java - Is having a single threadpool better design than multiple threadpools

Question

What are the advantages and disadvantages of having more than one threadpool in Java? I have seen code where there are multiple threadpools for different "types" of tasks, and I'm not sure whether its better design or just developers being lazy. One example is using a ScheduledThreadPoolExecutor for tasks that are executed periodically or have a timeout, and use another ThreadPoolExecutor for everything else.

Answer 1

The purpose of having separate dedicated threadpools is so that an activity doesn't get starved for threads because other activities took all the threads. If some service has its own threadpool then it is assured of having a certain number of threads at its disposal and it's not as sensitive to demands made by other services.

With the multiple dedicated threadpools if a service needs too many threads then it has to wait for threads to be available, introducing back-pressure into the system so that it degrades gradually, and since other parts have their own thread pools they have a chance to catch their parts up. So the idea is that the system should have more stable characteristics as load changes. In the case you describe having a separate threadpool for scheduled tasks makes sure that those tasks get run regardless of how busy the rest of the system is.

The multiple threadpools would require tuning to make sure each pool had enough threads and not too many. With a single threadpool you wouldn't need the tuning and might make better use of all the threads sometimes, but you might not have the predictability of knowing some important task would get the threads it needed to finish in a timely manner.