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

[Y. Srinivas Ramakrishna]

On Fri, 12 Sep 2025 21:29:39 GMT, William Kemper <wkemper at openjdk.org> wrote:

> This chart shows the distribution (as a box plot) of all concurrent evacuation times across all benchmarks and platforms for the past 24 hours (approximately 15K data points in total). Each facet of the chart shows 3 configurations:
> 
> * top: The diagnostic option in this PR is enabled
> * middle: The diagnostic option is disabled
> * bottom: The baseline - this behavior does not exist in the product build.
> 
> The first facet shows the effect of this option on _global_ GCs in Genshen. The second facet shows the effect of this option on _global_ GCs in Shenandoah. The last option shows the effect on _young_ evacuation in GenShen. 

If I am reading the data right, it seems to me that the impact is most noticeable (percentage-wise) in Shenandoah. (The effect on GenShen seems minimal.) Is it understood why there should be a bigger impact on Shenandoah, or is the data telling us something else about why it looks the way it does?

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PR Comment: https://git.openjdk.org/jdk/pull/27238#issuecomment-3305110048