An overview of Threading in the .Net framework, including classes that provide threading and classes which handles mutual exclusion of resources.

1. .Net Threading
Erik Ralston
BIS Birds of a Feather
July 15th, 2010
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2. How do I thread?
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3. What is a thread?
A single path of execution in a process
A stream of instructions given a time-slice by the CPU
Logically independent from state and resources
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4. Death of Moore’s Law?
“Number of transistors on a chip doubles every 2 years”
Speed
Memory
Pixels
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5. Rise of Amdahl
“Speed increase from multi-tasking is not linear”
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6. System.ComponentModel.BackgroundWorker
Simple event-oriented threading
Intended to run background task of a dialog
Ideal for WinForms apps, but not available for WebForms
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7. Invoke & BeginInvoke
Synchronous and asynchronous calling of delegates
Also, method for returning data to the UI thread
Good for small, sporadically occurring tasks
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8. System.Threading.ThreadPool
Group of self-recycling threads that call queued delegates
Intended for numerous short duration tasks
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9. Timers
System.Threading.Timer, System.Timers.Timer, System.Windows.Forms.Timer
Many implementations of periodically calling an event
What kind of thread is called varies
Windows.Forms calls UI Thread (Not True Multithreading)
Threading & Timers call worker threads
MSDN Comparison of Timer Classes
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10. System.Threading.Thread
Dedicated instance of a thread
Requires the most effort and management
For long-running, intensive tasks with custom purpose
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11. Thread Class Overview
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12. How do I make them
share?
(Safely)
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13. Race Conditions
Race conditions happen when shared state depends on
sequence of reads and writes (thread operations)
Read and write are always separate actions
Side-effect of time-slice and separating read & write
Thread-Safe – Will not cause race conditions
“Thread-Safe” is not the same as “Concurrent”
Only using local variables/resource is naturally concurrent
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14. Ownership & UI Threads
Abort all attempts by other threads to use a resource
If any other thread tries to update a WinForm, it will crash
Utilize Invoke and BeginInvoke on the Controls objects
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15. System.Threading.Interlocked
Class with shared “atomic” functions
Read and write happen together
For simple operations like add, compare, increment, etc
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16. SyncLock
(Lock in C#)
Creates a block that only one thread at a time can enter
Code block is called “critical section”
Other callers must wait in line
Exclusive locking eliminates both the danger and the
benefits of multi-threading for the code enclosed
Plays into the “parallel portion” for Amdah’s law
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17. System.Threading.Mutex
Provides mutual
exclusion on a single
resource
Child of WaitHandle
class, a class for
basic locking
A “Global Mutex” can
be used for inter-
process
communication
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18. System.Threading.ReaderWriterLock
Like a mutex, but provides “many reader, one writer”
Ensure thread-safety while better preserving concurrency
Similar to “S” versus “U” locking in SQL databases
Read vs. write behavior must be controlled manually
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19. System.Threading.Semaphore
Provides for a pool of resources for many threads
Uses functionality from the WaitHandle class
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20. System.Threading.Monitor
Forces threads to wait until
another thread wakes them
Must be used in conjunction
with another lock
Lack of automatic unlocking
makes it dangerous
Dependence on being in a
critical section means
concurrency is tough
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21. Sharing Isn’t Easy
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22. Stop Starvation!
Starvation – When a thread is locked so long without a
resource it has failed at its duty
Deadlock - When two or more threads are preventing
progress because they won’t (or can’t) share
Timeouts and priority schemes help prevent
Livelock – When two or more threads prevent progress
for each other while haplessly trying to share
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23. Priority Inversion
Not all threads execute with the same priority
Any thread may access mutually exclusive resources
When a low threads locks on a resource, high threads
may have to wait for the low thread to finish
The scheduler and resource system do not communicate
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24. Strategies for Preventing Starvation
Use Timeouts – If a thread can’t get a resource for X
milliseconds, let it go and try again later
Fixed Order – If a thread needs multiple resources, then
force them to acquire them in a specific order
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25. Other Technologies
Parallel Extensions in .Net 4.0 – Adds more language-
oriented parallel constructs (parallel loops) and tasks
OpenMP – Compiler extensions for C++ that offers
language-oriented parallelism
Thread Building Blocks – C++ Library with parallel loops
and task pipeline without compiler extensions
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26. Questions?
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27. Thank You!
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