// those subtasks to finish.
            // This ensures the number of running workers is no more than
            // the defined parallism, while making sure the pool will not
            // be exhausted
            //
            return new ThreadPoolExecutor(
                    parallelism, Integer.MAX_VALUE, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<>()) {

 * graph construction.
 * <p>
 * Extend this class {@code createXXX()} methods and override to customize, if needed. The contract of this class makes
 * sure that these (potentially overridden) methods are invoked only once, and instance created by those methods are
 * memorized and kept as long as supplier instance is kept open.
 * <p>