Ndtyjky

According to docs.microsoft.com [1, 2, 3, 4]:

A background thread executes only when the number of foreground threads executing is smaller than the number of processors.

However, if I start 4 foreground threads on a 4-core CPU (without hyper-threading) and then start 4 background threads, the foreground threads and background threads will run side-by-side, which seems to contradict the statement above.

Code sample:

static void Main(string args)

{

    int numberOfProcessors = 4;



    for (int i = 0; i < numberOfProcessors; i++)

    {

        int threadNumber = i;



        new Thread(() =>

        {

            Console.WriteLine($"Foreground thread {threadNumber} started.");



            for (int j = 1; j <= 100; j++)

            {

                for (long k = 0; k < 10000000000; k++);



                Console.WriteLine($"Foreground thread {threadNumber} progress: {j}%.");

            }

        })

        .Start();

    }



    for (int i = 0; i < numberOfProcessors; i++)

    {

        int threadNumber = i;



        var backgroundThread = new Thread(() =>

        {

            Console.WriteLine($"Background thread {threadNumber} started.");



            for (int j = 1; j <= 100; j++)

            {

                for (long k = 0; k < 10000000000; k++);



                Console.WriteLine($"Background thread {threadNumber} progress: {j}%.");

            }

        });

        backgroundThread.IsBackground = true;

        backgroundThread.Start();

    }



    Console.ReadLine();

}

Output:

Foreground thread 1 started.

Foreground thread 0 started.

Foreground thread 3 started.

Foreground thread 2 started.

Background thread 0 started.

Background thread 1 started.

Background thread 2 started.

Background thread 3 started.

Foreground thread 2 progress: 1%.

Foreground thread 0 progress: 1%.

Foreground thread 1 progress: 1%.

Foreground thread 3 progress: 1%.

Background thread 1 progress: 1%.

Background thread 0 progress: 1%.

Background thread 2 progress: 1%.

Background thread 3 progress: 1%.

Foreground thread 0 progress: 2%.

Foreground thread 2 progress: 2%.

Foreground thread 1 progress: 2%.

Foreground thread 3 progress: 2%.

Background thread 0 progress: 2%.

Background thread 1 progress: 2%.

Background thread 3 progress: 2%.

Background thread 2 progress: 2%.

Foreground thread 0 progress: 3%.

Foreground thread 2 progress: 3%.

Foreground thread 1 progress: 3%.

Foreground thread 3 progress: 3%.

Background thread 1 progress: 3%.

Background thread 0 progress: 3%.

Background thread 3 progress: 3%.

Background thread 2 progress: 3%.

...

Is the statement incorrect or do I simply get it wrong?

In my opinion the statement is incorrect and in addition you are looking not at the lastest version of the documentation. If you lookup the lastest version of the documentation about Managed Threading / Foreground and Background Threads it states:

A managed thread is either a background thread or a foreground thread. Background threads are identical to foreground threads with one exception: a background thread does not keep the managed execution environment running. Once all foreground threads have been stopped in a managed process (where the .exe file is a managed assembly), the system stops all background threads and shuts down.

Note

When the runtime stops a background thread because the process is shutting down, no exception is thrown in the thread. However, when threads are stopped because the AppDomain.Unload method unloads the application domain, a ThreadAbortException is thrown in both foreground and background threads.

Use the Thread.IsBackground property to determine whether a thread is a background or a foreground thread, or to change its status. A thread can be changed to a background thread at any time by setting its IsBackground property to true.

Important

The foreground or background status of a thread does not affect the outcome of an unhandled exception in the thread. In the .NET Framework version 2.0, an unhandled exception in either foreground or background threads results in termination of the application. See Exceptions in Managed Threads.

Threads that belong to the managed thread pool (that is, threads whose IsThreadPoolThread property is true) are background threads. All threads that enter the managed execution environment from unmanaged code are marked as background threads. All threads generated by creating and starting a new Thread object are by default foreground threads.

If you use a thread to monitor an activity, such as a socket connection, set its IsBackground property to true so that the thread does not prevent your process from terminating.

The same goes for the documentation of the Thread.IsBackground property:

Remarks

A thread is either a background thread or a foreground thread. Background threads are identical to foreground threads, except that background threads do not prevent a process from terminating. Once all foreground threads belonging to a process have terminated, the common language runtime ends the process. Any remaining background threads are stopped and do not complete.

By default, the following threads execute in the foreground (that is, their IsBackground property returns false):

• The primary thread (or main application thread).


• All threads created by calling a Thread class constructor.

By default, the following threads execute in the background (that is, their IsBackground property returns true):

• 
  

  Thread pool threads, which are a pool of worker threads maintained by the runtime. You can configure the thread pool and schedule work on thread pool threads by using the ThreadPool class.

  
  
  

  Note

  
  
  

  Task-based asynchronous operations automatically execute on thread pool threads.

  
  



• All threads that enter the managed execution environment from unmanaged code.