RFR: 8325403: Add SystemGC JMH benchmarks [v3]

[Stefan Johansson]

On Tue, 13 Feb 2024 14:12:06 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

>> Please review this change to add some System GC JMH benchmarks.
>> 
>> **Summary**
>> When developing and improving the Full GC implementation for the different GCs, using `System.gc()` to test performance is sometime very useful. This change adds a set of System GC benchmarks using JMH. The tests all have the same structure, a `@Setup` function that generates heap contents and a `@Benchmark` function which only calls `System.gc()`. The different scenarios covers some basic use-cases but more can certainly be added going forward. 
>> 
>> Since the benchmark function is just a single call to trigger the GC it makes sense to use the single shot JMH mode and fork a new JVM for each run. The tests also specify default heap options to avoid triggering GC during object creation to give all runs more or less the same heap layout.
>> 
>> **Testing**
>> Local and aurora testing verifying that results are fairly stable.
>
> Stefan Johansson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add AlwaysPreTouch to jvmArgsAppend

Thanks @turbanoff, will include fix these before integrating. After some more offline discussions I will also update the package name to `systemgc` instead of `system_gc`.

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PR Comment: https://git.openjdk.org/jdk/pull/17784#issuecomment-1943422425