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.

I cloned the `InputStream/TransferTo` test in a way which uses providers, but when started to implement the providers I noticed that a huge amount of *empty* source code is needed just to make the compiler happy: `FileChannel` already has dozens of abstract methods I have to override but they never will be used by any of the tests... Having that said, it would be good if I would be allowed to spare us thousands of useless codelines:
* May I just implement the `content()` test or do you insist on me really implementing *all* the tests found in `InputStream/TransferTo` for *each* of the overloaded `transferTo` implementations?
* May I use a mocking framework to actually just provide *partial* implementations of the channels, or do you insist on me actually overloading *all* methods of *all* channels in actual Java source code just for the sake of making the compiler happy?

While I understand the necessity of tests, and while I am convinced that the performance benefit outweighs the weeks of work this would imply just for adding all those test code, I actually like to propose that I only cover the `content()` tests plus using Mockito, so we would have 80% of the benefit for 20% of the coding costs.

WDYT?

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