C# course

Lecture 13

Multithreading in C#

Agenda

  • Introduction and Concepts
  • Threads
  • Creating and starting threads
  • Thread lifecycle
  • Error handling
  • Thread Pooling
  • Synchronization concepts
  • Immutable objects and synchronization

What is multithreading

Multithreading - parallel execution of code, leveraging threads.


C# enables multithreading using following namespace:

1: 

using System.Threading;

When to parallel?

When multithreading might be useful?

  • there are few independent tasks that do not intersect (usually calculation)
  • separate heavy calculation from UI (to avoid freezes)
  • constantly query external service and notify application if new data arrived
  • to avoid stop processing waiting for user's input
  • separating processing workflow by threads (collect - filter - process - save)

When not to parallel?

When multithreading might be awkward?

  • when number of threads is big enought
  • when modules are tightly coupled and there are a lot of commmon data across modules (refactor first)
  • when it won't bring any value (switching threads is a heavy operation)

Thread

  • a thread is an independent execution path, able to run simultaneously with other threads and can be managed independently by a scheduler
  • threads might run in parallel on differen physical cores or creatign multicore illusion (preemptive or context switching)

Context switching

Context switching

Threads vs Processes

Do not mix Threads and Processes

Task manager in Windows

Thread vs Process - Processes

Thread - execution path within a process

  • contains running code
  • share memory of containing process
  • share resources dedicated for containing process
  • has own stack withing process memory
  • requiere synchronization

Thread vs Process - Threads

Process - is an instance of a program

  • each process has own dedicated memory space
  • contains at least 1 thread
  • few processes might represent the same program
  • syncronization almost not required

Example of code with multiple threads

Threads in C#

Threads are represented in C# by class Thread

1: 

    System.Threading.Thread

Constructors:

Run parameterless method in separate thread

1: 

    Thread(ThreadStart)

Run method with an object-type parameter in separate thread

1: 

    Thread(ParameterizedThreadStart)

Creating and running threads

Thread should be created with an entry point when new thread will start execution.

This entry point is outlined by following delegates:

1: 
2: 

    public delegate void ThreadStart();
    public delegate void ParameterizedThreadStart(Object obj);

Once thread created it should be run for execution:

1: 
2: 

    var thread = new Thread(someEntryPoint);
    thread.Start();
Create thread - demo

Thread lifecycle

Thread lifecycle

Thread lifecycle details

Thread states with descriptions

More about multithreading

Stopping and aborting threads

There are a number of ways to stop thread:

Thread characteristics

Error handling

  • exception could be cought within a thread (not outside it)
  • unhandled exception occured at any thread terminates entire application
  • it does not make sense to catch ThreadAbortException (caused by Thread.Abort() method)
  • you can catch globally unhandled exception by event AppDomain.CurrentDomain.UnhandledException, however it's impossible to prevent application from closing

Thread pooling

Creating thread consumes is an expensive operation. Thread pool cuts these overheads.

Ways to enter the thread pool:

Thread pooling - schema

Thread Pool

Common synchronization problems

  • atomicity - make critical code atomic
  • visibility - if read/write operations are in diff threads, no guarantee that reading thread will read written value
  • reordering - code order might be changed by CLR - this might be critical for app logic
  • race condition - occurs when correctness of computation depends on data access time
  • livelock - thread fails operation and returns data for re-processing
  • starvation - mutual lock caused by inapropriate thread priorities
  • deadlock - mutual lock of 2 or more threads

Synchronization primitives

  • Simple blocking methods
  • Locking constructs
  • Signaling constructs
  • Nonblocking synchronization constructs
    • volatile keyword
    • Interlocked class
    • use immutable objects

Simple blocking methods

  • Sleep - pause thread execution for specified time
  • Join - invoking thread pauses until other thread finish its execution

Locking

Exclusive locking is used to ensure that only one thread can enter particular sections of code at a time

Signaling