net threading

.Net ThreadingErik Ralston

BIS Birds of a Feather

July 15th, 2010

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How do I thread?

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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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Death of Moore’s Law?

“Number of transistors on a chip doubles every 2 years”

Speed

Memory

Pixels

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Rise of Amdahl“Speed increase from multi-tasking is not linear”

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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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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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System.Threading.ThreadPool

Group of self-recycling threads that call queued delegates

Intended for numerous short duration tasks

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TimersSystem.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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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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Thread Class Overview

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How do I make them share?

(Safely)

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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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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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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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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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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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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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System.Threading.Semaphore

Provides for a pool of resources for many threads

Uses functionality from the WaitHandle class

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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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Sharing Isn’t Easy

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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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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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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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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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Questions?

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Thank You!

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