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

[Rui Li]

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;

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Commit messages:
 - 8372543: Shenandoah: undercalculated the available size when soft max takes effect

Changes: https://git.openjdk.org/jdk/pull/28622/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28622&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372543
  Stats: 226 lines in 7 files changed: 157 ins; 44 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/28622.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28622/head:pull/28622

PR: https://git.openjdk.org/jdk/pull/28622