code: public synchronized void start(long period, TimeUnit timeUnit) {
  		if (scheduledFuture == null) {
  			PurgerRunnable runnable = new PurgerRunnable(manager);
  			scheduledFuture = executorService.scheduleWithFixedDelay(runnable, period, period, timeUnit);
  		} else {
  			throw new IllegalStateException("IncrementSimonPurger has already been started");
  		}
  	}
docs: Start periodical Simons purging with the specified period.
  
  @param period duration of purging period
  @param timeUnit time unit of period duration