In workloads with almost zero dirty cache usage we can occasionally get into a state where eviction doesn't evict much or anything for a while despite the cache fill ration being >90%. It appears this is related to heuristics around revisiting pages for eviction, as wiredtiger will visit a page for eviction and then skip it up to 5 times following on from that. Additionally the target page ratio isn't high as there isn't dirty content in the cache which means we give up eviction walks quickly.