ShenandoahGarbageThreshold drives the collection set selection. It defaults to 60%, which means we can end up with regions that have 60% garbage that would not be collected until the memory is tight. This means that Shenandoah accepts 60% heap fragmentation in normal conditions: so, for the live data of 20GB, it might require the heap of 33GB.

Instead, we are better off defragmenting the heap more proactively by lowering the ShenandoahGarbageThreshold.