RFR: 8324649: Shenandoah: refactor implementation of free set [v6]

[Kelvin Nilsen]

On Thu, 1 Feb 2024 02:55:49 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

>> src/hotspot/share/gc/shenandoah/shenandoahFreeSet.hpp line 184:
>> 
>>> 182:   HeapWord* allocate_single(ShenandoahAllocRequest& req, bool& in_new_region);
>>> 183: 
>>> 184:   // While holding the heap lock, allocate memory for a humongous object which will span multiple contiguous heap
>> 
>> `which will` or `which may`? (Is a humongous object allowed to span just a single region as well?)
>> 
>> Or are objects humongous only if they won't fit in a region? In which case the "will" is correct.
>> 
>> I was confused by tests that use `ShenandoahHumongousThreshold=50` , `=90`, etc.
>> 
>> May be in those cases, we go through the `allocate_single()` despite allocating an object (or block) bigger than `ShenandoahHeapRegion::humongous_threshold_words()` ? (That would make the pre-condition of the previous method suspect, though.)
>
> Same remark applies to the precondition comment below (which is correct, but could be made stronger to say `req.size() > ShenandoahHeapRegion::RegionSizeWords` or such?

Thanks for prodding with these questions.  My comment was not accurate.  I've endeavored to fix the comment.  A humongous object may span 1 or more regions.  The extra memory within the region that is not used to represent the humongous object is wasted.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/17561#discussion_r1482111007