RFR: JDK-8273038: ChannelInputStream.transferTo() uses FileChannel.transferTo(FileChannel) [v8]

[Brian Burkhalter]

On Mon, 30 Aug 2021 18:03:09 GMT, Markus KARG <github.com+1701815+mkarg at openjdk.org> wrote:

>> As proposed in JDK-8265891, this PR provides an implementation for `Channels.newInputStream().transferTo()` which provide superior performance compared to the current implementation. This PR is a spin-off from https://github.com/openjdk/jdk/pull/4263 and https://github.com/openjdk/jdk/pull/5179 and deliberately concentrates **only** on the case where both, source **and** target, are actually `FileChannel`s. Other types of channels will be discussed in future PRs.
>> 
>> * 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 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. A rather similar approach in different use casse was recently implemented by https://github.com/openjdk/jdk/pull/5097 which was reported by even provide 5x performance (https://github.com/openjdk/jdk/pull/5097#issuecomment-897271997).
>
> Markus KARG has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Skipping bytes instead of reading them
>   
>   Signed-off-by: Markus Karg <markus at headcrashing.eu>

As I understand it, if a `j.u.Random` instance is constructed from a given seed, then a series of invocations on that instance will always produce the same sequence of outputs. `RandomFactory` prints the seed in the test output and allows a seed to be provided to the test via a `-D` option. So if the test fails with a given combination of initial positions and buffer size, then it should be straightforward to reproduce the failure by providing the seed used when the test failed.

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PR: https://git.openjdk.java.net/jdk/pull/5209