RFR: 8369111: G1: Determining concurrent start uses inconsistent predicates [v2]

[Ivan Walulya]

On Mon, 3 Nov 2025 07:31:37 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> Hi all,
>> 
>>   please review this change that fixes an inconsistency between requesting a concurrent start garbage collection during humongous object allocation and then actually starting it.
>> 
>> I.e. in `G1CollectedHeap::attempt_allocation_humongous` we check whether the allocation would cross the IHOP threshold taking the current allocation into account, and if so, see if G1 should start a concurrent marking, eventually starting a GC pause.
>> 
>> That GC pause did not take the prospective allocation into account, so we could do that GC for nothing (i.e. not start a concurrent marking although we already knew that the allocation would cause one).
>> 
>> This, in conjunction with JDK-8368959 can cause hundreds of extra GCs for the test in the CR (without eager reclaim of humongous arrays with references); otherwise it could cause the marking starting too late.
>> 
>> There is a second bug in the calculation whether G1 crossed the threshold: for humongous objects it only takes the actual size into account, not the size that is needed for allocating it. The same issue existed for determining to start a concurrent mark after any other collection too.
>> 
>> The change also tries to unify naming of the parameter to pass the allocation size (`alloc_word_size` -> `allocation_word_size`) and the parameter order where this size is passed along in multiple related methods.
>> 
>> Testing: mentioned test case now behaving correctly, tier1-5
>> 
>> Thanks,
>>   Thomas
>
> Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:
> 
>   * walulyai review

Marked as reviewed by iwalulya (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/27789#pullrequestreview-3410126117