RFR: 8265891: (ch) InputStream returned by Channels.newInputStream should override transferTo [v3]

[Markus KARG]

On Fri, 2 Jul 2021 06:20:29 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 refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR.

Sorry for the delay. I now have pushed a first draft of the test. IIUC then five implementation have to be tested just for the actualy API, i. e. whether they throws NPE when `out` is `null`, and whether they transfer small, single turn, and multiple turn loads correctly. The tests are taken from `InputStream/TransferTo.java` as you proposed, and I am not using mocking but instead use `Channels.new*` as the existing tests you proposed as a blueprint. I hope I understood your requests and proposals correctly, if not please tell me. The tests pass well and proof that the new implementations work as expected and API-compliant.

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