RFR: Add generations to freeset [v8]

[Kelvin Nilsen]

On Thu, 20 Apr 2023 19:04:41 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> src/hotspot/share/gc/shenandoah/shenandoahFreeSet.cpp line 540:
>> 
>>> 538:     } else if (r->free() < PLAB::min_size() * HeapWordSize) {
>>> 539:       // Permanently retire this region if there's room for a fill object
>>> 540:       size_t waste = r->free();
>> 
>> Are we writing a filler object here to keep this region out of the old collector freeset when it's rebuilt?
>
> yes.  Putting the region back into the old_collector set at rebuild time just makes work for us, because then we'll have to rediscover that it needs to be retired, and will need to retire it, and will need to adjust the bounds again.

I'm adding a comment to the code to clarify.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/250#discussion_r1173022974