Integrated: 8342640: GenShen: Silently ignoring ShenandoahGCHeuristics considered poor user-experience

[Rui Li]

On Wed, 27 Aug 2025 18:26:22 GMT, Rui Li <duke at openjdk.org> wrote:

> 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)

This pull request has now been integrated.

Changeset: 8051aaf0
Author:    Rui Li <ruiamzn at amazon.com>
Committer: SendaoYan <syan at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/8051aaf0685f7bb23bf3e23d32ad45b0bffbce7b
Stats:     7 lines in 1 file changed: 7 ins; 0 del; 0 mod

8342640: GenShen: Silently ignoring ShenandoahGCHeuristics considered poor user-experience

Reviewed-by: ysr, wkemper

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