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Multi-Threading

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.NET Multi-Threading Introductory UsageProven Practices

Overture

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DEMO

Hello to the threading world

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BlockingThread.Sleep & Join

Lockinglock, Mutex & Semaphore

SinglingEventWaitHandle & Wait/Pulse

Non-BlockingMemory Barrier, Interlocked & Volatile

Managing

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DEMO

Managing threads

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static void Main() { Thread t = new Thread (delegate() { Console.ReadLine(); }); t.Start(); t.Join(); // Wait until thread finishes /* Do next step */}

Join

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lock(locker){ gate.Add(primeCounter);}

Monitor.Enter(locker);try{ gate.Add(primeCounter);}finally{ Monitor.Exit(locker);}

Sugary lock

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Same as a lockAdvantage: Can work across processes, meaning

multiple applications can use a single mutex

Mutex

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static EventWaitHandle wh = new AutoResetEvent(false);

static void Main() { new Thread (Waiter).Start(); Thread.Sleep (1000); // Wait for some time... wh.Set(); // OK - wake it up}

static void Waiter() { Console.WriteLine("Waiting..."); wh.WaitOne(); // Wait for notification Console.WriteLine("Notified");}

Signalling

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lock (locker){ gate.Add(primeCounter);}

Thread.MemoryBarrier();try{ gate.Add(primeCounter);}finally{ Thread.MemoryBarrier();}

Memory Barrier

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static void Main() { Thread t = new Thread(delegate() { try { Thread.Sleep(Timeout.Infinite); // This is blocking } catch (ThreadInterruptedException) { Console.Write("Forcibly "); } Console.WriteLine("Woken!"); }); t.Start(); t.Interrupt(); }

Interrupt

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Similar usage to InterruptThrows – ThreadAbortExceptionDoes not wait for blocking

Abort

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DEMO

Using non-blocking management

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DEMO

Using the built in ThreadPools

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Single thread pool per processBuilt in logic to grow and shrink poolBuilt-in Limits

Worker threads – 25 per CPUI/O threads – 1000 per CPU

Thread Pools

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DEMO

Threaded controls in WinForms

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Forms Timer Timers Timer Threading Timer

Thread? UI UI or Worker Worker

Thread safe No Yes No

Object Model Yes Yes No

WinForm Req. Yes No No

Accurate No Yes Yes

State Support No No Yes

Schedule Support No No Yes

Inheritance Yes Yes No

Timers

Source Alex Calvo - http://msdn.microsoft.com/en-us/magazine/cc164015(printer).aspx

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DEMO

Talking to the UI thread from a worker thread

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“Multi-threaded programming needs a little care”

Patricia Shanahan

Understatement?

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CostThread Safety

Race conditionsDead locks

Exception ManagementDebugging

Common Problems

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1 Mb of Address space12k for kernel mode stackNotification of every DLL in the process

Threads are expensive

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Reads value (5)Adds 1 – (6)Assumes value 6

Reads value (6)Adds 1 – (7)Assumes value 7

Thread 1 Thread 2

Race Conditions

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If file does not existCreate itPopulate it with initial data

Read data from fileProcess dataWrite to file

Logical Process I

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If file does not existCreate itPopulate it with initial data (Write Lock)

Read data from file (Read Lock)Process dataWrite to file (Write Lock – if needed)Release locks

Logical Process II

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Creates fileLocks writerAttempts to lock

readerWaits…

Locks ReaderProcesses dataAttempts to lock writerWaits…

Thread 1 Thread 2

Dead Locks

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Exceptions are limited to a thread

Exceptions

try{}catch{}

MainChild

1

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DEMO

Debugging in Visual Studio

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Threads are expensive – use wisely Use managed threads over native threads. Use Timers or ThreadPool where possible.

Or the new parallel extensions in .NET 4.0 Avoid mutex, unless you need cross process. Avoid Thread.Abort as it can have high side effects Avoid Thread.Suspend/Resume as a blocking system

Rather use lock

Proven Practises I

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Be careful what you lock lock’s are type based shared across all AppDomains. Use static

as the solution this (instances) have a high chance of deadlocks

Use lock over Monitor If you must use Monitor, use try...finally

Inside a lock do as little as possible If you are doing math inside the lock rather change to

Interlocked Never perform long running operations on the UI thread

Proven Practises II

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Q&A