Low query rate with heavy cache pressure and an idle collection

• single-node replicat set, 3 GB cache, 20 GB oplog
• insert 10 M x 1 kB documents (10 GB total size, plus index)
• then 100 threads querying documents at random, observe low query rate
• then restart mongod, same queries are now much faster

• A-B: collection is being created
• B-C: random queries
  • query rate is very low, ~6 k/s
  • rate of evicting from and reading into cache is ~9 k pages/s, ~1.5 pages per query, so very high miss ratio
  • rate of pages walked for eviction is very high, ~21 M/s, so about 2300 pages walked for every page evicted, or 2-3% of pages in cache walked for every page evicted
  • no pages are being evicted from oplog, but it is uncertain whether that is because all pages have already been evicted
• C-D: after restart
  • query rate is much higher, ~34 k/s
  • rate of evicting from and reading into cache is 26 k pages/s, ~0.75 pages per query, so lower miss ratio than before restart
  • rate of pages walked for eviction is much lower

The issue does not reproduce on a standalone node.

Possibly related to SERVER-22423?