How accurate is the task scheduling of a ScheduledThreadPoolExecutor

Question

I was reading ScheduledThreadPoolExecutor JavaDoc and came across the following thing:

Delayed tasks execute no sooner than they are enabled, but without any real-time guarantees about when, after they are enabled, they will commence. Tasks scheduled for exactly the same execution time are enabled in first-in-first-out (FIFO) order of submission.

So, if I write something like this:

ScheduledExecutorService ses = Executors.newScheduledThreadPool(4); //uses ScheduledThreadPoolExecutor internally
Callable<Integer> c;
//initialize c
ses.schedule(c, 10, TimeUnit.SECONDS);

there's no any guarantees that the execution of the callable will start in 10 seconds after the scheduling? As far as I got, the specification allows it to execute even in hour after scheduleing (without any real-time guarantees, as stated in the documentation).

How does it work in practice? Should I excepct some really long delay?

Answer 1

Your understanding is correct. The Executor is not claiming to be a real-time system with any sort of timing guarantees. The only thing it will guarantee is that it doesn't run tasks too early.

In practice, the timing of well-tuned Executors are very accurate. Typically they start within 10ms after the scheduled time from my experience. The only time you will see scheduling get pushed back very far is if your Executor is lacking the appropriate resources to run it's workload. So this is more of a tuning issue.

Realistically, if you give your Executor enough resources to work with, the timing will be quite accurate.

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Some things that you don't want to do with an Executor is use the scheduling as part of a rate-based calculation. For example, if you schedule a task to run every 1 second and you use that to compute <somemetric> per second without factoring in what time the task is actually running at.

Another thing to be mindful of is the cost of context switching. If you schedule multiple tasks to run every 1ms, the Executor will not be able to keep up with running your task and context switching everyone 1ms.