RFR: 8348278: Trim InitialRAMPercentage to improve startup in default modes [v4]

[Stefan Johansson]

On Tue, 2 Sep 2025 12:57:47 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> See bug for discussion. This is the code change, which is simple. What is not simple is deciding what the new value should be. The change would probably require CSR, which I can file after we agree on the value.
>> 
>> I think cutting to 0.2% of RAM size gets us into good sweet spot:
>>  - On huge 1024G machine, this yields 2G initial heap
>>  - On reasonably sized 128G machine, this gives 256M initial heap
>>  - On smaller 1G container, this gives 2M initial heap
>> 
>> Additional testing:
>>  - [x] Linux AArch64 server fastdebug, `all`
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:
> 
>  - Also man page
>  - Fix

> Anyway, changing the _way_ Xms is selected feels way too much work, given the benefits we are after.

I agree, and if we want to do a re-haul of the sizing code we should probably do it after the AHS work. Hopefully that can simplify some things. 

We've discussed this a bit more internally and while there are some hesitations on some points we do see some benefits with the changed default as well. So will approve it.

Thanks for the discussion.

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Marked as reviewed by sjohanss (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/23262#pullrequestreview-3244258740