Multi-core processor pegged even when all threads are just sleeping

Question

I'm simulating some threading in a Windows Service, and the Thread.Start routine for each of my threads points directly to the following:

Private WithEvents CheckForOrdersTimer As System.Threading.Timer

Private Sub timerCheckForContracts_Tick(ByVal stateInfo As Object)
    ' Ticks every 5 seconds, then spawns threads until we're at our max
    Do

        If ThreadCollection.Count < MaxThreads Then

            Dim t As New Threading.Thread(AddressOf SomeThreadingCode()
            ThreadCollection.Add(t)
            t.Start()

        End If
    Loop

End Sub


Private Sub SomeThreadingCode()
    Do
        Thread.Sleep(1000)
        If Me.ThreadsShouldContinue = False Then  ' Global thread-stopper
            Exit Sub
        End If
        If (New Random).NextDouble > 0.8 Then  ' On average, wait 5 seconds
            Exit Do
        End If

    Loop

    ' Remove this thread from the main collection
    ThreadCollection.Remove(Thread.CurrentThread)

End Sub

Pretty simple - the threads aren't even doing anything yet, but with more than two threads running at the same time, my processor (Core 2 Duo 2.4 w/ 4GB) gets pegged and Windows gets really sluggish. According to what I've read, Thread.Sleep shouldn't be consuming any resources at all while it waits, but it may as well be running in a tight timing loop.

Can anybody explain to me what's going on here?

EDIT: Per the requests, I've expanded the amount of code I'm using. I was initially doing some database work before spawning each thread, but I've removed it and the processor maximization still occurs with just the code here (and, of course, the OnStart method for the Windows Service.

Answer 1

There is not enough here to help really. How are the threads being created? What is happening on the main thread? Console or WinForm app? If you are creating and destroying threads every one-five seconds, that is going to take resources. Research Threadpool for ways to cache threads so you can avoid the costs of creating and destroying them.

Answer 2

Well, there are 4 statements in your code, 3 of them are wrong. ThreadsShouldContinue cannot work well because VB.NET doesn't have the "volatile" keyword. The Random statement cannot work well because you create a new instance of the Random class every time, preventing it from being truly random. And the Remove() method call cannot work well because you don't use the SyncLock statement.

The Sleep() call is the only statement without an issue. Does that explain the problem? Probably not. Use Debug + Break All to see what the threads are doing.

Answer 3

This code should not be looping, the sleeps look ok. And sleep uses no CPU. YOu other threads are spinning somewhere.

Plus you should use an Event to signal termination rather than looping on a sleep/bool test

Silently ignoring all exceptions is not going to make things easier to debug either

Answer 4

timerCheckForContracts_Tick is running an infinite loop, and never goes to sleep, etc. Even if you are "ticking" it every 5 seconds, there is no code within the loop to prevent it from consuming CPU cycles, forever checking:

    If ThreadCollection.Count < MaxThreads Then