RFR: Avoid divide by zero error, improve variable names

[William Kemper]

On Wed, 21 Dec 2022 23:59:40 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

>> Depending on when the periodic thread runs the accounting task and how much CPU time the process receives, we may see a very small elapsed process time. Such a time should not be used to compute MMU.
>
> src/hotspot/share/gc/shenandoah/shenandoahMmuTracker.cpp line 106:
> 
>> 104:   double elapsed_collector_time_s = collector_time_s - _collector_reference_time_s;
>> 105:   _collector_reference_time_s = collector_time_s;
>> 106:   double minimum_mutator_utilization = ((elapsed_process_time_s - elapsed_collector_time_s) / elapsed_process_time_s) * 100;
> 
> Alternative: You could check if result `isnan()` and not use it, although I don't know if divide by zero is costly.

I'd just as soon catch it earlier and I'm not 100% certain that other values won't also cause this error. Also, `isnan` won't flag the result of division by zero (`isinf` would though).

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PR: https://git.openjdk.org/shenandoah/pull/188