Java Performance Tuning

Thread Pools

The VM is optimized for creating threads, so you can usually create a new thread when you need to without having to worry about performance. But in some circumstances, maintaining a pool of threads can improve performance. For example, in a case where you would otherwise create and destroy many short-lived threads, you are far better off holding on to a (variable-sized) pool of threads. Here, the tasks are assigned to an already created thread, and when a thread completes its task, it is returned to the pool, ready for the next task. This improves performance because thread creation (and, to some extent, destruction) does have a significant overhead that is better avoided for short-lived threads.

A second situation is where you want to limit the number of threads in your application. In this case, your application needs to make all thread requests through a centralized pool manager. Although a pool manager does not prevent other threads from being started, it is a big step towards that goal. (Strictly speaking, limiting threads does not require a pool of threads, just a centralized pool manager, but the two usually come together.) Every system has a response curve with diminishing returns after a certain number of threads are running on it. This response curve is different for different systems, and you need to identify values for your particular setup. A heavy-duty server needs to show good behavior across a spectrum of loads, and at the high end, you don’t want ...