RFR (S): 8079555: REDO - Determining the desired PLAB size adjusts to the the number of threads at the wrong place

[sangheon.kim]

Hi all,

Please review this REDO change to determine the desired PLAB size for 
current gc worker threads.
The reason for back out of previous 
change(https://bugs.openjdk.java.net/browse/JDK-8073204) was it wouldn't 
guarantee to be '>= PLAB minimum size'. So added MAX2(min_size(),) to 
guarantee that.

Currently we calculate an optimal PLAB size with current number of gc 
workers.
When the number of workers changes dynamically 
(-XX:+UseDynamicNumberOfGCThreads), the desired PLAB size returned(by 
desired_plab_sz()) is still tuned to the number of gc workers that has 
been used previously.

This change is, firstly calculate the desired PLAB value for a single gc 
worker and then return desired PLAB size which is bigger(or equal) than 
PLAB min size according to the current number of threads.

CR:
https://bugs.openjdk.java.net/browse/JDK-8079555

Webrev:
http://cr.openjdk.java.net/~sangheki/8079555/webrev.00/

Test:
JPRT

Thanks,
Sangheon