RFR: 8272773: Investigate making card table size configurable [v2]

[Thomas Schatzl]

On Fri, 8 Oct 2021 15:34:04 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> I'm trying to identify the usecases where we can see better performance with small card sizes. Right now, I have none. So, I'll set the minimum card size to 512 right now. Intention is to have a design such that if someone finds a usecase which gives better performance with smaller cards, only the minimum card size needs to be changed in the code, nothing else. Shall I keep this current behavior of overriding card_size based on region size till there's some feedback from others? or Do I need to change this?
>
> Let's keep the minimum card size to 512 for now, this can be investigated further separately. As you said, this can be changed easily.

Also please do not scale card table entry size with region size - there are some applications (many references, large remembered sets; e.g. the one in [JDK-8152438](https://bugs.openjdk.java.net/browse/JDK-8152438)) that with a 1024 card table size show 30% longer pause times. (Actually, they benefit a lot from smaller card table entry size, i.e. they get around the same pause time improvement with e.g. 256 byte sized card table entry size if heap region size permits).

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