RFR: 8351405: G1: Collection set early pruning causes suboptimal region selection [v2]

[Thomas Schatzl]

On Wed, 19 Mar 2025 08:32:52 GMT, Ivan Walulya <iwalulya at openjdk.org> wrote:

> > Looks good to me, thanks for removing the double-pruning from the initial protoype!
> > Did you ever try to get statistics about differences in marking length and changes to the cache hits in the mark stats cache? (Just curious)
> 
> For the Small Heap Sizes (24-26G) used for testing, it is a slight drop of 100 to 99.98** hit rate in a few marking cycles. Will try with larger heaps

I would actually expect a larger decrease with smaller heaps (or region sizes) - with larger heaps regions are larger, so the amount of cross-references/jumping around in the object graph potentially smaller, and so the amount of cache evictions.

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PR Comment: https://git.openjdk.org/jdk/pull/24076#issuecomment-2736113234