RFR: 8314599: [GenShen] Couple adaptive tenuring and generation size budgeting [v4]

[Y. Srinivas Ramakrishna]

On Tue, 7 Oct 2025 22:32:56 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> Notable changes:
>> * Improvements to logging
>> * More accurate tracking of promotion failures
>> * Use shared allocation for promotions only when the size is above the maximum plab size (not the minimum size)
>> * Use census information gathered during mark to size promotion reserves and old generation
>> 
>> With these changes, GenShen is expected to have fewer promotion failures and this is indeed the case. As a result of this, we expect less time to be spent in concurrent marking and update refs for young collections. We may also expect shorter concurrent evacuation phases because GenShen will have fewer densely packed regions stuck in the young generation. With more objects being promoted, we also expect to see longer remembered set scan times. This is generally the case across all benchmarks, but we do also see some counter-intuitive results.
>> 
>> Here we are comparing 20 executions (10 on x86, 10 on aarch64) of the changes in the PR (experiment) against 20 executions of the same benchmarks results from tip. This is a summary of statistically significant changes of more than 5% across all benchmarks:
>> 
>> 
>> Concurrent Evacuation: 7 improvements, 3 regressions
>> • Best improvements: extremem-large-45g (-29.6%), neo4j-analytics (-26.9%)
>> • Worst regression: xalan (+53.7%)
>> 
>> Concurrent Marking: 15 improvements, 1 regression  
>> • Best improvements: hyperalloc_a2048_o4096 (-30.1%), crypto.rsa (-27.3%)
>> • Only regression: serial (+8.9%)
>> 
>> Concurrent Scan Remembered Set: 7 improvements, 2 regressions
>> • Best improvements: xalan (-49.4%), pmd (-49.0%), crypto.rsa (-41.8%)
>> • Worst regression: extremem-phased (+52.4%)
>> 
>> Concurrent Update Refs: 5 improvements, 4 regressions
>> • Best improvements: crypto.rsa (-36.4%), mnemonics (-28.4%)
>> • Worst regression: xalan (+89.4%)
>
> William Kemper has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Review feedback, bug fixes

> Here we are comparing 20 executions (10 on x86, 10 on aarch64) of the changes in the PR (experiment) against 20 executions of the same benchmarks results from tip.

@earthling-amzn : Can the comparisons be done in each of the two classes, i.e. x86 and aarch64, separately? Not sure if it'll give more statistically significant differences between test and control.

Was there any significant difference (on x86 or aarch64 separately) on SPECjbb?

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PR Comment: https://git.openjdk.org/jdk/pull/27632#issuecomment-3387799702