Présentation sur la programmation parallèle en .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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