When a server is experiencing high load a DBA may issue a killOp on a session or multiple sessions in order to try to restore the overall health of the server. In some cases this is needed if the client has timed-out on a query, but the server is still running it. Thus, killOp() is a key tool in the DBA's hands. For instance, let's say a bad query is hitting the live DB, the DBA may killOp() that query over and over in order to not block other queries while the index is created, we have a script that does just that.

However, sometimes when killOp() is run it doesn't actually kill the session, especially under high load and/or high queue activity. It actually appears the killOp() gets queued along with other requests and MongoDB will get to it when it can.

The killOp() operation must have a higher priority than the operations being killed or it's not an effective tool.

I am sorry I don't have logs or more definitive data surrounding the bug, if it's needed please let me know an I can build a test case to illustrate the problem.