RFR: 8265891: (ch) InputStream returned by Channels.newInputStream should override transferTo [v6]

[Brian Burkhalter]

On Mon, 26 Jul 2021 20:45:14 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.

src/java.base/share/classes/sun/nio/ch/ChannelInputStream.java line 179:

> 177:             for (long n = srcSize - srcPos; bytesWritten < n;)
> 178:                 bytesWritten += src.transferTo(srcPos + bytesWritten, Long.MAX_VALUE, dest);
> 179:             return bytesWritten;

If `src` is extended at an inconvenient point in time, for example between the return from a call to `src.transferTo()` that makes `bytesWritten < n` false and the evaluation of that terminating condition, then it appears that not all the content of `src` will be transferred to `dest`. I am not however sure that this violates any contract but it could be a behavioral change from the existing implementation.

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