ceph/ceph — pull request #1328 by liewegas (2014-02-28 15:22:53+00:00)
Title: On our test 'large mon' cluster (many pgs, osds) there was lots of CPU burn calculating this value. It was also observed on a DH cluster back on bobtail.

Cache the value in memory and be somewhat intelligent about when we invalidate
our cached notion on PGMap updates.