6. Parallelism: threads, C#

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//6. Parallelism: threads

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using System.Threading;
using System.Threading.Tasks;
namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            //Modern operating systems support multitasking. This means that multiple programs (or processes) may run simultaneously even on a single cpu.
            //This is achieved by switching from one process to another many times per second (i.e. one process gets to execute its instructions, then another one and so on).
            //Threads are simillar to processes, but they run inside one process.
            //So threads are isolated chunks of code that run "simultaneously". They are usefull because they let utilise many cpus and this way speedup algorithms.
            //Imagine you need to fetch 1000 web pages from the web. You could do it in serial: first fetch one, then the second one and so on.
            //Or you could spawn 1000 threads and fetch them all in parallel.
            
            //The oldest way to start a thread in C# is to use Thread class:
            var thread = new Thread(f =>
                                    {
                                        Thread.Sleep(5000); //this will put current thread to sleep for 5 seconds. 
                                                            //This means that this thread won't be given any cpu time for the next 5 seconds.
                                        Console.WriteLine("Hello from another thread!");                                        
                                    });
            thread.Start();
            Console.WriteLine("Hello from main thread!");
            thread.Join(); //wait for spawned thread to end
            
            //One thing to keep in mind is creating threads with Thread class is an expensive way since it takes for some time and memory to initialise new thread.
            //Much cheaper way is to use already initialised thread from a ThreadPool. After your thread ends it is put back to the pool for later usage.
            ThreadPool.QueueUserWorkItem(f =>
                                         {
                                             Console.WriteLine("Hello from another thread (taken from the pool this time!)"); 
                                         });
            
            //Finally the latest and the hottest way to launch threads is by using Tasks from System.Threading.Tasks namespace. Thread from a pool will be used, so it is as cheap as before.
            var task = Task.Run(() =>
                                {
                                    Console.WriteLine("Hello from task-created thread!");
                                });
            task.Wait(); //wait for the task to finish
            
        }
    }
}

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Compilation time: 0.14 sec, absolute running time: 5.13 sec, cpu time: 0.11 sec, average memory usage: 22 Mb, average nr of threads: 7