RFR(X): Reduce region retirement during tlab allocation

[Zhengyu Gu]

Current implementation of unsafe_max_tlab_alloc() peeks current region 
size in free set and tries to use its remaining space. However, the 
current region is very likely just been consumed with very little or no 
memory left, then it returns region size as fallback. This can result 
the calculated tlab size too big, and causes too many regions to retire 
while they still have fair amount of free space.

When current region is too small for tlab, we can look ahead of next 
available region and use its size as hint. The approach seems to cut 
down retired region with usable space. As the result, we see less GC cycles.


Ran Derby test with 1g heap.

Before: 
http://cr.openjdk.java.net/~zgu/shenandoah/region_retirement/unpatched.stats.txt

After: 
http://cr.openjdk.java.net/~zgu/shenandoah/region_retirement/patched.stats.txt

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/region_retirement/webrev.00/index.html

Test:

   hotspot_gc_shenandoah.


Thanks,

-Zhengyu