RFR: 8314599: [GenShen] Couple adaptive tenuring and generation size budgeting [v14]

[William Kemper]

On Tue, 6 Jan 2026 23:18:02 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 with a new target base due to a merge or a rebase. The pull request now contains 81 commits:
> 
>  - Merge remote-tracking branch 'jdk/master' into promotion-budget-improvements
>  - Merge remote-tracking branch 'jdk/master' into promotion-budget-improvements
>  - Fix comments, add back an assertion
>  - Merge remote-tracking branch 'jdk/master' into promotion-budget-improvements
>  - Accommodate behavior of global heuristic
>  - Restore missing update for inplace promotion padding
>  - Remove reference to adaptive tuning flag
>  - Remove commented out assertion
>  - Merge remote-tracking branch 'jdk/master' into promotion-budget-improvements
>  - Adaptive tenuring is no longer optional
>    
>    We are using age census data to compute promotion reserves. The tenuring threshold may still be fixed by setting the min/max threshold to the same value.
>  - ... and 71 more: https://git.openjdk.org/jdk/compare/7c979c14...f460f115

This PR will need considerable rework after recent changes in mainline. I'm closing this PR and will follow up with others.

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PR Comment: https://git.openjdk.org/jdk/pull/27632#issuecomment-3837259118