RFR: 8253779: Amalloc may be wasting space by overaligning [v2]

[Coleen Phillimore]

On Fri, 9 Jul 2021 01:25:15 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Thanks to @kimbarrett for noticing this. The alignment was changed to 64 bits for 32 bit platforms, but overalign for 64 bits platforms.  I changed this to BytesPerLong to cover both, since the long case is why it was changed on 32 bits in the first place in JDK-4526490.
>> I also removed Amalloc_D since I don't know what D stands for and it's the same as Amalloc_4.  That's not a great name either. I'm open to suggestions!
>> Tested with tier1-3.
>
> Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Improvements ala Kim

I'll try to answer the main comments in one message.  I hope I haven't lost any.

1. I don't want to add a switch to keep the original alignment.  I think we should be confident that this change is correct or not make it!  It's the beginning of the release.  We have time to find any bugs that are unlikely to fall out.

2.  Funny that metaspace was originally inherited from Arena, as you know, but I'd rather see a simpler implementation for Arena if rewritten than using all of Metaspace.  Metaspace needs to deallocate from the middle and Arena doesn't.  Arena uses malloc which is better imo except for on one platform.  Rewriting Arena isn't something that's on the top of our list, please only file a bug if you think you're going to do it @tstuefe :)

3.  I'll file another RFE for renaming (ignore some comment above that I can't find now).

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PR: https://git.openjdk.java.net/jdk/pull/4732