RFR: Pauses that do not affect heap occupancy should not report heap

[Aleksey Shipilev]

On 10/11/2017 06:34 PM, Christine Flood wrote:
> I think it's confusing that phases that don't do any reclamation report heap sizes increasing.  
> Maybe we should only report the heap changes that go down at concurrent cleanup?
> 
> [19.889s][info][gc] GC(5) Concurrent partial GC 21740M->21828M(102400M) 10.035ms
> 
> To someone used to reading hotspot logs, that looks like an unsuccessful gc.

I don't think it is confusing for concurrent phases: during concurrent phase the heap occupancy
grows because either mutator or the GC itself allocates. "Concurrent marking" and "Concurrent
evacuation", for example, used to report increased heap occupancy since day 1, and it is useful,
because we know how much heap is left at the end of concurrent phases, thus eyeballing if there is
enough headroom.

We can consider changing that and reporting only heap occupancy improvements, but this particular
change is about the paused phases, not the concurrent phases -- do you have problem with those?

-Aleksey