RFR: 8199674: Improve G1 Full GC array marking

[Stefan Johansson]

Hi,

Please review this change to improve the Full GC marking code.

Links
JBS: https://bugs.openjdk.java.net/browse/JDK-8199674
Webrev: http://cr.openjdk.java.net/~sjohanss/8199674/00/

Summary
Currently when marking an object array it is split up to avoid one 
worker doing all the work. The code iterates over a chunk of the array 
and then push a task for the rest of the array to the marking stack. The 
problem with this is that if the iteration it self don't find any new 
pointers to follow, the next thing to do is to handle the next chunk in 
the array. This make work-stealing problematic and the performance far 
from optimal.

There are many schemes that can be used to improve the situation and we 
might want to dig into this more at some point. The fix I propose here 
is simple and easy but gives good improvements to the micro benchmarks 
I've tested it on. The fix is to push the task for the rest of the array 
onto the mark stack before doing the iteration of the current chunk.

Testing
Functional testing through mach5 and local performance testing. Created 
two small JMH benchmarks that create a big object array, one fills it 
with object the other one is completely empty.
Baseline:
Benchmark                  Mode  Cnt    Score    Error  Units
SystemGCOneBigEmpty.bench    ss   25   179.740 ±   8.497  ms/op
SystemGCOneBigFull.bench     ss   25  5220.095 ± 192.680  ms/op

With patch applied:
Benchmark                  Mode  Cnt    Score     Error  Units
SystemGCOneBigEmpty.bench    ss   25   111.854 ±  7.596  ms/op
SystemGCOneBigFull.bench     ss   25  1713.544 ± 36.956  ms/op

These benchmarks measure the whole time of the Full GC and not only the 
marking times but the improvement is still pretty clear.

Thanks,
Stefan