ExecutorService that executes tasks sequentially but takes threads from a pool

Question

I am trying to build an implementation of the ExecutorService, let\'s call it SequentialPooledExecutor, with the following properties.

Answer 1

If you have thousands of keys which must be processed sequentially, but you don't have thousands of cores you can use a hashing strategy to distribute the work like this

ExecutorService[] es = // many single threaded executors

public  Future submit(String key, Callable calls) {
    int h = Math.abs(key.hashCode() % es.length);
    return es[h].submit(calls);
}

In general you only need 2 * N threads to keep N cores busy, if your task is CPU bound, more than that just adds overhead.