G1 AHS JEP draft + Request for Feedback and Testing on G1 Heap Resizing Prototype

[Thomas Schatzl]

Hi all,

   just an update on AHS for G1 update:

- I started a draft of a JEP for this effort [0]. The scope of this 
effort for G1 is very different and reduced compared to the ZGC one: G1 
already has adaptive heap sizing, and while we are fixing minor issues 
around it, it is not in scope for that. Instead the change is be about 
determining the maximum heap size based on external memory availability, 
some tweaks of default min/max heap size, and G1GCIntensity.

The fixes for heap sizing are collected in the bug tracker using the 
gc-g1-heap-sizing label [1].

There are some (mostly minor) open questions still, that are all marked 
inline; the only major one is G1GCIntensity (the equivalent of 
ZGCIntensity), not completely sure whether we want/need that because all 
of that could be achieved the same using the (to be) managed 
GCTimeRatio/CPUUsagePercent flag.

- Ivan posted the PR about G1 better following GCTimeRatio for resizing 
yesterday [2].

_Now_ is the time to test this, so we encourage you to give it a try.

Thanks,
   Thomas

[0] https://bugs.openjdk.org/browse/JDK-8359211
[1] https://bugs.openjdk.org/issues/?jql=labels%20%3D%20gc-g1-heap-resizing
[2] https://github.com/openjdk/jdk/pull/25832