RFR: 8361520: Stabilize SystemGC benchmarks [v2]

[Thomas Schatzl]

On Tue, 8 Jul 2025 10:13:17 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Noticed this while working on a related bug ([JDK-8359960](https://bugs.openjdk.org/browse/JDK-8359960)):
>> 
>> First, I see the benchmark executes a single shot per fork. As such, I believe the benchmark really tests the cost of initial GC, that probably drags a lot of (potentially non-benchmark-related) objects through new (possibly awkwardly wired, despite +AlwaysPreTouch) memory. The first iteration is 80 ms/op for me here, and the second one is -- whoosh -- only 3 ms/op! Second, the benchmark is really, really noisy. Part of it is due to first iteration being noisy, but also we want more samples to shrink the estimated errors.
>> 
>> Additional testing:
>>  - [x] Linux x86_64 server fastdebug, `gc.systemgc` benchmark runs
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Revert accidental comment removal

Marked as reviewed by tschatzl (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/26182#pullrequestreview-2997204680