RFR: 8325673: GenShen: Share Reserves between Old and Young Collector [v2]

[Kelvin Nilsen]

On Mon, 15 Jul 2024 16:32:59 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> Kelvin Nilsen has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 70 commits:
>> 
>>  - Remove unreferenced local variable
>>  - Fix whitespace
>>  - Remove debug instrumentation and deprecated code
>>  - Turn off instrumentation
>>  - Verifier should only count non-trashed committed regions
>>  - Ignore generation soft capacities when adjusting generation sizes
>>    
>>    Soft capacities are established, for example, by setting NewRatio or New size on the
>>    JVM command line.  GenShen, for now at least, does not honor these settings.  Better
>>    performance is obtained by allowing GenShen to expand and shrink generation sizes
>>    according to application behavior.
>>    
>>    This commit also tidies up various aspects of the implementation to make adjustments
>>    to generation sizing more consistent:
>>    
>>    1. ShenandoahGlobalHeuristics::choose_global_collection_set(): share the reserves
>>       between young and old collection to maximize evacuation of garbage-first
>>       regions, regardless of whether most garbage is found in old or young
>>    2. ShenandoahConcurrentGC::entry_final_roots(): do not balance generations before
>>       invoking finish_rebuild() because finish_rebuild will balance generations.
>>    3. ShenandoahFreeSet::flip_to_old_gc(): invoke force_transfer_to_young() instead
>>       of transfer_to_young() so we can override soft-capacity limits
>>    4. ShenandoahFullGC::phase5_epilog(): Do not invoke compute_balances() or
>>       balance_generations_after_rebuilding_free_set().  Allow the free-set
>>       rebuild() implementation to do this work in a more consistent fashion.
>>    5. ShenandoahGeneration::adjust_evacuation_budgets(): replace transfer_to_youn()
>>       with force_transfer_to_young() to avoid enforcement of soft capacity limits.
>>    6. ShenandoahGenerationSizer::force_transfer_to_young(): new method
>>    7. ShenandoahGenerationalFullGC::balance_generations_after_gc(): establish
>>       reserves() so that free-set rebuild() can adjust balance.  Do not redundantly
>>       force transfer of regions here.
>>    8. ShenandoahGenerationalFullGC::balance_generations_after_rebuilding_free_set():
>>       deprecate this method.
>>    9. ShenandoahGenerationalFullGC::compute_balances(): deprecate this method.
>>    10. ShenandoahGenerationaStatsClosure::validate_usage() (part of Shenandoah
>>        Verification): add consistency check for generation capacities
>>  - Fix budget...
>
> src/hotspot/share/gc/shenandoah/heuristics/shenandoahOldHeuristics.cpp line 179:
> 
>> 177: }
>> 178: 
>> 179: void ShenandoahOldHeuristics::initialize_piggyback_evacs(ShenandoahCollectionSet* collection_set,
> 
> Can we continue using `mixed` instead of `piggyback` to describe collections that include young and old regions? I think `mixed` is an accepted term and `piggyback` feels a little bit too colloquial (also not sure if the phrase is commonly known outside of English).

Good suggestion.  Thanks.  I'm replacing piggyback with mixed throughout (where piggyback means mixed evac).  There are a few places where piggyback in Shenandoah where piggyback is used for other purposes and I'm leaving those as is.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/395#discussion_r1680001115