If you groupBy with an upper bound of substreams at Int.MaxValue you can at
some point have more than 256 substreams, which is what you limit
mergeSubstreamsWithParallelism with, this means that it will backpressure
indefinitely (as none of the substreams will complete until upstream
completes and
I'm trying to figure out why this is hanging/idling indefinitely :
Source.fromIterator(() => Iterator.from(0).take(500).map(_ -> 1))
.groupBy(Int.MaxValue, _._1)
.mergeSubstreamsWithParallelism(256)
.runWith(Sink.seq)
This is the only way how to avoid instantiating ridiculous amounts of
