RFR (XS): 8205043: Make parallel reference processing default for G1

[Kim Barrett]

> On Jun 14, 2018, at 8:08 AM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> 
> Hi all,
> 
>  can I have reviews for this split-off of JDK-8043575 to make parallel
> reference processing in conjunction with dynamic number of thread
> sizing default for G1?
> 
> We think that with recent changes to parallel reference processing it
> is useful to do so, further nowadays we expect that most VMs are run
> with more than one GC thread. So reference processing should benefit
> from that as well by default.
> 
> Threads are by default automatically limited by the functionality
> introduced with JDK-8043575 to avoid actually being slower than before
> if using too many threads.
> 
> This is also the reason why we only suggest to make
> ParallelRefProcEnabled default for G1: the thread sizing does not work
> with other collectors. 
> 
> There is also a linked CSR for that change.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8205043
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8205043/webrev/
> Testing:
> new included test case
> 
> Thanks,
>  Thomas

Looks good.