RFR: 8325403: Add SystemGC JMH benchmarks

[Albert Mingkun Yang]

On Fri, 9 Feb 2024 09:36:35 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.

Since all of them are related to system-gc, does it make sense to put them inside a folder, `system_gc`, and a text file containing info on how to run them altogether?

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PR Comment: https://git.openjdk.org/jdk/pull/17784#issuecomment-1935794125