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Thread pooling is the process of creating a collection of threads during the initialization of a multithreaded application, and then reusing those threads for new tasks as and when required, instead of creating new threads. The number of threads for the process is usually fixed depending on the amount of memory available, and the needs of the application. However, it might be possible to increase the number of available threads. Each thread in the pool is given a task and, once that task has completed, the thread returns to the pool and waits for the next assignment.
Thread pooling is essential in multithreaded applications for the following reasons.
Thread pooling improves the response time of an application as threads are already available in the thread pool waiting for their next assignment and do not need to be created from scratch
Thread pooling saves the CLR from the overhead of creating an entirely new thread for every short-lived task and reclaiming its resources once it dies
Thread pooling optimizes the thread time slices according to the current process running in the system
Thread pooling enables us to start several tasks without having to set the properties for each thread
Thread pooling enables us to pass state information as an object to the procedure arguments of the task that is being executed
Thread pooling can be employed to fix the maximum number of threads for processing a particular request
One of the major problems affecting the responsiveness of a multithreaded application is the time involved in spawning threads for each task.
For example, a web server is a multithreaded application that can service several client requests simultaneously. Let's suppose that ten clients are accessing the web server at the same time:
If the server operates a one thread per client policy, it will spawn ten new threads to service these clients, which entails the overhead of first creating those threads and then of managing them throughout their lifetime. It's also possible that the machine will run out of resources at some point.
Alternatively, if the server uses a pool of threads to satisfy those requests, then it will save the time involved in the spawning of those threads each time a request from a client comes in. It can manage the number of threads created, and can reject client requests if it is too busy to handle them. This is exactly the concept behind thread pooling.
The .NET CLR maintains a pool of threads for servicing requests. If our application requests a new thread from the pool, the CLR will try to fetch it from the pool. If the pool is empty, it will spawn a new thread and give it to us. When our code using the thread terminates, the thread is reclaimed by .NET and returned to the pool. The number of threads in the thread pool is limited by the amount of memory available.
To recap then, the factors affecting the threading design of a multithreaded application are:
The responsiveness of the application
The allocation of thread management resources
Responsiveness of the application and resource sharing are addressed by this chapter on thread pooling. The remaining factors have been covered in the previous chapters of this book.
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