8
  • I've got an I/O intensive operation.
  • I only want a MAX of 5 threads ever running at one time.
  • I've got 8000 tasks to queue and complete.
  • Each task takes approximately 15-20seconds to execute.

I've looked around at ThreadPool, but 

        ThreadPool.SetMaxThreads(5, 0);

        List<task> tasks = GetTasks();

        int toProcess = tasks.Count;
        ManualResetEvent resetEvent = new ManualResetEvent(false);

        for (int i = 0; i < tasks.Count; i++)
        {
            ReportGenerator worker = new ReportGenerator(tasks[i].Code, id);
            ThreadPool.QueueUserWorkItem(x =>
            {
                worker.Go();
                if (Interlocked.Decrement(ref toProcess) == 0)
                    resetEvent.Set();
            });
        }

        resetEvent.WaitOne();

I cannot figure out why... my code is executing more than 5 threads at one time. I've tried to setmaxthreads, setminthreads, but it keeps executing more than 5 threads.

What is happening? What am I missing? Should I be doing this in another way?

Thanks

Mike
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5 Answers5

9

There is a limitation in SetMaxThreads in that you can never set it lower than the number of processors on the system. If you have 8 processors, setting it to 5 is the same as not calling the function at all.

inafume
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    Here's the quote from [MSDN](https://learn.microsoft.com/en-us/dotnet/api/system.threading.threadpool.setmaxthreads): ```You cannot set the maximum number of worker threads or I/O completion threads to a number smaller than the number of processors on the computer. To determine how many processors are present, retrieve the value of the Environment.ProcessorCount property.``` – Sal Jan 08 '20 at 15:44
7

Task Parallel Library can help you:

List<task> tasks = GetTasks();

Parallel.ForEach(tasks, new ParallelOptions { MaxDegreeOfParallelism = 5 }, 
  task => {ReportGenerator worker = new ReportGenerator(task.Code, id); 
           worker.Go();});

What does MaxDegreeOfParallelism do?

Community
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Dmitry Harnitski
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1

I think there's a different and better way to approach this. (Pardon me if I accidentally Java-ize some of the syntax)

The main thread here has a lists of things to do in "Tasks" -- instead of creating threads for each task, which is really not efficient when you have so many items, create the desired number of threads and then have them request tasks from the list as needed.

The first thing to do is add a variable to the class this code comes from, for use as a pointer into the list. We'll also add one for the maximum desired thread count.

// New variable in your class definition
private int taskStackPointer;
private final static int MAX_THREADS = 5;

Create a method that returns the next task in the list and increments the stack pointer. Then create a new interface for this:

// Make sure that only one thread has access at a time
[MethodImpl(MethodImplOptions.Synchronized)] 
public task getNextTask()
{
    if( taskStackPointer < tasks.Count )
        return tasks[taskStackPointer++];
    else
        return null;
}

Alternately, you could return tasks[taskStackPointer++].code, if there's a value you can designate as meaning "end of list". Probably easier to do it this way, however.

The interface:

public interface TaskDispatcher
{
     [MethodImpl(MethodImplOptions.Synchronized)] public task getNextTask();
}

Within the ReportGenerator class, change the constructor to accept the dispatcher object:

public ReportGenerator( TaskDispatcher td, int idCode )
{
    ...
}

You'll also need to alter the ReportGenerator class so that the processing has an outer loop that starts off by calling td.getNextTask() to request a new task, and which exits the loop when it gets back a NULL.

Finally, alter the thread creation code to something like this: (this is just to give you an idea)

taskStackPointer = 0;
for (int i = 0; i < MAX_THREADS; i++) 
{ 
    ReportGenerator worker = new ReportGenerator(this,id);
    worker.Go(); 
}

That way you create the desired number of threads and keep them all working at max capacity.

(I'm not sure I got the usage of "[MethodImpl(MethodImplOptions.Synchronized)]" exactly right... I am more used to Java than C#)

Mike Fulton
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  • Thanks for taking the time to answer my question, it does make sense. This method is more verbose though :P – Mike Jul 15 '12 at 03:36
  • It might be a little more verbose but once you've got it in place it's pretty efficient and easy to understand. – Mike Fulton Jul 15 '12 at 03:49
1

Your tasks list will have 8k items in it because you told the code to put them there:

List<task> tasks = GetTasks();

That said, this number has nothing to do with how many threads are being used in the sense that the debugger is always going to show how many items you added to the list.

There are various ways to determine how many threads are in use. Perhaps one of the simplest is to break into the application with the debugger and take a look at the threads window. Not only will you get a count, but you'll see what each thread is doing (or not) which leads me to...

There is significant discussion to be had about what your tasks are doing and how you arrived at a number to 'throttle' the thread pool. In most use cases, the thread pool is going to do the right thing.

Now to answer your specific question...

To explicitly control the number of concurrent tasks, consider a trivial implementation that would involve changing your task collection from a List to BlockingCollection (that will internally use a ConcurrentQueue) and the following code to 'consume' the work:

var parallelOptions = new ParallelOptions
{
    MaxDegreeOfParallelism = 5
};

Parallel.ForEach(collection.GetConsumingEnumerable(), options, x =>
{
    // Do work here...
});

Change MaxDegreeOfParallelism to whatever concurrent value you have determined is appropriate for the work you are doing.

The following might be of interest to you:

Parallel.ForEach Method

BlockingCollection

Chris

Chris Keller
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1

Its works for me. This way you can't use a number of workerthreads smaller than "minworkerThreads". The problem is if you need five "workerthreads" maximum and the "minworkerThreads" is six doesn't work. {

ThreadPool.GetMinThreads(out minworkerThreads,out minportThreads);
ThreadPool.SetMaxThreads(minworkerThreads, minportThreads);

}

MSDN

Remarks

You cannot set the maximum number of worker threads or I/O completion threads to a number smaller than the number of processors on the computer. To determine how many processors are present, retrieve the value of the Environment.ProcessorCount property. In addition, you cannot set the maximum number of worker threads or I/O completion threads to a number smaller than the corresponding minimum number of worker threads or I/O completion threads. To determine the minimum thread pool size, call the GetMinThreads method.

If the common language runtime is hosted, for example by Internet Information Services (IIS) or SQL Server, the host can limit or prevent changes to the thread pool size.

Use caution when changing the maximum number of threads in the thread pool. While your code might benefit, the changes might have an adverse effect on code libraries you use.

Setting the thread pool size too large can cause performance problems. If too many threads are executing at the same time, the task switching overhead becomes a significant factor.

Santiago de Pena
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