c# - Why is my multi-threading slower than my single threading?

Question

I know a couple of people asked a question similar to this, but I can’t find any response that would make me understand why it's slower.

So, I made a little console program for my own understanding of the threading object in Visual Studio 2013. My CPU is an Intel Core i7 that supply can use multiple threading.

My code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Threading;
using System.Diagnostics;

namespace ConsoleApplication1
{
    class Program
    {

        static TimeSpan MTTime;
        static TimeSpan STTime;

        static void Main(string[] args)
        {
            Stopwatch stopwatch = new Stopwatch();
            stopwatch.Start();


            Console.WriteLine(Environment.NewLine + "---------------Multi Process-------------" + Environment.NewLine);

            Thread th1 = new Thread(new ParameterizedThreadStart(Process));
            Thread th2 = new Thread(new ParameterizedThreadStart(Process));
            Thread th3 = new Thread(new ParameterizedThreadStart(Process));
            Thread th4 = new Thread(new ParameterizedThreadStart(Process));

            th1.Start("A");
            th2.Start("B");
            th3.Start("C");
            th4.Start("D");

            th1.Join();
            th2.Join();
            th3.Join();
            th4.Join();

            stopwatch.Stop();
            MTTime = stopwatch.Elapsed ;

            Console.WriteLine(Environment.NewLine + "---------------Single Process-------------" + Environment.NewLine);


            stopwatch.Reset();
            stopwatch.Start();

            Process("A");
            Process("B");
            Process("C");
            Process("D");

            stopwatch.Stop();
            STTime = stopwatch.Elapsed;

            Console.Write(Environment.NewLine + Environment.NewLine + "Multi  : "+ MTTime + Environment.NewLine + "Single : " + STTime);


            Console.ReadKey();
        }

        static void Process(object procName)
        {
            for (int i = 0; i < 100; i++)
            {
                Console.Write(procName);
            }
        }
    }
}

Result image:

We can clearly see that the multi-treading process is total random and the single one just do all presses on after the other, but I don't think this have an impact on the speed.

At first, I thought my thread was just bigger than the process needed for running the program, but after changing for a bigger process the single treading still was still the fastest in a big way. So, do i miss a concept in multi-threading? Or it normal that is slower?

Answer 1

Note that Process writes to the console (and basically does nothing else), and that output to the console (which here acts as a kind of shared resource) is slow and needs to be synchronized with the other threads.

To my understanding, the parallelization you use creates a huge overhead but gains no speedup, because all of the time the threads are apparently mostly waiting for the other process to finish writing to the console.