RFR: 8372543: Shenandoah: undercalculated the available size when soft max takes effect [v3]

[Rui Li]

On Thu, 4 Dec 2025 02:19:32 GMT, Rui Li <duke 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
>> ``` 
>> 
>> Tests:
>> - Ran the repro app `StableLiveSet.java` in https://bugs.openjdk.org/browse/JDK-8372543. Without fix, tip had ~2910 times gc in 20 sec with `-XX:+UseShenandoahGC -XX:ShenandoahGCHeuristics=adaptive -XX:SoftMaxHeapSize=512m -Xmx31g` jvm args. With the fix, only 18 times in 20 sec.
>> - GHA passed.
>> 
>> -------
>> 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;
>
> Rui Li has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove unused freeset includes

src/hotspot/share/gc/shenandoah/heuristics/shenandoahAdaptiveHeuristics.cpp line 255:

> 253:   size_t min_threshold = min_free_threshold();
> 254:   if (available < min_threshold) {
> 255:     log_trigger("Free (Soft) (%zu%s) is below minimum threshold (%zu%s)",

Forgot to use log format macros `PROPERFMT` & `PROPERFMTARGS` here. Will update.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28622#discussion_r2600564531