RFR: 8367473: Shenandoah: Make the detailed evacuation metrics a runtime diagnostic option

[William Kemper]

On Thu, 11 Sep 2025 22:43:23 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Non-product builds currently have a feature to produce detailed metrics about evacuations (number of objects, number of bytes, split out by age (young, old, promotion) and thread (mutator or worker). This change makes this feature accessible in release builds as a diagnostic option.

I think we can ignore the outliers (the long "whisker" in the box plot represents p100 (max) time). The `master` branch likely has more benchmark runs than the feature branches so it may in turn be more likely to experience a long evacuation phase. The colored portion of the box plot represents P25 to P75 and aligns more with my expectations for the performance. I don't have an explanation for why enabling the diagnostic feature seems to reduce the severity of the outliers.

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PR Comment: https://git.openjdk.org/jdk/pull/27238#issuecomment-3292981876