mark phase typically consumes the large majority of the
collection time.  Thus this largely parallelizes the garbage collector
itself, though not the allocation process.  Currently the marking is
performed by the thread that triggered the collection, together with
<I>N</i>-1 dedicated
threads, where <I>N</i> is the number of processors detected by the collector.
The dedicated threads are created once at initialization time.
<P>
A second effect of this flag is to switch to a more concurrent
implementation of