RFR: 8358529: GenShen: Heuristics do not respond to changes in SoftMaxHeapSize

[William Kemper]

On Mon, 23 Jun 2025 21:42:36 GMT, Rui Li <duke at openjdk.org> wrote:

> Generational shenandoah currently doesn't pick up the changes of managed flag `SoftMaxHeapSize` when the app is running. This is because the value of `_soft_max_capacity` in `shenandoahGeneration` is never changed.
> 
> This change delegates the soft max heap size in `shenandoahGeneration` to `ShenandoahGenerationalHeap::heap()->soft_max_capacity()`, which does pick up the flag value changes.
> 
> Also, found `ShenandoahHeap:: initialize` uses `_num_regions * reg_size_bytes` rather than user input flag value. Updated to using actual flag value.

Some nits.

src/hotspot/share/gc/shenandoah/heuristics/shenandoahCompactHeuristics.cpp line 60:

> 58:   size_t min_threshold = capacity / 100 * ShenandoahMinFreeThreshold;
> 59: 
> 60:   log_debug(gc)("should_start_gc? available: %zu, soft_max_capacity: %zu", available, capacity);

Can we remove these, or at least make them `log_develop_debug`?

src/hotspot/share/gc/shenandoah/shenandoahGeneration.cpp line 1017:

> 1015: size_t ShenandoahGeneration::soft_max_capacity() const {
> 1016:   size_t capacity = ShenandoahGenerationalHeap::heap()->soft_max_capacity();
> 1017:   log_debug(gc)("soft_max_capacity: %zu", capacity);

Can we add a comment here saying that we need this message for the `TestDynamicSoftMaxHeapSize`? Also, this property will be read far more often than it will be written. Can we put the log message in the place where the value changes?

src/hotspot/share/gc/shenandoah/shenandoahGeneration.hpp line 129:

> 127:   virtual ShenandoahHeuristics* initialize_heuristics(ShenandoahMode* gc_mode);
> 128: 
> 129:   size_t soft_max_capacity() const override;

Can we take out the `_soft_max_capacity` member from `ShenandoahGeneration` now?

src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp line 101:

> 99: #include "utilities/globalDefinitions.hpp"
> 100: #include "utilities/powerOfTwo.hpp"
> 101: #include "logging/log.hpp"

Let's keep the includes alphabetized.

test/hotspot/jtreg/gc/shenandoah/TestDynamicSoftMaxHeapSize.java line 68:

> 66: 
> 67:             ProcessBuilder satbPb = ProcessTools.createLimitedTestJavaProcessBuilder(satbGCModeArgsNoDegeneratedGC);
> 68:             (new OutputAnalyzer(satbPb.start())).shouldHaveExitValue(0);

I don't think we need parentheses around `new OutputAnalyzer(...)`.

test/hotspot/jtreg/gc/shenandoah/TestDynamicSoftMaxHeapSize.java line 89:

> 87:         genShenGCModeArgs.add("-XX:ShenandoahGCMode=generational");
> 88: 
> 89:         for (String heuristic : HEURISTICS) {

The generational mode will quietly ignore other heuristics: https://bugs.openjdk.org/browse/JDK-8342640

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

Changes requested by wkemper (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/25943#pullrequestreview-2951652830
PR Review Comment: https://git.openjdk.org/jdk/pull/25943#discussion_r2162604159
PR Review Comment: https://git.openjdk.org/jdk/pull/25943#discussion_r2162605400
PR Review Comment: https://git.openjdk.org/jdk/pull/25943#discussion_r2162605932
PR Review Comment: https://git.openjdk.org/jdk/pull/25943#discussion_r2162606483
PR Review Comment: https://git.openjdk.org/jdk/pull/25943#discussion_r2162607534
PR Review Comment: https://git.openjdk.org/jdk/pull/25943#discussion_r2162612757