be touched only just before it is used for allocation.
Hence any paging is essentially unavoidable.
<P>
Except in the case of pointer-free objects, we maintain the invariant
that any object in a small object free list is cleared (except possibly
for the link field).  Thus it becomes the burden of the small object
sweep routine to clear objects.  This has the advantage that we can
easily recover from accidentally marking a free list, though that could
also be handled by other means.  The collector currently 