Expensive flatMap() operation on streams originating from Stream.emits() - scala

Question

I just encountered an issue with degrading fs2 performance using a stream of strings to be written to a file via text.utf8encode. I tried to change my source to use chunked strings to increase performance, but the observation was performance degradation instead.
As far as I can see, it boils down to the following: Invoking flatMap on a stream that originates from Stream.emits() can be very expensive. Time usage seems to be exponential based on the size of the sequence passed to Stream.emits(). The code snippet below shows an example:
/*
Test done with scala 2.11.11 and fs2 version 0.10.0-M7.
*/
val rangeSize = 20000
val integers = (1 to rangeSize).toVector
// Note that the last flatMaps are just added to show extreme load for streamA.
val streamA = Stream.emits(integers).flatMap(Stream.emit(_))
val streamB = Stream.range(1, rangeSize + 1).flatMap(Stream.emit(_))
streamA.toVector // Uses approx. 25 seconds (!)
streamB.toVector // Uses approx. 15 milliseconds
Is this a bug, or should usage of Stream.emits() for large sequences be avoided?

Answer 1

TLDR: Allocations.
Longer answer:
Interesting question. I ran a JFR profile on both methods separately, and looked at the results. First thing which immediately attracted my eye was the amount of allocations.
Stream.emit:
Stream.range:
We can see that Stream.emit allocates a significant amount of Append instances, which are the concrete implementation of Catenable[A], which is the type used in Stream.emit to fold:
private[fs2] final case class Append[A](left: Catenable[A], right: Catenable[A]) extends Catenable[A]
This actually comes from the implementation of how Catenable[A] implemented foldLeft:
foldLeft(empty: Catenable[B])((acc, a) => acc :+ f(a))
Where :+ allocates a new Append object for each element. This means we're at least generating 20000 such Append objects.
There is also a hint in the documentation of Stream.range about how it produces a single chunk instead of dividing the stream further, which may be bad if this was a big range we're generating:
/**
* Lazily produce the range `[start, stopExclusive)`. If you want to produce
* the sequence in one chunk, instead of lazily, use
* `emits(start until stopExclusive)`.
*
* #example {{{
* scala> Stream.range(10, 20, 2).toList
* res0: List[Int] = List(10, 12, 14, 16, 18)
* }}}
*/
def range(start: Int, stopExclusive: Int, by: Int = 1): Stream[Pure,Int] =
unfold(start){i =>
if ((by > 0 && i < stopExclusive && start < stopExclusive) ||
(by < 0 && i > stopExclusive && start > stopExclusive))
Some((i, i + by))
else None
}
You can see that there is no additional wrapping here, only the integers that get emitted as part of the range. On the other hand, Stream.emits creates an Append object for every element in the sequence, where we have a left containing the tail of the stream, and right containing the current value we're at.
Is this a bug? I would say no, but I would definitely open this up as a performance issue to the fs2 library maintainers.