RFR: 8339611: GenShen: Simplify ShenandoahOldHeuristics::trigger_collection_if_fragmented [v2]

[William Kemper]

On Fri, 6 Sep 2024 19:39:44 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> src/hotspot/share/gc/shenandoah/heuristics/shenandoahOldHeuristics.cpp line 570:
>> 
>>> 568:     if ((old_span_percent >= 0.50) && (old_density / old_span_percent_squared < 0.75)) {
>>> 569:       // We trigger old defragmentation, for example, if:
>>> 570:       //  old_span_percent is 100% and old_density is below 75.0%, or
>> 
>> Note that old_span_percent may be greater than 100%.  Can add comment:
>>   old_span_percent is 110% and old_density is below 90.8%, or
>>   
>> At higher existing utilization, there is less benefit to defragmenting, because there are diminishing returns for the larger efforts.  We're just copying nearly full regions around...
>
> But if we are at 110% old_span_percent, we have violated our intended Humongous Reserves, so I'm thinking we should try to squeeze that last 10% out of old so we don't have to STW on the next humongous allocation request...

I have a few broader questions:
* why is `allowed_old_gen_span` defined in terms of humongous reserve?
* why do we want to square `old_span_percent` and divide it into `old_density`? The comments suggest we ought to be comparing `old_density` directly?

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/492#discussion_r1747737832