RFR: 8342640: GenShen: Silently ignoring ShenandoahGCHeuristics considered poor user-experience [v2]

[Rui Li]

> When generational shenandoah is enabled, it ignores the value of ShenandoahGCHeuristics input silently:
> 
> java -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:ShenandoahGCMode=generational -XX:ShenandoahGCHeuristics=junk -version
> openjdk version "25" 2025-09-16 LTS
> OpenJDK Runtime Environment Corretto-25.0.0.36.1 (build 25+36-LTS)
> OpenJDK 64-Bit Server VM Corretto-25.0.0.36.1 (build 25+36-LTS, mixed mode, sharing)
> 
> 
> Adding additional guard rail to gen shen is not an option - it is actually by design that gen shen does not support non adaptive heuristics due to the complexity brought by generational design. 
> 
> So print out a warning to users to indicate that their `ShenandoahGCHeuristics` input is ignored.
> 
> 
> x64 (8342640) % java -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:ShenandoahGCMode=generational -XX:ShenandoahGCHeuristics=static -version
> [0.001s][warning][gc] Ignoring -XX:ShenandoahGCHeuristics input: static, because generational shenandoah only supports adaptive heuristics
> openjdk version "26" 2026-03-17
> OpenJDK Runtime Environment Corretto-26.0.2.1.1 (slowdebug build 26+2-FR)
> OpenJDK 64-Bit Server VM Corretto-26.0.2.1.1 (slowdebug build 26+2-FR, mixed mode)
> 
> x64 (8342640) % java -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:ShenandoahGCMode=generational -XX:ShenandoahGCHeuristics=junk -version
> [0.001s][warning][gc] Ignoring -XX:ShenandoahGCHeuristics input: junk, because generational shenandoah only supports adaptive heuristics
> openjdk version "26" 2026-03-17
> OpenJDK Runtime Environment Corretto-26.0.2.1.1 (slowdebug build 26+2-FR)
> OpenJDK 64-Bit Server VM Corretto-26.0.2.1.1 (slowdebug build 26+2-FR, mixed mode)
> 
> x64 (8342640) % java -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:ShenandoahGCMode=generational -XX:ShenandoahGCHeuristics=adaptive -version
> openjdk version "26" 2026-03-17
> OpenJDK Runtime Environment Corretto-26.0.2.1.1 (slowdebug build 26+2-FR)
> OpenJDK 64-Bit Server VM Corretto-26.0.2.1.1 (slowdebug build 26+2-FR, mixed mode)
> 
> x64 (8342640) % java -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -XX:ShenandoahGCMode=generational -version  
> openjdk version "26" 2026-03-17
> OpenJDK Runtime Environment Corretto-26.0.2.1.1 (slowdebug build 26+2-FR)
> OpenJDK 64-Bit Server VM Corretto-26.0.2.1.1 (slowdebug build 26+2-FR, mixed mode)

Rui Li has updated the pull request incrementally with one additional commit since the last revision:

  Update

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/26968/files
  - new: https://git.openjdk.org/jdk/pull/26968/files/c999d47f..247dce42

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26968&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26968&range=00-01

  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26968.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26968/head:pull/26968

PR: https://git.openjdk.org/jdk/pull/26968