RFR (S): 8073204: Determining the desired PLAB size adjusts to the the number of threads at the wrong place

[Sangheon Kim]

Hi all,

Please review this change to determine the desired PLAB size for current 
gc worker threads.

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 first calculate the desired PLAB value for a single gc 
worker and then return desired PLAB size according to the current number 
of threads.

CR:
https://bugs.openjdk.java.net/browse/JDK-8073204

Webrev:
http://cr.openjdk.java.net/~sangheki/8073204/webrev.01

Test:
JPRT

Thanks,
Sangheon