During the review of JDK-8309403 [1], we found the Parallel GC doesn't decide if it should clear soft references according to gc overhead limit and other relevant metrics.

The ZGC has a comfortable implementation in `zDriver.cpp::should_clear_soft_references`. It is good to implement the related feature in other GCs.


Extract from the PR [1] of JDK-8309403:

```
I am not sure that using SoftRefGenPolicy is the cleanest way to solve the issue in Parallel. Can we check gc-overhead (and other relevant metrics) at gc-pause-start to decide soft-ref-policy, instead of relying on the global bool? (IOW, can GC make the more active decidion, in contrast to letting others tell GC what to do regarding to soft-refs?)

The approach in should_clear_soft_references of zDriver seems less intrusive to the global env.
```

[1] https://github.com/openjdk/jdk/pull/14296#discussion_r1221624327