lling <TT>GC_enable_incremental()</tt>.
This does not result in fully incremental collection, since parallel mark
phases cannot currently be interrupted, and doing so may be too
expensive.
<P>
Gcj-style mark descriptors do not currently mix with the combination
of local allocation and incremental collection.  They should work correctly
with one or the other, but not both.
<P>
The number of marker threads is set on startup to the number of
available processors (or to the value of the <TT>GC_NPROCS</tt>
envir