RFR: 8305896: Alternative full GC forwarding [v25]

[Roman Kennke]

On Wed, 3 May 2023 21:29:20 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix type narrowing
>
> src/hotspot/share/gc/shared/gc_globals.hpp line 699:
> 
>> 697:                                                                             \
>> 698:   product(bool, UseAltGCForwarding, false, EXPERIMENTAL,                    \
>> 699:           "Use alternative GC forwarding that preserves object headers")    \
> 
> I would strongly prefer if this were not a product flag at this time, but a develop flag.
> 
> It potentially decreases performance of serial gc full gcs by a significant amount with no upside at all (not that worried about g1 or other concurrent gcs). Can you give me reasons why an end user would ever consciously enable this flag?
> 
> Using a develop flag is only a minor annoyance for development - we already do that for other features like evacuation failure injection in G1. For end users this would result in (guaranteed) zero performance impact. 
> 
> Only when adding compressed object headers with Lilliput this should be changed to a product flag.
> 
> I do not know your schedule for upstreaming Lilliput, but if it would miss JDK 21, people would suffer from this for the entire lifetime of JDK 21.... which is an LTS release. (Fwiw I would suggest the same for a non-LTS release, it seems to be worse in this situation though).

Ok that is reasonable, I will do that.

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PR Review Comment: https://git.openjdk.org/jdk/pull/13582#discussion_r1184571039