.Width - 1
            Dim succesfull As Boolean = False
            Do While succesfull = False
                Try
                    COPY.SetPixel(X, Y, IMG.GetPixel(X, Y))
                    succesfull = True
                Catch
                End Try
            Loop
        Next
        finishcount += 1
    End Sub

    Private Sub CopyImage()
        IMG = Image.FromFile("c:\test.png")
        COPY = New Bitmap(IMG.Width, IMG.Height)
        finishcount = 0
        For Y As Integer = 0 To IMG.Height - 1
            line = Y
            started = False
            Dim t As New Threading.Thread(AddressOf CopyLine)
            t.IsBackground = True
            t.Start()
            Do While started = False
                Threading.Thread.Sleep(50)
            Loop
        Next
        'all threads launched, now waiting for it to finish
        Do While finishcount < IMG.Height
            Threading.Thread.Sleep(500)
        Loop
        MessageBox.Show("Copying finished.", "Finished.", MessageBoxButtons.OK)
    End Sub

This is, of course, very buggy and causes a lot of violation errors etc. But that doesn't matter right now. How come the multi-threading way is so much faster than the regular routine? Is the processor not running on full speed with the first code, and is it working harder on the second?

I find it weird to see this type of "threading performance increase", as if every single thread gets their own piece of processor speed allocated like it is a new process. Is there anyone who knows more of this?