RFR: 8280915: Better parallelization for AbstractSpliterator and IteratorSpliterator when size is unknown

[Brian Goetz]

Are you proposing dropping SIZED from the spliterator for arrays?  This 
would undermine all the array-based optimizations (e.g., toArray), which 
seems a bad trade.  I realize the splitting heuristics are frustrating 
for a number of use cases, but this seems like throwing the baby out 
with the bathwater.

(Loom is coming, and that is a good time to revisit streams support for 
blocking operations, which is a big part of what people complain about 
with parallel streams.)

On 1/29/2022 11:38 AM, Tagir F.Valeev wrote:
> See the bug description for details.
>
> I propose a simple solution. Let's allow ArraySpliterator to be non-SIZED and report artificial estimatedSize(), much bigger than the real one. This will allow AbstractSpliterator and IteratorSpliterator to produce prefix whose size is comparable to Long.MAX_VALUE (say, starting with Long.MAX_VALUE/2), and this will enable further splitting of the prefix. This change will drastically improve parallel streaming for affected streams of size <= 1024 and significantly improve for streams of size 1025..20000. The cost is higher-grained splitting for huge streams of unknown size. This might add a minor overhead for such scenarios which, I believe, is completely tolerable.
>
> No public API changes are necessary, sequential processing should not be affected, except an extra field in ArraySpliterator which increases a footprint by 8 bytes.
>
> I added a simple test to ensure that at least two threads are actually used when parallelizing Stream.iterate source. More testing ideas are welcome.
>
> -------------
>
> Commit messages:
>   - JDK-8280915 Better parallelization for AbstractSpliterator and IteratorSpliterator when size is unknown
>
> Changes:https://git.openjdk.java.net/jdk/pull/7279/files
>   Webrev:https://webrevs.openjdk.java.net/?repo=jdk&pr=7279&range=00
>    Issue:https://bugs.openjdk.java.net/browse/JDK-8280915
>    Stats: 128 lines in 2 files changed: 96 ins; 0 del; 32 mod
>    Patch:https://git.openjdk.java.net/jdk/pull/7279.diff
>    Fetch: git fetchhttps://git.openjdk.java.net/jdk  pull/7279/head:pull/7279
>
> PR:https://git.openjdk.java.net/jdk/pull/7279