RFR: 8265891: ChannelInputStream.transferTo() uses FileChannel.transferTo()

[Markus KARG]

On Thu, 19 Aug 2021 06:16:42 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 deliberately concentrates **only** on `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).

@AlanBateman @bplb This PR is a spin-off for FileChannels only, as suggested by you. I kindly request your review. Thanks.

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PR: https://git.openjdk.java.net/jdk/pull/5179