Zombie Interpreter processes

When we restart Zeppelin server (main process), in many cases, Interpreter
process keeps running, essential becoming a zombie processes.

In case of Spark interpreter, it also holds SparkContext - consuming server-side resources too.

As discussed in users@ and dev@ mailing lists, other users have confirmed this problem too.
Please see [Interpreter zombie processes|http://apache-zeppelin-users-incubating-mailing-list.75479.x6.nabble.com/Interpreter-zombie-processes-td4738.html.

luciano resende

I have also seen similar issues even using zeppelin-ddeamon but didn't have much time to investigate the issue when it was happening to me.

zjffdu

I believe I see this before too.

bzz@apache.org

Have similar experience, although hard to say what's the reason as all process supposed to killed, as Moon pointed out.
Also noticed that with `mvn tests`, after almost every run, there are 1-2 zombie RemoteInterpreter processes hanging around.

@ blaubaer <rene.pfitzner@nzz.ch>

We are seeing this problem as well, regularly actually. Especially in
situations when we have many concurrent interpreters running.

How to reproduce
1. Start Zeppelin
2. Run a paragraph \w any interpreters -> RemoteInterpreterServer process will be shown up (check by ps -ef | grep RemoteInterpreterServer)
3. Stop Zeppelin
4. check ps -ef | grep RemoteInterpreterServer again