Prior to r254304, we only began scanning the active page queue when the

amount of free memory was close to the point at which we would begin
reclaiming pages.  Now, we continuously scan the active page queue,
regardless of the amount of free memory.  Consequently, we are continuously
calling pmap_ts_referenced() on active pages.

Prior to this change, pmap_ts_referenced() would always demote superpage
mappings in order to obtain finer-grained reference information.  This made
sense because we were coming under memory pressure and would soon have to
begin reclaiming pages.  Now, however, with continuous scanning of the
active page queue, these demotions are taking a toll on performance.  To
address this problem, I have replaced the demotion with a heuristic for
periodically clearing the reference flag on superpage mappings.

Approved by:	re (kib)
Sponsored by:	EMC / Isilon Storage Division

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