RFR: 8272773: Investigate making card table size configurable

[Thomas Schatzl]

On Fri, 8 Oct 2021 09:59:24 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> **FLAG_SET_ERGO** is needed for G1 GC. In case of G1, min card size is determined by the region size. 
>> For the following cmdline arguments: -XX:G1HeapRegionSize=32M -XX:GCCardSizeInBytes=256 
>> ---- card_size is ergonomically set to 512 instead of 256, as min. card size for 32M region is 512.
>
> Let's leave this question out for now: I am not sure it makes sense to allow a card size of 256; however the common reaction when specifying incompatible command line options would be to bail out.
> The question is what to do if the user selects GCCardSizeInBytes=256 and G1 ergonomically decides to use 32m regions. Silently use 16m regions, which are very likely worse than 32m regions in that case?
> That is why we should first determine if card size of 256 makes sense somewhere and then decide (or somebody else has a particular opinion on this).

That same question will come up if we allow 64m regions later, but the step from 64->32m may not be that big. Another option would be to go away from the 1:1 mapping of cards in the remembered set and cards in the card table. This could probably be fixed, but is a completely different issue of course.

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PR: https://git.openjdk.java.net/jdk/pull/5838