RFR: Add generations to freeset [v5]

[Kelvin Nilsen]

On Thu, 13 Apr 2023 16:28:49 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

>> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Remove debug instrumentation
>
> src/hotspot/share/gc/shenandoah/shenandoahFreeSet.cpp line 994:
> 
>> 992:         size_t ac = alloc_capacity(r);
>> 993:         _capacity -= ac;
>> 994:         _old_capacity += ac;
> 
> This piecemeal adjustment of capacities following change of set membership indicates that the set abstraction should be pushed down to `old`, `mutator` and `young` (?) subsets, which then carry both membership & capacity that is adjusted at the point of membership changes.

There are some inconsistencies, I agree.  We do not keep track of capacity for the collector_set, just the old_collector_set and the mutator_set.  This is historical also.  I'm inclined to leave as is for now.  Maybe we can address this when/if we refactor to improve the "set" abstraction.

For now, I'll place a comment in the source code.

> src/hotspot/share/gc/shenandoah/shenandoahFreeSet.hpp line 147:
> 
>> 145:   void print_on(outputStream* out) const;
>> 146: 
>> 147:   void reserve_regions(size_t young_reserve, size_t old_reserve);
> 
> Documentation comment, preferably here, but if not then in the implementation as a block header.

Added documentation.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/250#discussion_r1171398698
PR Review Comment: https://git.openjdk.org/shenandoah/pull/250#discussion_r1171398913