Re: Flight recorder mode

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.)

Thanks a lot,
Jan

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