title: Jenkins running at very high CPU usage
body: <p>I recently upgraded from Jenkins 1.6 to 2.5.  After I did this, I noticed very high CPU usage, sometimes over 300% (there are only 4 cores, so I don't think it could go over 400%).  I'm not sure where to begin debugging this, <a href="https://gist.github.com/MKorostoff/f5d4ae8380c60bd66db43ac16dc6c187" rel="noreferrer">but here's a thread dump</a> and some screenshots from top/htop</p>
  
  <p><strong>htop</strong>
  <a href="https://i.stack.imgur.com/il3qj.png" rel="noreferrer"><img src="https://i.stack.imgur.com/il3qj.png" alt="htop"></a></p>
  
  <p><strong>top:</strong>
  <a href="https://i.stack.imgur.com/B89Sj.png" rel="noreferrer"><img src="https://i.stack.imgur.com/B89Sj.png" alt="top"></a></p>