Experimental Physics and
Industrial Control System

Richard Fearn <[email protected]> · Wed, 12 Nov 2014 11:18:02 +0100

Hi Geoff,

Since I'm new to CSS and Java I'm interested in debugging techniques
others have found successful.  For example, I was unprepared for not
being able to see all the threads as with a C or python program.

Do you mean that you can't see all the threads in the output from top(1) ?

First thing to do is find the pid of the VM itself. You can get this

from ps(1) or using the jps tool [1] that comes with the JDK.

This main process is the one that top(1) will show by default. However

if you type 'H' (upper case, not lower case) top will switch to showing

threads. You will then see one line for each Java thread. This might

allow you to identify particular threads that are busy.

Another JDK tool, jstack [2], will give you a thread dump showing what

each thread is doing. Unfortunately the process IDs in the jstack output

(nid=0xNNNN) are in hex, whereas in jps/top they're decimal :-)

The other thing that jstack might show you, at the bottom of its output,

is that there's a deadlock.

Obviously the user has to leave the hanging program running for you to

be able to get in there and (try to) diagnose the problem. At Diamond,

we developed a simple bash script for users to run that would run jstack

a few times and save the output somewhere for us to have a look at.

[1] https://docs.oracle.com/javase/8/docs/technotes/tools/unix/jps.html

[2] https://docs.oracle.com/javase/8/docs/technotes/tools/unix/jstack.html

Rich

--

Richard Fearn
Senior Software Engineer
Integrated Control Systems division
European Spallation Source ESS AB
[email protected]

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System