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what's the proper way to use a ThreadPool?

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If my understanding of the way the ThreadPool works is correct, one of its purposes is to limit the number of worker threads within a process that can be created at a given time. For example, if you set MaxThreads to 5 and then call QueueUserWorkItem 30 times, 30 requests will be made to the ThreadPool, but only 5 of those requests will be serviced by a new thread, while the other 25 requests will be added to the queue and serviced one at time as previous requests complete and existing threads becomes available.

In the code below, however, the call to Thread.Sleep(-1) guarantees that the DoSomething() method will never return, meaning that the current thread will never become available to subsequent requests.

But my understanding of the way a ThreadPool works cannot be correct, because if it were correct the code below would print only the numbers 0-4, rather than 0-29.

Can someone please explain how the ThreadPool works and why the below code isn't doing what I thought it should be doing?

    static void DoSomething(object n)     {         Console.WriteLine(n);         Thread.Sleep(-1);     }      static void Main(string[] args)     {         ThreadPool.SetMaxThreads(5, 5);         for (int x = 0; x < 30; x++)         {             ThreadPool.QueueUserWorkItem(new WaitCallback(DoSomething), x);         }         Console.Read();     } 
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John Smith Avatar asked Jan 25 '13 04:01

John Smith


People also ask

How do you implement a ThreadPool?

The thread pool implementation consists of two parts. A ThreadPool class which is the public interface to the thread pool, and a PoolThread class which implements the threads that execute the tasks. To execute a task the method ThreadPool. execute(Runnable r) is called with a Runnable implementation as parameter.

When should you not use ThreadPool?

Thread pools do not make sense when you need thread which perform entirely dissimilar and unrelated actions, which cannot be considered "jobs"; e.g., One thread for GUI event handling, another for backend processing. Thread pools also don't make sense when processing forms a pipeline.

What is the use of ThreadPool?

The thread pool is primarily used to reduce the number of application threads and provide management of the worker threads. Applications can queue work items, associate work with waitable handles, automatically queue based on a timer, and bind with I/O.

What is the difference between thread and ThreadPool?

A thread pool is - as the name suggests - a pool of worker threads which are always running. Those threads then normally take tasks from a list, execute them, then try to take the next task. If there's no task, the thread will wait.


2 Answers

ThreadPool.SetMaxThreads(5, 5)

means the number of active thread is 5 (if you have more than 5 cpu core), does not mean that the ThreadPool can only create 5 threads. The ThreadPool maximum number of threads = CPU Core * 250.

After Thread.Sleep, the thread is inactive, so it will not affect the execution of other threads.

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shalongbus Avatar answered Oct 10 '22 19:10

shalongbus


But my understanding of the way a ThreadPool works cannot be correct, because if it were correct the code below would print only the numbers 0-4, rather than 0-29.

Yes your assumption is very much correct.

Since u have queued 30 jobs in a ThreadPool and the Jobs will sleep for InfiniteTime, they will never finish, the ThreadPool class will wait for a certain interval to create new thread, but will not exceed the max number of threads.

NOTE

Console.Read() is keeping your Background thread alive.

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From MSDN

Many applications create threads that spend a great deal of time in the sleeping state, waiting for an event to occur. Other threads might enter a sleeping state only to be awakened periodically to poll for a change or update status information. Thread pooling enables you to use threads more efficiently by providing your application with a pool of worker threads that are managed by the system. One thread monitors the status of several wait operations queued to the thread pool. When a wait operation completes, a worker thread from the thread pool executes the corresponding callback function.


When all thread pool threads have been assigned to tasks, the thread pool does not immediately begin creating new idle threads. To avoid unnecessarily allocating stack space for threads, it creates new idle threads at intervals. The interval is currently half a second, although it could change in future versions of the .NET Framework.


The threads in the managed thread pool are background threads. That is, their IsBackground properties are true. This means that a ThreadPool thread will not keep an application running after all foreground threads have exited.

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Parimal Raj Avatar answered Oct 10 '22 17:10

Parimal Raj