RFR: JDK-8312018: Improve reservation of class space and CDS [v2]

[Thomas Stuefe]

On Fri, 11 Aug 2023 04:34:43 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> Thomas Stuefe has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - factor out shuffle and hemi-split
>>  - feedback Ioi
>
> Some preliminary comments. 
> 
> - I am not sure what "CDS" means in the PR title. It seems like CDS doesn't (yet) benefit from the improvements.
> - I would suggest breaking this PR into smaller steps (see my comments inside `Metaspace::reserve_address_space_for_compressed_classes`)

Hi @iklam, I addressed some of your concerns, please take another look. Thank you!

> src/hotspot/os/linux/os_linux.cpp line 4229:
> 
>> 4227:   // good protection against NULL references while still leaving enough of the lower
>> 4228:   // 4G addressable.
>> 4229:   return (char*)(MAX2(os::vm_allocation_granularity(), 16 * M));
> 
> The code doesn't do what the comment says.

I fixed the comment and made it hopefully much clearer

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PR Comment: https://git.openjdk.org/jdk/pull/15041#issuecomment-1675064685
PR Review Comment: https://git.openjdk.org/jdk/pull/15041#discussion_r1291567824