Spring team feedback on Leyden-premain optimizations

[Sebastien Deleuze]

Hi,

I would like to share the Spring team feedback on Leyden-premain
optimizations.

I have updated my spring-boot-leyden-demo repository [1] to leverage the
new Leyden 1 step workflow [2] that is much simpler to use than the
previous 5 steps workflow. It is great to see that we can reuse the same
`-Dspring.context.exit=onRefresh` property introduced by Spring Framework
6.1 for Class Data Sharing [3] with this new Class Data Store arrangement.

I am not sure if all optimizations are enabled yet, but the data points on
my MacBook with Spring Petclinic are already promising, and pretty close to
what we observed with the 5 steps workflow:
 - Executable JAR: 1.55s
 - Unpacked: 1.26s
 - Unpacked + Premain: 0.80s
 - Unpacked + Premain + Spring AOT: 0.56s

A few remarks:
 - We currently observe various warnings [4]
 - We have to use `-XX:CachedCodeMaxSize` in order to have cached code
generation with the "Unpacked + Premain" variant since its generates a
cached code of 12M (more than current 10M default limit)
 - We plan to do more benchmarks on cheap production servers (1 CPU 2G RAM
and 2 CPU 4G RAM)

I have for now only measured the startup time improvements, not yet the
warm up time improvements. I would be interested to know if the Leyden
premain branch already provides warm up improvements worth benchmarking
with Spring applications.

Best regards,
Sébastien Deleuze

[1] https://github.com/sdeleuze/spring-boot-leyden-demo
[2]
https://github.com/openjdk/leyden/tree/premain/test/hotspot/jtreg/premain/javac_new_workflow
[3]
https://spring.io/blog/2023/12/04/cds-with-spring-framework-6-1/#initial-cds-support-introduced-in-spring-framework-61
[4] https://gist.github.com/sdeleuze/8d442f9e85b069be0a5b24f3454d39a8

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