Since each Observable has a cache which can be traced back to the very first emitted value it seems an amount of memory used to store this cache is not bounded.
I've tested this assumption with the following code:
Observable.interval(1.microsecond).map(_ => System.currentTimeMillis)
.subscribe(x => ())
And indeed the memory usage has been steadily rising during the whole 10 minute period while an app was running.
My question is if it's possible to instantiate a special Observable without cache or maybe instruct it to cap it's cache at some level?
Only a specific set of Observables (ReplaySubject, replay(), GroupedObservable for example) tend to cache items, but not Observable.interval().
What you are likely experiencing here is the hundreds of thousands of boxed Long values. If you have a lot of RAM, GC might not kick in but just increase the heap size up to a maximum. Assuming you can really get a 1 microsecond timer, you have roughly 24 MB/s allocation rate or 1.4 GB/minute. If left alone for 10 minutes, you'd likely see a sawtooth like shape in memory usage.