This document summarizes a presentation on parallel programming in .NET. It introduces parallel programming and why it is useful for performing work faster and creating responsive user interfaces. It discusses common issues like deadlocks and race conditions. It then covers different techniques for parallel programming in C#, including using threads, thread pools, tasks, Parallel class, async programming, and PLINQ. It also discusses synchronization techniques like locks, mutexes, and signaling with wait/pulse and barriers. Finally, it discusses storing shared items and provides an overview of thread-safe collection classes in .NET.

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MeetingsMERI
Programmation
Parallèle en .NET
Animé par
Sylvain Rescourio – Expert .NET
Olivier Navarre – Directeur Technique
Mardi 27 Mai 2014

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 Introduction
 What is parallel programming ?
 Why parallel programming ?
 The issues that come with parallel
programming ?
 How to do parallel programming
in C# ?
 Thread
 ThreadPool
 Task
 Parallel
 Async
 PLinq
 How to synchronize in C# ?
 Lock
 Mutex
 SpinLock
 Semaphore
 ReaderWriterLock
 Semaphore Slim
 ReaderWriterLockSlim
 How to signal in C# ?
 Wait / Pulse
 Barrier
 How to store shared items in C# ?
 Wait / Pulse
 Barrier
 Conclusion
Sommaire

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What is parallel programming ?
Parallel computing is the simultaneous use of multiple compute resources
Problem Result
Compute resource 1
Compute resource 2
Compute resource m
Sub Problem 1
Sub Problem 2
Sub Problem n

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Why parallel programming ?
Do the same work but in less time
8 sec
4 sec
Work 1 2 * (0,5 * Work 1)
at the same time
Sequential Parallel
Start

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Why parallel programming ?
Responsive UI
Long Action
Refresh interface
Refresh interface Refresh interface
Sequential Parallel
See example
Click Click Long Action

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The issues that come with parallel programming
A is a situation in which two or more competing actions are
each waiting for the other to finish, and thus neither ever does.
Waiting for Task 2 to finish
Task 1
Task 2
End
Waiting for Task 1 to finish

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The issues that come with parallel programming
A is the behavior of a system where the output is
dependent on the sequence or timing of other uncontrollable events.
Task 1
Task 2
Result 1
Task 1
Task 2
Result 2
Race Condition if
Result 1 != Result 2

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How to do parallel
programming in C# ?
Thread
ThreadPool
Task
Parallel
Async
PLinq

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Thread
A is the basic unit to which an operating system allocates
processor time to execute a sequence of instruction.
Process
Thread 1
Instruction 1
Instruction 2
Instruction n
Thread k
Instruction 1
Instruction 2
Instruction n

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Thread
Code

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Solutions Thread, ThreadPool, Task, Parallel,
Async, Plinq, …
Differents solutions
 ThreadPool : A pool of threads that can be used to execute tasks.
 Task : Represents an asynchronous operation.
 Parallel : Provides support for parallel loops.
 Async : A keyword to specify that a method is asynchronous.
 Plinq : Execute LINQ queries in parallel.

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ThreadPool
Code

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Task
Code

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Parallel
Code

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Parallel Foreach
Code

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Async
Code
See example

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PLinq
Code

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How to synchronize in C# ?
Lock
Mutex
SpinLock
Semaphore
ReaderWriterLock
Semaphore Slim
ReaderWriterLockSlim

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Different solutions
 Lock
 Protect a section of code.
 Mutex
 Locking across processes.
 SpinLock
 Locking few instructions (Sum, variable copy, …).
 Semaphore
 Cap the access of a resource to a fixed number of
thread.
 ReaderWriterLock
 Control the access of a resource by multiple thread,
allowing 1 writer or many reader at the same time.
Lock
Locking for a long time
can degrade
performances !
Be aware of deadlock !

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Lock
Comparison of Different solutions
Reference : http://www.albahari.com/threading/part2.aspx#_Locking

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Lock
Code

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SemaphoreSlim
Code

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ReaderWriterLockSlim
Code

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How to signal in C# ?
Wait / Pulse
Barrier

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How to signal in C# ?
Reference : http://www.albahari.com/threading/part2.aspx#_Signaling_with_Event_Wait_Handles
Differents solutions
 Wait / Pulse methods
 Let a thread wait for a signal or send a signal (Pulse).
 Barrier
 Allow threads to wait for each other.

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Wait / Pulse
Code

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Barrier
Code

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A quick word about
atomicity in C#

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A quick word about atomicity in C#
An atomic operation
 An atomic operation cannot be interrupted by other thread.
 Example : Incrementing a variable is not atomic
 Read the variable
 Add 1
 Store the result in the same variable
 What if in the middle, another thread access this variable ?
 Solution
 Interlocked.Add, Interlocked.Increment
 Interlocked.Exchange, Interlocked.CompareExchange
 Interlocked.Read

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DEMO

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DEMO - Image crawler
Image crawler :
http://www.hdwallpapers.in
GUI Processing
Get image count
Download pages
Download images
Update GUI
Wait for all images to be downloaded

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How to store shared items
in C# ?

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How to store shared items in C# ?
Different Solutions
 Standard Collection + Lock
If you only add elements to the collection
 Standard Collection + ReaderWriterLockSlim
If you modify & read simultaneously the collection
 Native ThreadSafe collections
If you modify & read simultaneously the collection
 For highly concurrent scenario
Native ThreadSafe collections
Concurrent collection Non Concurrent equivalent
ConcurrentStack<T> Stack<T>
ConcurrentQueue<T> Queue<T>
ConcurrentDictionary<TKey,TValue> Dictionary<TKey,TValue>
BlockingCollection<T>
ConcurrentBag<T>

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Conclusion

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Parallel programming can help
you to
 Compute faster
 Create responsive application
 Parallel programming has a cost :
 Increase code complexity.
 Too much synchronization can lead to
worse performance than single
threaded application.
Conclusion
Don’t rush into parallel programming, profile your code if necessary

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