RFR: Usage tracking cleanup

[William Kemper]

On Tue, 18 Apr 2023 00:03:34 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

>> src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp line 727:
>> 
>>> 725: // * The bottom of a PLAB must be aligned on card size. In some cases this will
>>> 726: //   require padding in front of the PLAB (a filler object). Because this padding
>>> 727: //   is included in the region's used memory we include the padding in the accounting.
>> 
>> ... in the accounting as waste.
>
> So, for humongous objects waste isn't included in used, but for non-humongous objects waste is included in used?

We don't really get waste in the strictest sense for non-humongous objects. However, the _bottom_ of PLABs need to be aligned to card size (and this alignment itself needs to be large enough to hold a "filler" object). When this happens, this filler object is included in the "used" memory for the region (and so, is included in the "used" memory for the old generation). I will clarify this comment.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/260#discussion_r1170643232