Question

I’ve read a lot of different questions on SO about multithreaded applications and how to split work up between them, but none really seem to fit what I need for this. Here’s how my program currently basically works:

Module Module1
'string X declared out here
    Sub Main()
'Start given number of threads of Main2()
    End Sub
    Sub Main2()
'Loops forever
'Call X = nextvalue(X), display info as needed
    End Sub
    Function nextvalue(byval Y as string)
'Determines the next Y in the sequence
    End Function
End Module

This is only a rough outline of what actually happens in my code by the way.

My problem being that if multiple threads start running Main2(), they’re dealing with the same X value as in the other threads. The loop inside of main2 executes multiple times per millisecond, so I can’t just stagger the loops. There is often duplication of work done.

How can I properly divide up the work so that the two threads running simultaneously never have the same work to run?

Answer 1

You should synchronize the generation and storage of X so that the composite operation appears atomic to all threads.

Module Module1

  Private X As String
  Private LockObj As Object = New Object()

  Private Sub Main2()

    Do While True

      ' This will be used to store a snapshot of X that can be used safely by the current thread.
      Dim copy As String

      ' Generate and store the next value atomically.
      SyncLock LockObj
        X = nextValue(X)
        copy = X
      End SyncLock

      ' Now you can perform operations against the local copy.
      ' Do not access X outside of the lock above.
      Console.WriteLine(copy)

    Loop

  End Sub

End Module