RFR: 8372543: Shenandoah: undercalculated the available size when soft max takes effect

[Rui Li]

On Wed, 3 Dec 2025 18:43:48 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> Detailed math and repro see https://bugs.openjdk.org/browse/JDK-8372543. 
>> 
>> Currently in shenandoah, when deciding whether to have gc, how we calculate available size is:
>> 
>> 
>> available = (Xmx * (100 - ShenandoahEvacReserve) / 100) - used
>> soft_tail = Xmx - soft_max
>> if (available - soft_tail < ShenandoahMinFreeThreshold * soft_max) // trigger gc 
>> 
>> 
>> The if condition `available - soft_tail` will be reduced to: `-(ShenandoahEvacReserve/100) * Xmx - used + soft_max`, which means when soft max is the same, the larger Xmx is, the less free size the app would have and the more gc it would have, which does not make sense, especially for the case where the app is mostly idle. This caused one of our internal customers experienced frequent gc with minimal workload, when soft max heap size was set way lower than Xmx.
>> 
>> 
>> Suggested fix: when deciding when to trigger gc, use logic similar to below:
>> 
>> mutator_soft_capacity = soft_max * (100 - ShenandoahEvacReserve) / 100;
>> available = mutator_soft_capacity - used;
>> if (available < mutator_soft_capacity) // trigger gc
>> ``` 
>> 
>> -------
>> This change also improved gc logging:
>> 
>> Before:
>> 
>> [6.831s][info][gc          ] Trigger: Free (52230K) is below minimum threshold (52428K)
>> [6.831s][info][gc,free     ] Free: 1587M, Max: 1024K regular, 1539M humongous, Frag: 2% 
>> external, 18% internal; Used: 352M, Mutator Free: 1940 Collector Reserve: 103M, Max: 1024K; Used: 0B
>> 
>> 
>> After:
>> 
>> [8.358s][info][gc          ] Trigger: Free (Soft mutator free) (51498K) is below minimum threshold (52428K)
>> [8.358s][info][gc,free     ] Whole heap stats: Total free: 1509M, Total used: 401M, Max free in a single region: 
>> 1024K, Max humongous: 1490M; Frag stats: External: 0%, Internal: 21%;  Mutator freeset stats: Partition count: 
>> 1911, Reserved: 1509M, Max free available in a single region: 1024K; Collector freeset stats: Partition count: 
>> 122, Reserved: 102M, Max free available in a single region: 1024K;
>
> src/hotspot/share/gc/shenandoah/heuristics/shenandoahAdaptiveHeuristics.cpp line 240:
> 
>> 238:   size_t allocated = _space_info->bytes_allocated_since_gc_start();
>> 239: 
>> 240:   log_debug(gc)("should_start_gc calculation: available: %zu%s, soft_max_capacity: %zu%s"
> 
> Can we add `ergo` tag to this message? Let's use the `PROPERFMT` and `PROPERFMTARGS` macros here and in other log messages we're changing.

Sure.

> src/hotspot/share/gc/shenandoah/heuristics/shenandoahCompactHeuristics.cpp line 52:
> 
>> 50:   size_t capacity = ShenandoahHeap::heap()->soft_max_capacity();
>> 51:   size_t available = _space_info->soft_available();
>> 52:   size_t allocated = _space_info->bytes_allocated_since_gc_start();
> 
> This shadows `bytes_allocated` below. Let's just use one variable for this.

Good catch. Removed one.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/28622#discussion_r2586856567
PR Review Comment: https://git.openjdk.org/jdk/pull/28622#discussion_r2586856298