Multithreading/Multitas king. Task Parallel Library. Patterns

sections of code simultaneously.

Or pretend like. //For 1 CPU core

button is pressed less often

Threads container

Thread

Basic unit to which the operating system allocates processor time.
Executes within the context of a process and shares the same resources allotted to the process by the kernel.

On an Operating System level

Insides:
PID
Memory (Code and Data, Stack, Heap,
Shared Memory…)
File Descriptors
Registers
Kernel State (Process State, Priority,
Statistics)

Insides:
Thread Kernel Object
Thread Environment Block (TEB)
Stacks (User-mode and Kernel-mode)

for x64
Thread environment block
1 memory page (4 Kb)
User-mode stack
1+ Mb
Kernel-mode stack
12 Kb for x86
24 Kb for x64

Note:

Also, whenever a thread is created in a process, all unmanaged DLLs loaded in that process have their DllMain method called. Similarly, whenever a thread dies.

the priority (process priority + thread priority)

Client-side and server-side applications where non-sequentially execution is possible. For performance improvements

Thread(ParameterizedThreadStart, Int32)
, where ThreadStart and ParameterizedThreadStart
are the delegates, lambdas, closures, Action,
Func, etc. Also, you may limit thread stack size
By passing second parameter.
Start() method to run the thread
Use IsAlive property to wait for the thread start
Join() method to wait till thread ends
Use closures to simplify value return

Set thread IsBackground property to true for
immediately suspension when parent foreground
thread ends
Exceptions can be caught only on the same thread

bool _isRunning;
private Thread _commandGetThread;
private object _commandGetterLocker = new object();
private int _sleepInterval;
private Consumer _executor;
public Producer(Consumer executor, int sleepInterval) {
… //Set defaults
_isRunning = true;
_commandGetThread = new Thread(CommandRequestSend);
_commandGetThread.Start();
}
private void CommandRequestSend() {
while (_isRunning) {
lock (_commandGetterLocker) {
… //GetCommands code goes here
_executor.EnqueueCommands(webCommands);
}
Thread.Sleep(_sleepInterval);
}
}
public void Dispose() { … } //use Join() instead of Abort()
}

public class Consumer : IDisposable {
private volatile bool _isRunning;
private object locker = new object();
private Thread[] executants;
private ICommandRepository _commandsRepo = new CommandListRepository();
public Consumer(int executorsCount) {
_isRunning = true;
executants = new Thread[executorsCount];
for (int i = 0; i < executorsCount; i++)
(executants[i] = new Thread(Execute)).Start();
}
public void EnqueueTask(List commands) {
lock (locker) {
_commandsRepo.AddCommands(commands);
Monitor.PulseAll(locker);
}
}
void Execute() {
while (_isRunning) {
lock (locker) {
while (_commandsRepo.IsEmpty()) Monitor.Wait(locker);
commandClient = _commandsRepo.GetCommand();
}
if (commandClient == null) return;
… //Execute Command Code (better wrap with try-catch)
}
}
public void Dispose() { … } //enque null in each thread and join
}

approach will use it in 4.0 version
ThreadPool works on CLR level. It has highly intelligent algorithm for thread management.
Only busy threads in pool
To perform asynchronous operation: just call ThreadPool.QueueUserWorkItem()

manages thread number?

How the Thread Pool Manages Its Threads?

when operation completes
Task class for void and Task generic for T object return
No-headache with exception handling. Throws AggregateException with inner exceptions tree that corresponds to Tasks tree
Task start does not guarantee execution in separate thread!

ThreadPool.QueueUserWorkItem(SomeLongTermFunction);
var task = new Task(SomeLongTermFunction);
task.Start();

for children’s completion) states

you need all results

Cancelling

Create CancellationTokenSource object and pass its Token property to task constructor
Start task and call Cancel() method on CancellationTokenSource object
Task will stop and throw AggregateException

threads block.
Calling task.Wait() will pause current thread until Result property became available
Its better for performance to start next task immediately after previous.
For this case, there are .ContinueWith() extension for task.
Usage sample:

var task = new Task(SomeLongTermFunction, cancelToken.Token);
task.ContinueWith(parentTask => AnotherLongTermFunction(),
TaskContinuationOptions.NotOnFaulted);
task.Start();

will construct a TaskFactory.
If you want to create a bunch of tasks that have a specific return type, then you will construct a
TaskFactory

Schedulers

TaskScheduler object is responsible for executing scheduled tasks and also exposes task information
to the Visual Studio debugger
The FCL ships with two TaskScheduler-derived types:
the thread pool task scheduler
synchronization context task scheduler.
By default, all applications use the
thread pool task scheduler.

Tasks are very flexible

var factory =
new TaskFactory(TaskScheduler
.FromCurrentSynchronizationContext());
factory.StartNew(() => GetFibonacciNumber(1));
factory.StartNew(() => GetFibonacciNumber(2));
factory.StartNew(() => GetFibonacciNumber(3));

toConclusive, index => method(index));

Parallel.ForEach(IEnumerable, item => method(item));

Parallel.Invoke(method0(), method1(), method2()…);

They all have overloaded versions that takes ParallelOption object as parameter. ParallelOption contains such settings:
- MaxThreadNumbers
- CancellationToken
- TaskScheduler

parts of algorythm.
Use localInit and localFinal parameters.
Code sample:

Tasks interaction in Parallel

var files = Directory.EnumerateFiles(path, searchPattern, searchOption);
var masterTotal = 0;
var result = Parallel.ForEach(
files,
() => { return 0; /* Set taskLocalTotal initial value to 0*/ },
(file, loopState, index, taskLocalTotal) =>
{
// body: Invoked once per work item
// Get this file's size and add it to this task's running total
var fileLength = 0;
FileStream fs = null;
try
{
fs = File.OpenRead(file);
fileLength = (int) fs.Length;
}
catch (IOException) { /* Ignore any files we can't access */ }
finally
{
if (fs != null) fs.Dispose();
}
return taskLocalTotal + fileLength;
},
taskLocalTotal =>
{
// localFinally: Invoked once per task at end
// Atomically add this task's total to the "master" total
Interlocked.Add(ref masterTotal, taskLocalTotal);
});
Console.WriteLine(masterTotal);

processing of ParallelQuery collection.
To transform IEnumerable into ParallelQuery - just call AsParallel() on it (AsSequential() for vice versa)
Supports almost the same functionality, as the ordinar LINQ.
Also, offers some additional ParallelEnumerable methods that you can call to control how the query is processed:
WithCancellation(CancellationToken)
WithDegreeOfParalelism(Int32)
WithExecutionMode(ParallelExecutionMode)
WithMergeOptions(ParallelMergeOption)

timers in .Net:

System.Threading.Timer
System.Windows.Forms.Timer
System.Windows.Threading.DispatcherTimer (Silverlight and WPF)
Windows.UI.Xaml’s DispatcherTimer (Windows Store Apps)
System.Timers.Timer. Obsolete class. Wrapper for System.Threading.Timer.

private static Timer s_timer; public static void Main() { Console.WriteLine("Checking status every 2 seconds");
// Create the Timer ensuring that it never fires. This ensures // that s_timer refers to it BEFORE Status is invoked by a
// thread pool thread s_timer = new Timer(Status, null, Timeout.Infinite, Timeout.Infinite);
// Now that s_timer is assigned to, we can let the timer fire // knowing that calling Change in Status will not throw a
// NullReferenceException s_timer.Change(0, Timeout.Infinite); Console.ReadLine(); // Prevent the process from terminating }
// This method's signature must match the TimerCallback delegate private static void Status(Object state) { // This method is executed by a thread pool thread Console.WriteLine("In Status at {0}", DateTime.Now); Thread.Sleep(1000); // Simulates other work (1 second)
// Just before returning, have the Timer fire again in 2 seconds s_timer.Change(2000, Timeout.Infinite);
// When this method returns, the thread goes back // to the pool and waits for another work item
}

method without awaits inside will be executed synchronously
Compiler will create continuations for code after await
There are a lot of async functions in FCL that can be easily found by suffix “Async”

Async/Await

Exception can be catched from main thread only if async method is awaited
Using await with a Task, the first inner exception is thrown instead of an AggregateException
“await” keyword inside of a catch{} and a finally {} blocks are supported from C# 6.0

Patterns

APM to TAP conversion:

await Task.Factory.FromAsync(
stream.BeginRead, stream.EndRead, null);