RFR: 8280915: Better parallelization for AbstractSpliterator and IteratorSpliterator when size is unknown [v5]

[Paul Sandoz]

On Thu, 10 Feb 2022 04:30:34 GMT, Tagir F. Valeev <tvaleev at openjdk.org> 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 using an artificial collector to ensure that at least two non-empty parts are created when parallelizing Stream.iterate source. More testing ideas are welcome.
>
> Tagir F. Valeev has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Update copyright year
>  - Cosmetic fixes

Getting back to this after much delay! Approving. But, i would like to try and document this design decision in comments, and maybe in implementation notes. We can do that as a follow on PR.

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Marked as reviewed by psandoz (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/7279