RxJava - Schedulers vs ExecutorService?

Question

I had a hunch that for highly computational, parallelized tasks in RxJava, a traditional ExecutorService would be faster than a Scheduler.

I had a theory that this code

Observable<MyItem> source = ...

source.flatMap(myItem -> myItem.process().subscribeOn(Schedulers.computation()))
.subscribe();

would run slower than this

final ExecutorService svc = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors() + 1);
Observable<MyItem> source = ...

source.flatMap(myItem -> myItem.process().subscribeOn(Schedulers.from(svc)))
.finallyDo(svc::shutdown)
.subscribe();

I compared these two approaches with a typical parallel process I do at work, and I got the following results.

EXECUTOR

START: 2016-01-25T09:47:04.350
END: 2016-01-25T09:48:37.181
TOTAL TIME (SEC): 92


COMPUTATION SCHEDULER

START: 2016-01-25T09:50:37.799
END: 2016-01-25T09:54:23.674
TOTAL TIME (SEC): 225

So my rough testing has shown the traditional ExecutorService is much faster than a Scheduler for computation.

Is there a reason for these results? Are RxJava schedulers just not optimized for parallelization? I've gotten the impression that computation schedulers use lesser threads than Executors.

Answer 1

I did several tests and discovered that creating your own ExecutorService can in fact increase parallelization performance. I wrote a blog post on it here.

Answer 2

When you use Schedulers.computation(), all the events are handled in the same thread. You can refer to the source code CachedThreadScheduler.java and NewThreadWorker.java. The benefit of this implementation is that if eventA is emitted after eventB, then eventA will be handled after eventB.

When you use Schedulers.from(), the events are handled in different threads.