ScheduledThreadPoolExecutor core thread pool / keepalive/timing out threads

Question

Hi,
I am using a ScheduledThreadPoolExecutor - java1.6 on Windows(currently).
I would like to minimise the number of threads in the pool running at any one time to around the number of threads required to run the number of tasks that are concurrently executing.
However, I don't seem to be able to get the thread pool to minimise the number of threads below the max threads in the constructor.
I want to do this mainly because I have a large delay queue and the performance of the pool seems to be better when the number of threads in the pool is minimised to roughly the number of threads required to service the concurrently executing tasks.
Any ideas?
Thanks,
Alex
                ThreadFactory tf = new MyThreadFactory();
          ScheduledThreadPoolExecutor scheduler = new ScheduledThreadPoolExecutor(50,tf);
          scheduler.setCorePoolSize(10);
          //scheduler.setMaximumPoolSize(50);
          scheduler.setKeepAliveTime(1, TimeUnit.NANOSECONDS);
          scheduler.allowCoreThreadTimeOut(true);
          scheduler.schedule(aTask, 10, TimeUnit.SECONDS);
...

Answer 1

because of how a scheduledthreadpoolexecutor works, it doesn't really make sense to have a variable number of threads. since threads are only added when tasks are added to the pool (aka, when submit is called), once the core threads timed out they would never get replaced (e.g. you would eventually get down to 1 thread and never get anymore added). one way to achieve what you want would be to have a single threaded ScheduledThreadPoolExecutor, where the scheduled jobs do the real work in a separate, variable size thread pool (e.g. the scheduled tasks do not do anything but submit a task to the real work thread pool). this solution would work fine for "schedule at fixed rate" but not for "schedule with fixed delay". you would actually have to simulate the latter by scheduling a one-off task which resubmits itself to the scheduler each time it completes.