RFR: JDK-8312018: Improve reservation of class space and CDS

[Thomas Stuefe]

On Fri, 11 Aug 2023 04:30:59 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> Also, for removing the old AARCH64/PPC code, perhaps you should do this in a prerequisite PR (taking just this "if" block, plus os::attempt_reserve_memory_between). That way we can make sure there's no regression for AARCH64/PPC, before implementing this full PR.

I would rather avoid that, since it could mean unnecessary churn and an unclearer PR goal.

Replacing the AARCH/PPC code would need both this and the preceding if-block, since PPC piggybacked on the AARCH block for its ability to reserve below 32 GB. 

So that hypothetical PR would need: 
- the full new API, including os::vm_min_address, 
- all gtests, -
-  90% of Metaspace::reserve_address_space_for_compressed_classes

This accounts for the largest part of this patch, there is not much more I could leave out. 

I have tested the patch well, which is currently going through CI at SAP. I am confident in its working as a whole, but I would have to rethink it if we only take parts over. Note that I already left out a lot of changes I originally planned; this is the very much cut-down version.

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PR Review Comment: https://git.openjdk.org/jdk/pull/15041#discussion_r1290908833