RFR: 8165443: Free Collection Set serial phase takes very long on large heaps

[Stefan Johansson]

Hi,

Please review this fix to improve freeing the collection set when having 
a lot of regions.

Issue: https://bugs.openjdk.java.net/browse/JDK-8165443
Webrev: http://cr.openjdk.java.net/~sjohanss/8165443/00/

Summary
For heaps with many regions freeing the collections set takes a 
considerable amount of time and the big bottleneck is freeing regions 
and creating the free list. The approach to fix the problem is to split 
the collection set freeing and the free list rebuild into two different 
phases. This makes it easier to parallelize the free collection set 
phase and to work around sorting regions into the free list we now 
rebuild the free list from scratch, handing chunks of the heap to 
different workers and then appending them together.

This changes removes the simple heuristic around how many workers to use 
for the collection set freeing and instead just caps in the number of 
workers to regions in the collection set. I did this because I saw not 
improvement using a work chunk size.

I also added a new event and logging for the collection set freeing that 
captures the whole parallel time for each worker. I left the old events 
and logging for how much time has been spent working on young vs old 
regions, but I'm not sure how useful this information is in the logs. I 
get that it is needed for predictions, but maybe we can skip the logs 
and events? Current version keeps them but I'm very willing to remove 
them if others agree.

Testing
Manual performance testing on various benchmarks show good results for 
heap with a lot of regions and no regression for heap with a few 
regions. Functional tests using mach5 tier 1-5.

Thanks,
Stefan