.Net Threading

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 15
th
, 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
(Lockin 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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