Failure to execute all of the steps in the Parallel class

Question

Hi guys,
Since I'm concerned about speed and efficiency, I began reading Gastón C. Hillar's Professional Parallel Programming with C# book and want to rewrite my project from the ground up using the parallel programming approach.
In the following exercise, I noticed that not all steps of the parallel loop are executed while comparing the execution speed of the sequential and parallel loops.
I noticed this by analyzing the text length.
In the sequential loop, the length is always 200,000, but in the parallel loop it is variable.

Sequential loop

            System.Diagnostics.Stopwatch StopWatch = new System.Diagnostics.Stopwatch();  
            StringBuilder SB = new StringBuilder();  
            Console.ReadKey();  
            StopWatch.Start();  
            for (int i = 0; i < 200000; i++)  
            {  
                SB.Append("B");  
            }  
            StopWatch.Stop();  
            Console.WriteLine("Total seconds: " + StopWatch.Elapsed.TotalSeconds + ", Length: " + SB.Length);  
            Console.ReadKey();  

Result:

Parallel.For

            System.Diagnostics.Stopwatch StopWatch = new System.Diagnostics.Stopwatch();  
            StringBuilder SB = new StringBuilder();  
            Console.ReadKey();  
            StopWatch.Start();  
            Parallel.For(0, 200000, range =>  
            {  
                SB.Append("B");  
            });  
            StopWatch.Stop();  
            Console.WriteLine("Total seconds: " + StopWatch.Elapsed.TotalSeconds + ", Length: " + SB.Length);  
            Console.ReadKey();  

One of the results:

Parallel.ForEach

            System.Diagnostics.Stopwatch StopWatch = new System.Diagnostics.Stopwatch();  
            StringBuilder SB = new StringBuilder();  
            Console.ReadKey();  
            StopWatch.Start();  
            Parallel.ForEach(Partitioner.Create(0, 200000, ((200000 / Environment.ProcessorCount) + 1)), range =>  
            {  
                for (int i = range.Item1; i < range.Item2; i++)  
                {  
                    SB.Append("B");  
                }  
            });  
            StopWatch.Stop();  
            Console.WriteLine("Total seconds: " + StopWatch.Elapsed.TotalSeconds + ", Length: " + SB.Length);  
            Console.ReadKey();  

One of the results:

1. What is the reason for this, and how can I know if the entire parallel loop has been implemented in the projects? Please explain with an example.
2. Also, what exactly is the concept of overhead?

I use the following tools:

• .NET Framework 4.5
• Console app

Thank you for your time.
Best regards

Answer 1

First a StringBuilder is not thread safe, you need to take a lock before accessing. Second the Append method is not too cpu intensive, so the overheard of creating and managing threads is greater. And once you add proper locking, only 1 thread at a time will do the append, so you are just adding needless overheard.

Now if there was a lot of cpu involved in generating what to append, this example might make sense.

Update:

List is not thread safe and requires a lock. there are thread safe collections.

the following code would work:

             System.Diagnostics.Stopwatch StopWatch = new System.Diagnostics.Stopwatch();  
             var List = new ConcurrentBag<object>();  
             Console.ReadKey();  
             StopWatch.Start();  
             Parallel.For(0, 200000, range =>  
             {  
                 List.Add("A");  
             });  
             StopWatch.Stop();  
             Console.WriteLine("Total seconds: " + StopWatch.Elapsed.TotalSeconds + ", Length: " + List.Count);  
             Console.ReadKey();  

but this code will be slower due to the locking and thread magement than a non parallel version. again this is because the parallel cpu work is trivial.