RFR: 8335932: GenShen: Fix old heuristic unit test

[Kelvin Nilsen]

On Mon, 8 Jul 2024 21:49:51 GMT, William Kemper <wkemper at openjdk.org> wrote:

> This isn't really a unit test and it must be run with Shenandoah enabled, but it exercises a hard to reach code path. The test has suffered some bit-rot* of late that needs to be addressed.
> 
> * We no longer set soft-max-capacity on the old generation.

Marked as reviewed by kdnilsen (Committer).

test/hotspot/gtest/gc/shenandoah/test_shenandoahOldHeuristic.cpp line 89:

> 87:     _heap->heap_region_iterate(&reset);
> 88:     _heap->old_generation()->set_capacity(ShenandoahHeapRegion::region_size_bytes() * 10);
> 89:     _heap->old_generation()->set_evacuation_reserve(ShenandoahHeapRegion::region_size_bytes() * 4);

I'll approve these changes because I assume you've tested and are satisfied with the results.

Might be good to leave some comments in this code that warn the user that failure of this test does not necessarily mean we've broken old heuristics.

What causes me some concern is that setting the capacity and reserves of old generation is very "dangerous".  There are "assumptions" scattered throughout the implementation related to reserves being sufficient to handle anticipated evacuation, and furthermore, any attempt to take control over capacity and reserves may be defeated by the GC's ergonomics...

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PR Review: https://git.openjdk.org/shenandoah/pull/458#pullrequestreview-2164517412
PR Review Comment: https://git.openjdk.org/shenandoah/pull/458#discussion_r1669383515