title: mongodb memory release
body: <p>I am new to mongodb and doing some testing with it (1 master + 1 slave).
  We inserted 2 million documents in couple of collections (Ubuntu, with 6G RAM, Quad core) and mongodb's memory increased from 40m to 4G (we are ok with it). Now we deleted all data and we were expecting memory (RSS) to go down back to 40m. Is it normal? Does mongodb takes time to flush mem (based on some scheduling)? </p>
  
  <p>Just a fyi : performance (from frontend GUI ) was really good and improved a lot with adding indexes.</p>
  
  <p>thanks</p>