In computer programming, the thread pool pattern (also replicated workers or worker-crew model) is where a number of threads are created to perform a number of tasks, which are usually organized in a queue. The results from the tasks being executed might also be placed in a queue, or the tasks might return no result (for example, if the task is for animation). Typically, there are many more tasks than threads. As soon as a thread completes its task, it will request the next task from the queue until all tasks have been completed. The thread can then terminate, or sleep until there are new tasks available.
The number of threads used is a parameter that can be tuned to provide the best performance. Additionally, the number of threads can be dynamic based on the number of waiting tasks. For example, a web server can add threads if numerous web page requests come in and can remove threads when those requests taper down. The cost of having a larger thread pool is increased resource usage. The algorithm used to determine when to create or destroy threads will have an impact on the overall performance:
- create too many threads, and resources are wasted and time also wasted creating any unused threads
- destroy too many threads and more time will be spent later creating them again
- creating threads too slowly might result in poor client performance (long wait times)
- destroying threads too slowly may starve other processes of resources
The algorithm chosen will depend on the problem and the expected usage patterns.
If the number of tasks is very large, then creating a thread for each one may be impractical.
Another advantage of using a thread pool over creating a new thread for each task is thread creation and destruction overhead is negated, which may result in better performance and better system stability. Creating and destroying a thread and its associated resources is an expensive process in terms of time. An excessive number of threads will also waste memory, and context-switching between the runnable threads also damages performance. For example, a socket connection to another machine—which might take thousands (or even millions) of cycles to drop and re-establish—can be avoided by associating it with a thread which lives over the course of more than one transaction.
When implementing this pattern, the programmer should ensure thread-safety of the queue. In Java, you can synchronize the relevant method using the
synchronized keyword. This will bind the block modified with
synchronized into one atomic structure, therefore forcing any threads using the associated resource to wait until there are no threads using the resource. As a drawback to this method, synchronization is rather expensive. You can also create an object that holds a list of all the jobs in a queue, which could be a singleton.
Typically, a thread pool executes on a single computer. However, thread pools are conceptually related to server farms in which a master process, which might be a thread pool itself, distributes tasks to worker processes on different computers, in order to increase the overall throughput. Embarrassingly parallel problems are highly amenable to this approach.
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