[jira] [Created] (SIS-76) Remove (for now) the internal ThreadPoolExecutor

["Martin Desruisseaux (JIRA)" <ji...@apache.org>]

Martin Desruisseaux created SIS-76:
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             Summary: Remove (for now) the internal ThreadPoolExecutor
                 Key: SIS-76
                 URL: https://issues.apache.org/jira/browse/SIS-76
             Project: Spatial Information Systems
          Issue Type: Task
          Components: Utilities
            Reporter: Martin Desruisseaux
            Priority: Minor


SIS needs a thread for executing short tasks after some (potentially zero nanosecond) delay. This thread is reserved to internal SIS usage, mostly resources disposal. We tried to use the {{java.util.concurrent}} package in a previous version, especially {{ScheduledThreadPoolExecutor}}, but those executors seem more suitable to heavier tasks in applications controlling their own executor. Indeed, the Javadoc of the {{ForkJoinPool}} executor said "_Normally a single {{ForkJoinPool}} is used for all parallel task execution in a program or subsystem. Otherwise, use would not usually outweigh the construction and bookkeeping overhead of creating a large set of threads._" So we may be better to let users create their own executor, and potentially leverage it in SIS (this strategy is yet to be determined).

It seems difficult to instantiate an executor for casual SIS use without wasting resources. For example {{ScheduledThreadPoolExecutor}} acts as a fixed-sized pool, thus forcing us to use only one thread if we don't want to waste resources (profiling shows that even a single thread has very low activity for the SIS tasks that we need to submit in background). The {{ThreadPoolExecutor}} super-class is more flexible but still have a quite aggressive policy on threads creation, and doesn't handle delayed tasks by itself.

We could try to combine both worlds with a {{ThreadPoolExecutor}} using a {{DelayedQueue}}, but it forces us to declare a core pool size of 0 otherwise {{ThreadPoolExecutor}} tries to execute the tasks immediately without queuing them. Combined with the {{DelayedQueue}} characteristics (being an unbounded queue), this result in {{ThreadPoolExecutor}} never creating more than one thread (because it waits for the queue to reject a task before to create more threads than the pool size).

Given that it seems difficult to configure {{(Scheduled)ThreadPoolExecutor}} in such a way that two or more threads are created only when really needed, given that using those thread pools seems an overkill when the pool size is fixed to one thread, given that our profiling has show very low activity for that single thread anyway, and given that we do not need cancellation and shutdown services for house keeping tasks since they are executed in a daemon thread, a more lightweight solution seems acceptable here. Se we could remove the internal SIS executor, use a single thread as we do for {{ReferenceQueueConsumer}}, and wait to see later if we need an executor.

h2. Future evolution
We may revert to an executor in a future SIS evolution if we happen to need an executor anyway. However it may be better to wait and see what are the executor needs. Setting up an executor implies choosing many arbitrary parameter values like the number of core threads, maximum threads, idle time, queue capacity, _etc._ Furthermore some platforms (e.g. MacOS) provide OS-specific implementations integrating well in their environment. We may want to let the user provides the executor of his choice, or we way want to have more profiling data for choosing an appropriate executor. But we would need to find some way to give priority to SIS tasks, since most of them are for releasing resources - in which case quick execution probably help the system to run faster.


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