Overview of Threading with the .NET Framework

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Overview of Threadingwith the .NET Framework
Scalable Development, Inc. Building systems today
that perform tomorrow.

• Wallace B. McClure
• Scalable Development, Inc.

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.NET Resources

• ASP.NET www.asp.net
• AspAdvice www.aspadvice.com
• Windows Forms www.windowsforms.net
• Architecture msdn.microsoft.com/architecture
• .NET News www.dotnetwire.com

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What is MultiTasking /Multiprocessing?

• Ability of multiple applications to work at the
  same time.
• Cooperative multitasking. Windows 3.1 Mac OS
  pre-X applications were responsible for passing
  control of processing to each other. One
  application can cause problems in the whole
  system by blocking other running apps.
• Pre-emptive multitasking. Win32 Mac OSX
  applications have processing capabilities granted
  to them by the OS. One application is less like
  to cause a problem. TimeSlice is given to the app.

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What is Threading?

• Threading is the ability of a single application
  (process) to perform multiple units of work that
  are controlled by the parent application.

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Algorithms and Their Impact

• Dividing an application into threads will not
  automatically make an application faster.
• How an application runs will determine how well
  threading in an application will work.
• Threading is not a magic snake oil.

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Serial Processes

• Serial. One step within the algorithm must be
  completed before the next step.

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Parallel Processes

• Parallel. Steps within the algorithm are not
  dependent on other steps to be completed first.

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Algorithms

• Algorithms are neither Serial or Parallel but
  some combination.
• Assume an app that is 50 parallel.
• Only improve by up to 25 by adding an additional
  thread.
• Never double the speed.
• Must understand your algorithm.
• Where are your bottlenecks and your opportunities
  for improvement?

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Types of Suitable Apps /Algorithms

• Long running algorithms.
• Highly Parallel algorithms (FFT is the best
  parallel algorithm I know of).
• Responsive User Interface.
• Async operations.
• Windows Services (HTTP, Database).

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Types of Threads

• Managed Threads / Regular Threads.
• System.Threading.Thread() Class.
• ThreadPool.
• System.Threading.ThreadPool() Class.

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Create a Thread

• New System.Threading.Thread
• (AddressOf(method)).
• Some Methods
• Start(). Start processing on the Thread.
• Sleep(). Wait X milliseconds.
• Join(). Wait on a thread to finish.
• Resume(). Start processing again.
• Abort(). Quit the thread.

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Priority

• Normal is the default.
• Threading priority tells the OS how much relative
  time to a thread.

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Performance Monitor Integration

• .NET CLR LocksAndThreads
• Track the number of Threads.

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Uniprocessor Threading Hiccup

• On a uniprocessor, the thread does not get any
  processor time until the main thread yields.
  Call Thread.Sleep(0) for a thread to immediately
  start after calling the Thread.Start().

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Example 1

• Create a thread and send a message to the UI.

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Passing Parameters

• In Set a property of a class.
• In Use the constructor to set initial value.
• Out Raise an event and pass a param to that
  event.

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Example 2

• Instantiate a class.
• Set a property of the class.
• Thread runs.
• Raises event.
• Event is processed by the calling thread.

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Problem(s)

• Debugging.
• Management overhead.
• Deadlocks.
• Race Conditions.
• Order of Execution.
• What happens when Threads must access common
  variables?
• Exclusively lock access to any common objects.

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Locking Access to Objects (Synchronization)

• System.Threading.Monitor() Class.
• Methods
• Enter(obj).
• Exit(obj).
• Wait, Pulse, PulseAll.

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Other Ways to Lock Objects

• Synchronization Attribute.
• Lock (C) / SyncLock (VB).
• ReaderWriterLock() Class. Designed for reads with
  a small number of writes.
• Mutex. Works for threads and processes.

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Example 3

• Use the Monitor object to lock access to an
  object.

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Notes on the Monitor Object

• Only works on Reference types. Value types are
  not exclusively locked.
• The vbc and csc compilers put a try/catch/finally
  so that in the case of an error, the appropriate
  Exit() method is called.

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Managed ThreadRecommendations

• Don't use Thread.Abort to terminate threads.
  Thread state is unknown.
• Don't use Thread.Suspend and .Resume to
  synchronize threads. Use the appropriate
  objects.
• Monitor.Enter and Monitor.Exit are both called.
• Threads are great for tasks involving different
  resources.

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Overview of the Thread Pool
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ThreadPool

• Pool of threads.
• Managed by the CLR.
• Per Process.
• Built into the CLR.
• of Threads dependent on CPU usage. 25 threads
  per CPU default.
• Article on MSDN with ThreadPool guidelines.

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Of Interest

• WaitCallback.
• QueueUserWorkItem.
• Monitor the pool.
• GetAvailableThreads(out int Wthrds, out int
  iCompPortThrds).
• GetMaxThreads(out int Wthrds,out int
  iCompPortThrds).

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ThreadPool Worries

• Don't do operations that are not guaranteed to
  complete.
• Remember that the ThreadPool has a maximum number
  of threads.

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Personal ExperienceOverview of a Windows Service

• Goal Pull data from multiple sources.
• Multi-threaded Windows Service.
• Thread for each data source.
• Little contention for resources.
• Error processing is complicated.

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Things to Look at / Last Thoughts

• Windows Services.
• EventLog.
• Weak References.
• Performance Monitor Integration.
• Nothing wrong with Interop, if it will work for
  you.

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Questions?
Scalable Development, Inc. Building systems today
that perform tomorrow.

• Scalable Development, Inc.
• Consulting Development Services.
• http//www.scalabledevelopment.com
• 865-693-3004.
• wallym_at_scalabledevelopment.com

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