RFR: 8289822: G1: Make concurrent mark code owner of TAMSes [v2]

[Thomas Schatzl]

On Thu, 14 Mar 2024 10:16:45 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Thomas Schatzl has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains six commits:
>> 
>>  - Merge branch 'master' into 8289822-concurrent-mark-tams-owner
>>  - Do the mark information reset more fine grained (like before this change) to not potentially make HeapRegion::hr_clear() too slow
>>  - Fixes
>>  - some attempts to not have TAMSes always be updated for all regions
>>  - Remove _top_at_mark_start HeapRegion member
>>  - 8326781
>>    
>>    initial version
>>    
>>    initial draft
>>    
>>    some improvmeents
>>    
>>    Make things work
>>    
>>    forgotten fix
>
> src/hotspot/share/gc/g1/g1ConcurrentMark.hpp line 569:
> 
>> 567: 
>> 568:   inline HeapWord* top_at_mark_start(const HeapRegion* r) const;
>> 569:   inline HeapWord* top_at_mark_start(uint region) const;
> 
> Maybe call it `region_index` or sth alike?  (In the impl of these methods, having `region` being the index is a bit confusing, IMO.)

I can do that separately for all `region` indexes used in the file. Having both at the same time does not look good either.

> src/hotspot/share/memory/iterator.hpp line 44:
> 
>> 42: // The following classes are C++ `closures` for iterating over objects, roots and spaces
>> 43: 
>> 44: class Closure : public CHeapObj<mtGC> { };
> 
> Why is this required (in this PR)?

The changes need to allocate `G1CollectedHeap::_is_alive_closure_cm` when creating that argument for creating the concurrent reference processor. Previously the closure used `G1CollectedHeap` which is available when inlining it into the `G1CollectedHeap` instance, but now the closure needs `G1ConcurrentMark` which is not available at that time (i.e. null).
The alternative would have been introducing something like an `initialize` method to that closure which I did not like.

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PR Review Comment: https://git.openjdk.org/jdk/pull/18150#discussion_r1525101929
PR Review Comment: https://git.openjdk.org/jdk/pull/18150#discussion_r1525105894