RFR: 8334489: Add function os::used_memory [v3]

[David Holmes]

On Thu, 20 Jun 2024 17:23:40 GMT, Severin Gehwolf <sgehwolf at openjdk.org> wrote:

>> I think that is okay here. Sometimes its the lesser of two evils.
>> 
>> I actually understood David's remark that way, too (https://github.com/openjdk/jdk/pull/19772#pullrequestreview-2127842338)
>
> I understood the remark as "reduce the default implementation duplication". I.e. not have the same impl for all platforms. A pd_<foo> would just move the impl to `os/linux` rather than having it in `share/runtime`. But if that's fine, then I won't object, of course.

I am okay with the proposed code. If it were larger then I'd want it hidden behind a pd function but this is okay.

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PR Review Comment: https://git.openjdk.org/jdk/pull/19772#discussion_r1650390629