RFR: 8357445: G1: Time-Based Heap Uncommit During Idle Periods [v7]

[Thomas Schatzl]

On Fri, 1 Aug 2025 05:53:48 GMT, Monica Beckwith <mbeckwit at openjdk.org> wrote:

>> **Implements:** https://bugs.openjdk.org/browse/JDK-8357445
>> 
>> Implement time-based heap uncommit for G1 during idle periods.
>> 
>> Key changes:
>> - Added G1HeapEvaluationTask for periodic heap evaluation
>> - Switch from G1ServiceTask to PeriodicTask for improved scheduling
>> - Implemented time-based heap sizing policy with configurable uncommit delay
>> - Added region activity tracking with last access timestamps
>> - Integrated VM_G1ShrinkHeap operation for safe heap shrinking
>> - Added new G1 flags: G1UseTimeBasedHeapSizing, G1TimeBasedEvaluationIntervalMillis, G1UncommitDelayMillis, G1MinRegionsToUncommit
>> - Added 'sizing' log tag for heap sizing operations
>> 
>> Comprehensive Test Coverage:
>> - Enhanced TestG1RegionUncommit: minimum heap boundaries, concurrent allocation/uncommit scenarios
>> - Enhanced TestTimeBasedHeapSizing: humongous object handling, rapid allocation cycles, edge cases
>> - Enhanced TestTimeBasedRegionTracking: concurrent region access, lifecycle transition validation
>> - Enhanced TestTimeBasedHeapConfig: parameter boundary values, small heap configurations
>> 
>> This ensures time-based heap uncommit works correctly while maintaining all safety guarantees and test expectations.
>
> Monica Beckwith has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove unused static _uncommit_delay member and accessor

Also, garbage collections and the `VM_G1ShrinkHeap` operation need to reset the timestamp for all (free) regions too. Otherwise the mechanism will try to uncommit the wrong amount of regions, similar to the issue I described in `VM_G1ShrinkHeap`.

E.g. Regions A, B, C are free for a long time, GC happens that keeps A, B, C for various reasons, now idle-evaluation happens, and A, B, C are determined idle in addition to the other regions if their timestamp is not reset. So the idle uncommit will free too many regions.

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PR Comment: https://git.openjdk.org/jdk/pull/26240#issuecomment-3244703382