RFR: 8309191: Reduce JDK dependencies of cgroup support

[Aleksandar Pejovic]

On Thu, 1 Jun 2023 08:50:33 GMT, Severin Gehwolf <sgehwolf 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).
>
> src/java.base/linux/classes/jdk/internal/platform/CgroupInfo.java line 110:
> 
>> 108:      */
>> 109:     static CgroupInfo fromCgroupsLine(String line) {
>> 110:         String[] tokens = line.split("\t");
> 
> With this change, we now hard-code the expected delimiter and, thus, depend on what the kernel does. Do we have sufficient evidence this hasn't changed/won't change in the future?

As far as I can tell, the delimiter hasn't changed since the file was introduced, and judging by the kernel mailing list (e.g., see [the following](https://lore.kernel.org/all/Yr5JVHhSUCrbT8OH@mtj.duckdns.org/)), I don't think it will change any time soon.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14216#discussion_r1217759178