Integrated: 8361520: Stabilize SystemGC benchmarks

[Aleksey Shipilev]

On Tue, 8 Jul 2025 09:02:37 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

This pull request has now been integrated.

Changeset: a9bd1ad4
Author:    Aleksey Shipilev <shade at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/a9bd1ad40cb4e275d83b2e8b15e3c4be1551f7fc
Stats:     125 lines in 12 files changed: 125 ins; 0 del; 0 mod

8361520: Stabilize SystemGC benchmarks

Reviewed-by: tschatzl, ayang

-------------

PR: https://git.openjdk.org/jdk/pull/26182