RFR: 8254739: G1: Optimize evacuation failure for regions with few failed objects [v19]

[Albert Mingkun Yang]

On Fri, 5 Nov 2021 10:34:48 GMT, Hamlin Li <mli at openjdk.org> wrote:

>> src/hotspot/share/gc/g1/g1EvacFailureObjectsSet.cpp line 131:
>> 
>>> 129: 
>>> 130:   _offsets.drop_all();
>>> 131: }
>> 
>> Having some destructive operations (`drop_all`) inside a method named `iterate` could come as a surprise, IMO. If I understand this correctly, the following would be problematic.
>> 
>> 
>> evac_failed_objects.iterate(closure1);
>> ...
>> evac_failed_objects.iterate(closure2);
>
> drop_all just returns buffers to free list, it will not destruct the buffers. So, iterate multiple times is OK, because next time it will get memory from free list or allocate a new buffer. Hope this answer your question.

Since `drop_all()` resets all counters (e.g. `_num_allocated_nodes`), the subsequent iteration will think the array is empty, won't it?

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PR: https://git.openjdk.java.net/jdk/pull/5181