Re: Flight recorder mode

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
3. run some load
4. lttctl -n mytrace -l /sys/kernel/debug/ltt -f -m flight -t /tmp/trace
5. lttv -mtextDump -t /tmp/trace

What I get:
** ERROR **: Trace /tmp/trace has no facility tracefile

Which is true as "ls /tmp/trace/control" shows. Simply copying those
files from some full-fledge trace works around it, but only when the
facilities are in fact the same, I guess...

Jan

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