RFR: 8309191: Reduce JDK dependencies of cgroup support

[Severin Gehwolf]

On Tue, 30 May 2023 13:03:27 GMT, Aleksandar Pejovic <apejovic at openjdk.org> wrote:

> The current code for cgroup support in the JDK has large and expensive dependencies: it uses NIO, streams, and regular expressions. This leads to unnecessary class loading and slows down startup, especially when the code is executed early during an application startup. This is especially a problem for GraalVM, which executes this code during VM startup.
> 
> This PR reduces the dependencies:
> - NIO is replaced with regular `java.io` for file access.
> - Streams are replaced with loops (a side effect of this is that files are read in full whereas previously they could be read up to a certain point, e.g., until a match is found).
> - Regular expressions are replaced with manual tokenization (and for usages of `String.split`, the "regex" is changed to single characters for which `String.split` has a fast-path implementation that avoids the regular expression engine).

Not as far as I'm aware (it can deal with tabs vs. spaces differences as well as multiple spaces).

FWIW, I've done [this change](https://github.com/openjdk/jdk/compare/master...jerboaa:jdk:jdk-21+25-cgroups-change) as a PoC a while ago and it seems sufficient to use the JDK's metrics impl in native-image (barring some perf numbers; If somebody provides me with pointers, happy to provide those too).

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PR Comment: https://git.openjdk.org/jdk/pull/14216#issuecomment-1680514999