RFR: 8361520: Stabilize SystemGC benchmarks [v2]

[Aleksey Shipilev]

On Tue, 8 Jul 2025 12:07:57 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Maybe also related to [JDK-8343047](https://bugs.openjdk.org/browse/JDK-8343047)?

Maybe. The first iteration of the benchmark is very noisy, as it IMO heavily depends on initial heap conditions at benchmark start. For a mark-compact, a small dead or alive object near the bottom of the heap may be a difference between a substantial heap move, or hardly any moves at all. Throw in multithreaded work balancing, and things get even more interesting. This is why there is `System.gc()` in `@Setup` now, so that we get into the same heap conditions before we go for a "real" benchmarked `System.gc()`.

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PR Comment: https://git.openjdk.org/jdk/pull/26182#issuecomment-3048988773