[lttng-dev] Questions about JUL (was: Re: lttng-dev Digest, Vol 67, Issue 48)

[David Goulet]

On 04 Feb (18:59:02), Thibault, Daniel wrote:
> ----------------------------------------------------------------------
> Date: Tue, 4 Feb 2014 12:17:36 -0500
> From: David Goulet <dgoulet at efficios.com>
> To: Jim Dumont <jim.dumont at ericsson.com>
> Cc: "lttng-dev at lists.lttng.org" <lttng-dev at lists.lttng.org>
> 
> >> 3) add-context: does --jul have the same possible context types as --userspace (i.e. ip, pthread_id, procname, vpid, vtid)?
> >
> > Nope.
> 
>    What are the possible context types then?  Or is it just that you can't apply a filter to a jul event?

None, you can't add a context to the JUL domain.

> 
> >> 4) enable-channel:
> >> 4.1) Are the --jul --subbuf-size, --num-subbuf and --read-timer defaults the same as with --userspace?
> >
> > Nope. JUL uses it's internal channel so basically enable channel does NOT support the jul domain thus not having -j in the help/man.
> 
>    So you turn jul events on and off using enable/disable-event --all instead?  This is getting a tad confusing: jul events force you to use only enable/disable-event but kernel syscalls force you to use only enable/disable-channel...

No. For JUL you enable "Logger" names from the JUL API. For instance:

$ lttng enable-event -j MyLogger
or to enable all loggers
$ lttng enable-event -j -a

Using the lttng-ctl API, -a == "*".

For the kernel syscalls, yeah well this is a tracer limitation that we
can't enable/disable specific syscalls but fortunately this year we'll
bring this feature! :)

> 
> >> 4.4) If yes, won't they all degenerate to the virtual machine's user and process IDs?
> >
> > Not sure I understand this one?
> 
>    A Java application is interpreted and runs in a Java virtual machine (JVM).  The userID and processID of any jul events are thus likely to be those of the JVM process.  I would expect Java apps launched by separate users to launch separate JVM instances, but I would also expect two separate Java apps launched by the same user to share a single JVM (the user's) and, therefore, their process IDs.  This could get fun with multi-threaded Java apps.

Hrm, I've not observed that. If a specific Linux user with let say UID =
1000, the Java application registering to the session deamon is set with
UID = 1000. Don't forget that the JUL support uses JNI to link with
lttng-ust and that library registers as a the user.

> 
> > Now after answering all this, I feel like either creating a lttng-jul man page or adding a special section in "lttng.1" for the JUL domain explaining all this. Thoughts?
> 
>    We definitely need an additional section in "lttng.1".

Agreed. I'll be updating lttng.1 before the stable release.

THanks!
David

> 
> Daniel U. Thibault
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