Overview of Threadingwith the .NET Framework

Wallace B. McClure

Scalable Development, Inc.

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

.NET Resources

ASP.NET – www.asp.netAspAdvice – www.aspadvice.comWindows Forms –

www.windowsforms.netArchitecture –

msdn.microsoft.com/architecture.NET News – www.dotnetwire.com

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.

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.

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.”

Serial Processes

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

Parallel Processes Parallel. Steps within the algorithm are not

dependent on other steps to be completed first.

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?

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).

Types of Threads

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

ThreadPool.System.Threading.ThreadPool() Class.

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.

Priority

Normal is the default.

Threading priority tells the OS how much relative time to a thread.

Performance Monitor Integration

.NET CLR LocksAndThreads

Track the number of Threads.

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().

Example #1

Create a thread and send a message to the UI.

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.

Example #2

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

thread.

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.

Locking Access to Objects (Synchronization)

System.Threading.Monitor() Class.

Methods:Enter(obj).Exit(obj).Wait, Pulse, PulseAll.

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.

Example #3

Use the Monitor object to lock access to an object.

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.

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.

Overview of the Thread Pool

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.

Of Interest

WaitCallback.QueueUserWorkItem.Monitor the pool.

GetAvailableThreads(out int Wthrds, out int iCompPortThrds).

GetMaxThreads(out int Wthrds,out int iCompPortThrds).

ThreadPool Worries

Don't do operations that are not guaranteed to complete.

Remember that the ThreadPool has a maximum number of threads.

Personal Experience:Overview 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.

Things to Look at / Last Thoughts

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

work for you.

Questions?

Scalable Development, Inc.Consulting & Development Services.http://www.scalabledevelopment.com865-693-3004.wallym@scalabledevelopment.com

END

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