Re: Flight recorder mode

* Jan Kiszka ([email protected]) wrote:
> Mathieu Desnoyers wrote:
> > * Jan Kiszka ([email protected]) wrote:
> >> Mathieu Desnoyers wrote:
> >>> * Jan Kiszka ([email protected]) wrote:
> >>>> Jan Kiszka wrote:
> >>>>> Mathieu Desnoyers wrote:
> >>>>>> * Jan Kiszka ([email protected]) wrote:
> >>>>>>> Mathieu Desnoyers wrote:
> >>>>>>>> * Jan Kiszka ([email protected]) wrote:
> >>>>>>>>> Hi,
> >>>>>>>>>
> >>>>>>>>> I'm trying to understand the flight recorder mode (including the hybrid
> >>>>>>>>> one) of LTTng. I originally expected that this mode means:
> >>>>>>>>>
> >>>>>>>>>  1. Fixed set of buffers, older data will be overwritten on wrap-around
> >>>>>>>>>  2. Limited size of trace files
> >>>>>>>>>
> >>>>>>>>> While 1. seems to be true, 2. is neither what I observe in the output
> >>>>>>>>> directory (flight-cpu_* get larger and larger) nor what I can find in
> >>>>>>>>> lttd's code. Intentionally?
> >>>>>>>>>
> >>>>>>>> Yep, see the other lttctl options and the quickstart guide to see how to
> >>>>>>>> spawn one lttd for the "normal" channels and a different one for the
> >>>>>>>> flight recorder channels (should be done after you stop tracing).
> >>>>>>> Sorry, /me is too blind: The quickstart guide is not mentioning flight
> >>>>>>> recorder or hybrid mode, and also lttctl --help gives me no hint what
> >>>>>>> pattern to apply for achieving limited trace file sizes.
> >>>>>>>
> >>>>>> Hrm, or do I recall doing documentaion I haven't done, let's see :)
> >>>>>>
> >>>>>> Ah, it's in a mailing list, not in the documentation yet :
> >>>>>>
> >>>>>> http://osdir.com/ml/linux.kernel.tracing/2006-09/msg00015.html
> >>>>>>
> >>>>>> It explains how to use the hybrid mode.  I'll cut'n'paste it at the end
> >>>>>> of the quickstart guide.
> >>>>>>
> >>>>>> There is not trace file size limit you can give to lttctl nor lttd. You
> >>>>>> can only specify the buffer size. The idea is that you can decide at
> >>>>>> which moment you want to take you flight recorder "snapshot" by running
> >>>>>> lttctl -f at the correct moment.
> >>>>> OK, -EBRAINSUSPENDED, you just woke it up again:
> >>>>>
> >>>>> lttctl -b -n mytrace -m flight -l <debugfs>/ltt
> >>>>> <run application>
> >>>>> <when something happened:>
> >>>>> lttctl -f -n mytrace -t <trace-out> -l <debugfs>/ltt
> >>>> Hmm, in fact this won't fly yet. It looks like I need the hybrid mode,
> >>>> because flight recorder traces generated like above are lacking some
> >>>> basic information (stuff under /control I guess) to make lttv happy.
> >>>>
> >>>> That leaves me with a (for now) acceptable tracing procedure via hybrid
> >>>> traces. But I'm still lacking some howto for pure flight recorder
> >>>> traces. Any hints still appreciated... :)
> >>>>
> >>> Hrm, LTTV should be able to cope with the missing control information.
> >>> The only tracefiles that needs to be complete are the facilities
> >>> tracefiles.
> >>>
> >>> What do you get when you start lttv on a flight recorder trace ?
> >> What I did:
> >> 1. armall
> >> 2. lttctl -n mytrace -l /sys/kernel/debug/ltt -b -m flight
> > 
> > Ah, I see.. try to change -b for -d : it will spawn an lttd daemon which
> > will read the "normal" channels (in this case, only the facilities
> > tracefiles). (also add -l /sys/kernel/debug/ltt -t /tmp/trace)
> > 
> > It should then start working better.
> 
> Perfect! That's now the behavior we already had with an ancient version
> and that I wanted to recreate with latest LTTng.
> 
> You see what kind of howtos it takes for dummies like me? :->
> (Means: Please add both the flight and hybrid mode scenario to the
> quickstart.)
> 

hehe, I would not call yourself a dummy.. it's added to the quickstart,
thanks for the feedback.

Mathieu

> Thanks a lot,
> Jan
> 
> -- 
> Siemens AG, Corporate Technology, CT SE 2
> Corporate Competence Center Embedded Linux
> 

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
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