RFR: 8272773: Investigate making card table size configurable [v3]

[Vishal Chand]

On Fri, 8 Oct 2021 20:16:45 GMT, Vishal Chand <duke at openjdk.java.net> wrote:

>> Hi,
>> 
>> Please review the changes to make CardTable entry size configurable. The changes primarily consists of:
>> 
>> 1. Addition of a cmdline flag **GCCardSizeInBytes** to make the card size startup time configurable.
>> 2. Setting the card size based on the flag in G1, Parallel and Serial GC memory initialization paths.
>> 3. Setting BlockOffsetTable size and ObjectStartArray size based on the card size.
>
> Vishal Chand has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Merge branch 'card-size-configurable' of https://github.com/vish-chan/jdk into card-size-configurable
>  - Changes as per the first set of comments

Are there any problems with waiting a bit for this change on PR#5909?
[Vishal] No issues. I think it would be a cleaner and better approach. 

Maybe you are also interested to try out the combination of PR#5909 and this one, allowing card table entry sizes from 128 to 1024 and any combination of region size iirc.
[Vishal] Yes, I'll try on my end different combinations of card sizes and region sizes. I'll report my observations.

In PR#5909 there is also the question of how large regions G1 should allow with that - the patch currently allows up to 512M regions, but theoretically there is no limit - do you have any opinion on that?
[Vishal] Currently, I have no strong opinion regarding the max. region size. I'll run specjbb2015 with different region sizes > 32m with different card sizes. I'll report if I find anything interesting.  

Thanks,
Vishal

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