RFR: JDK-8296796: Provide clean, platform-agnostic interface to C-heap trimming [v3]

[David Holmes]

On Sat, 19 Nov 2022 06:48:24 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

>> This is a breakout from [JDK-8293114](https://bugs.openjdk.org/browse/JDK-8293114), which is starved for reviews. So I attempt to break up that fix into smaller units which are hopefully easier to review separately.
>> 
>> We can trim the C-heap manually using jcmd since [JDK-8268893](https://bugs.openjdk.org/browse/JDK-8268893). This patch reshapes this code, cleaning it up in an OS-agnostic way. That will allow us to add implementions for other platforms (I have this on my list for AIX at least) and make review of 8293114 easier.
>
> Thomas Stuefe has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
> 
>  - Merge branch 'master' into JDK-8296796-factor-out-os-trim-native-heap
>  - Feedback David
>  - JDK-8296796-factor-out-os-trim-native-heap

We had a strange quirk in our CI testing after this was pushed and it made me think a little differently about the change. If `rss_change` is only needed for the test then we should perhaps make it (and the test) debug only? Also the `rss_change` logic seems not to be thread-safe, so only useful for that single-threaded test.

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PR: https://git.openjdk.org/jdk/pull/11089