However, the parallel GC will be a problem if one or more of your cores is being used by other process(es) on the machine.  In that case, the GC synchronisation will stall and performance will go down the drain.  You can often see this on a ThreadScope profile as a big delay during GC while the other cores wait for the delayed core.  Make sure your machine is quiet and/or use one fewer cores than the total available.  It's not usually a good idea to use hyperthreaded cores either.