inMemory spins CPU when cache fills with dirty data

1. Start Enterprise mongod: ./mongod --dbpath=/tmp --storageEngine=inMemory --replSet rs 2. Run initiate in a shell: ./mongo rs.initiate() 3. Run a benchrun load to trigger lots of inserts: 3a. clone git@github.com:mongodb/mongo-perf.git 3b. python benchrun.py --includeFilter 'Insert.JustID' -f ./testcases/ insert .js --trialTime 60 --trialCount 1 --writeCmd=true -w 1 -t 24 -s /path/to/mongo/shell/mongo } 4. After about 30 seconds, observe that a few threads in mongod are spinning in eviction. Even after the benchrun completes, the threads continue to spin.