Summary: Use the same default number of GC threads as used by ParNewGC and ConcMarkSweepGC (i.e., the 5/8th rule).
Reviewed-by: ysr, tonyp

Summary: Use correct allocation path in expand_and_allocate() so object's mark and p-bits are set as appropriate.
Reviewed-by: jmasa, pbk

Summary: Backed out part of cahnge for 6624765 because of nightly testing regressions.  Reviewers below were for 6624765.
Reviewed-by: ysr, apetrusenko

Summary: Take lock conditionally (in asynchronous mode only) when updating the dead-object map.
Reviewed-by: jmasa

Summary: In verification take into account partial coverage of a region by a card and expansion of the card table.
Reviewed-by: ysr, apetrusenko

Summary: Disable parallel reference processing when there are no worker threads
Reviewed-by: apetrusenko, pbk, jmasa, tonyp