Qi Wang eeefdf3ce8 Fix # of unpurged pages in decay algorithm.
When # of dirty pages move below npages_limit (e.g. they are reused), we should
not lower number of unpurged pages because that would cause the reused pages to
be double counted in the backlog (as a result, decay happen slower than it
should).  Instead, set number of unpurged to the greater of current npages and
npages_limit.

Added an assertion: the ceiling # of pages should be greater than npages_limit.
2017-05-23 13:48:30 -07:00
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