RFR(S): 6818524: G1: use ergonomic resizing of PLABs

[John Cuthbertson]

Hi Everyone,

Can I have a couple of volunteers review the changes for this CR? The 
webrev can be found at: 
http://cr.openjdk.java.net/~johnc/6818524/webrev.0/. These changes have 
been contributed by Brandon Mitchell (at Twitter).

Summary:
PLABStats instances are now employed in G1 to record information about 
old and young PLABs, and they are then used to adjust the PLAB sizes for 
the next GC. As a result the ResizePLAB flag is now being observed by 
G1. The previous behavior (and PLAB sizes) can be obtained by specifying 
-XX:-ResizePLAB.

With these changes I see just over a net 1% performance gain, on our 
reference workload, when running with G1.

Testing:
GC test suite with and without ResizePLAB and PrintPLAB enabled to 
observe the changes in PLAB sizes; reference workload on multiple 
platforms; jprt.

Thanks,

JohnC