SoftMaxHeapSize has no effect with Cassandra

[Kornel Pal]

Hi,

Thank you for implementing SoftMaxHeapSize, it greatly simplifies 
keeping the heap utilization low while also providing a buffer for 
allocation spikes.

I've tried it with Cassandra using the latest 
openjdk-shenandoah-jdk8-linux-x86_64-server-release.tar.xz nightly build 
and the default adaptive heuristics, but unfortunately 
ShenandoahSoftMaxHeapSize does not seem to have any effect on the GC. 
Setting ShenandoahMinFreeThreshold=50 on the other hand resulted in a 
behavior similar to what I would expect from setting 
ShenandoahSoftMaxHeapSize to half the size of the heap.

Cassandra is allocating short-lived objects at a high rate, that might 
be a different allocation pattern than what SoftMaxHeapSize was tested with.

I am not very familiar with the Shenandoah code, but I think that while 
should_start_gc() removes soft tail from the available heap, 
choose_collection_set_from_regiondata() is not removing soft tail from 
actual_free (or increasing free_target with the soft tail), that results 
in no GC being performed, even when heap utilization is above the soft 
limit.

Could you please check whether ShenandoahSoftMaxHeapSize in sh/8u is 
behaving as expected.

Thank you,
Kornel