RFR: 8317371: GenShen: Needs improved support for humongous allocation [v6]

[William Kemper]

On Fri, 13 Oct 2023 17:25:02 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> Several improvements:
>> 1. If there is a humongous allocation failure, go immediately to Full GC rather than trying 64 degenerated GC first
>> 2. Reduce likelihood of humongous allocation errors by:
>>     a. Allocating regular objects from top end of memory while allocating humongous from bottom end of memory
>>     b. (Especially) in the case that regular regions that are promoted in place reside within the bottom end of memory, detect that old-gen memory has become fragmented and trigger old-gen GC, which will make a special effort to move these out-of-place old-gen regions to higher addresses.
>
> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Restore _humongous_alloc_failure_gc flag removed in merge conflict

> If there is a humongous allocation failure, go immediately to Full GC rather than trying 64 degenerated GC first

Can we set `ShenandoahFullGCThreshold` back to the upstream default of 3 now? It should have been done when we merged https://github.com/openjdk/jdk/pull/15500 .

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PR Comment: https://git.openjdk.org/shenandoah/pull/336#issuecomment-1762055716