title: how to avoid high CPU usage when no active queries
body: <p>On my linux server I ran into a problem this morning. I realized that after I killed all background tasks that were making ~200k database entries (1gb in size) throughout the night, my CPU usage was still at 80%, only due to MySQL. 
  Neither a reboot or restarting MySQL or nginx worked. </p>
  
  <p>"InnoDB saved the data in the rows it was changing, and queries against that changes that are still being rolled back should be answered transparently using the data that's still in the undo log."</p>
  
  <p>I am not too familiar with this topic, but it seems like that this is the answer of why there is a high CPU usage even when there are no queries. When I <code>SHOW PROCESSLIST</code> it shows three connections and it says in the "state"-column "Copying to tmp table".</p>
  
  <p>I guess right now I just have to wait out until all those rollback processes are finished, but why do they come up in the first place and how can I avoid this from happening again?</p>