nd notices that there is a collection
in progress, it immediately performs a small amount of marking work
as described above.
<P>
This change was made both because we wanted to easily accommodate
single-threaded environments, and because a separate GC thread requires
very careful control over the scheduler to prevent the mutator from
out-running the collector, and hence provoking unneeded heap growth.
<P>
In incremental mode, the heap is always expanded when we encounter
insufficient space for an allocation