RFR: 8265891: (ch) InputStream returned by Channels.newInputStream should override transferTo [v2]

[Markus KARG]

On Thu, 1 Jul 2021 21:59:33 GMT, Markus KARG <github.com+1701815+mkarg at openjdk.org> wrote:

>> This PR-*draft* is **work in progress** and an invitation to discuss a possible solution for issue [JDK-8265891](https://bugs.openjdk.java.net/browse/JDK-8265891). It is *not yet* intended for a final review.
>> 
>> As proposed in JDK-8265891, this PR provides an implementation for `Channels.newInputStream().transferTo()` which provide superior performance compared to the current implementation. The changes are:
>> * Prevents transfers through the JVM heap as much as possibly by offloading to deeper levels via NIO, hence allowing the operating system to optimize the transfer.
>> * Using more JRE heap in the fallback case when no NIO is possible (still only KiBs, hence mostl ynegligible even on SBCs) to better perform on modern hardware / fast I/O devides.
>> 
>> Using JMH I have benchmarked both, the original implementation and this implementation, and (depending on the used hardware and use case) performance change was approx. doubled performance. So this PoC proofs that it makes sense to finalize this work and turn it into an actual OpenJDK contribution. 
>> 
>> I encourage everybody to discuss this draft:
>> * Are there valid arguments for *not* doing this change?
>> * Is there a *better* way to improve performance of `Channels.newInputStream().transferTo()`?
>> * How to go on from here: What is missing to get this ready for an actual review?
>
> Markus KARG has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Draft: Renaming i and separating code into several methods
>   
>   Signed-off-by: Markus Karg <markus at headcrashing.eu>

I have renamed `i` (and other ambiguous variables) and separated the implementation of `transferTo` into multiple methods. I will look into the test examples next to learn what I can reuse when writing tests for all those implementations. Thank you for all the kind input and the good ideas.

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PR: https://git.openjdk.java.net/jdk/pull/4263