scheduleAtFixedRate not starting the task at specified delay

Question

I am using SchedulerExecuterService to execute a task after specified delay and at given intervals.

ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
scheduler.scheduleAtFixedRate(taskThread,60 ,120 ,TimeUnit.SECONDS);

What happening is first time taskThread is not starting after delay of 60 sec, it is started after delay of more than 60 secs. Whereas the next executions started at correct intervals of 120 secs(not exactly 120 secs but there is a very minute delay which can be ignored).

My query is why the first execution is delayed for more than 60 secs? Since the task is executed by a thread, Does the start up time depends on the thread priority?

How can I make it run at exact delay of 60 secs? What about Quartz library? Will this library solve my purpose(run the job at specific time without any delay)?

Thanks in advance.

Answer 1

Startup of a job depends on the scheduled time and the amount of threads available - if there are none available, it can be delayed. However, further executions will start at a scheduled time (delay + n * period) - which is takes place in your case.

Start will happen approximately at a scheduled time, I doubt there are real-time guarantees for this, it depends on the underlying OS. You could try changing a thread priority for a thread in this thread pool. This could help, but it won't give you a guarantee that it would work out in different environment.

You could also make sure you run a single task for a fixed thread pool of a single thread you use. Or try increasing the thread amount.

Answer 2

T.b.h. the delay you are seeing is unlikely to be ScheduledExecutorService's fault. Starting a thread won't take a second unless your machine is ridiculously overloaded, and I think the thread will get initialised up-front anyways if you use a fixed size thread pool.

I'm guessing it's the initialisation of your own tasks. I don't know what you are doing in your task, but getting the resource ready (starting DB connection pools) etc. can amount to significant time.

If you want you can test with a trivial task (like writing "hello" on the console) to see. And then you can measure each part of your task to see what's taking long. Once you have that you can think of ways to "warm" your system to prevent the delay.