RFR: JDK-8079556: BACKOUT - Determining the desired PLAB size adjusts to the the number of threads at the wrong place

[Jesper Wilhelmsson]

Hi,

Please review this backout of JDK-8073204 - Determining the desired PLAB size 
adjusts to the the number of threads at the wrong place

After the change the size returned by PLABStats::desired_plab_sz() can get below 
the minimum PLAB size due to truncating integer division in that function, and 
actually missing alignment. This results in several places getting the wrong 
results because of this broken invariant.

The fix will take more than a day and this change is now blocking a push of 
hs-gc to main.

Thomas prepared this backout, I have reviewed it (looks good), so we need 
another Reviewer to push it.

Backout bug: https://bugs.openjdk.java.net/browse/JDK-8079556
Webrev: http://cr.openjdk.java.net/~jwilhelm/8073204/webrev_backout.00/
Original change: http://hg.openjdk.java.net/jdk9/hs-gc/hotspot/rev/d57d15fb074b

The backout applies cleanly to latest hs-gc.

Thanks,
/Jesper