[lttng-dev] On lost events

[Yannick Brosseau]

On 2013-07-05 14:54, Matthew Khouzam wrote:
> Quick question. Some traces have overlapping packets, so in that case,
> the lost events are between the last event of a previous packet and the
> first event of the next packet, which just happens to be the previous
> packet's end time?
Babeltrace say that the losts are between the beginning of the next and
the end of the previous.

> On 13-07-04 06:29 PM, Mathieu Desnoyers wrote:
>> * Matthew Khouzam (matthew.khouzam at ericsson.com) wrote:
>>> Hi tracing werewolves,
>>>
>>> I want to clear up some stuff about lost events.
>>>
>>> Lost events are caused, in lttng 2.x when the tracer does not have
>>> enough memory available to write an event. This can be due to:
>>> * events coming in too fast (faster than we can write them)
>>> * a given event being too large
>>> * we have an event that does a nested wrap around
>>>
>>> LTTng will not write the whole packet that is lost.
>> LTTng does not write an _event_ discarded at all. However, a packet
>> "lost" is a different thing: it only happens in overwrite mode, when we
>> overwrite a packet.
>>
>>> It uses a ring
>>> buffer and the packet will just be overwritten in the buffer, but a
>>> counter will be incremented.
>> For an _event_ discarded, we're not even writing it into the buffer.
>>
>>> The counter is written in the packet header in the CTF trace.
>> Indeed, the event lost counters are written in the packet in the ctf
>> trace.
>>
>>> When reading a trace, if you just read the packet headers, you will see
>>> something as follows:
>>>
>>> Packet 1:
>>> ts_begin: 100
>>> ts_end: 200
>>> discarded: 0
>>>
>>> Packet 2:
>>> ts_begin: 300
>>> ts_end: 400
>>> discarded: 100
>>>
>>> Packet 3:
>>> ts_begin: 500
>>> ts_end: 600
>>> discarded: 100
>>>
>>> Packet 4:
>>> ts_begin: 650
>>> ts_end: 750
>>> discarded: 200
>>>
>>> With this example, the lost events are located between packets 1, 2 and
>>> packets 3,4.
>>>
>>> If the events are too large, they are discarded in the current packet,
>>> lttng does not split events into packets (yet?), and this is lost during
>>> a current packet.
>>>
>>> That means if the events are lost between 1 and 2, they are potentially
>>> in the range of [200-400] but probably in [200-300]
>>> The next lost event are between 3 and 4 (events discarded is a total
>>> count, not local to that packet), so 100 events are also lost between
>>> [600-750] but likely in between [600-650]. If you can confirm that all
>>> events WITH CONTEXTS are smaller than the packet sizes, I think it's a
>>> shoe in that it's between the packets and not containing the next packet.
>> "and not contained in the next packet".
>>
>> Thanks,
>>
>> Mathieu
>>
>>> Hope this clarifies some issues, it did for me!
>>>
>>> Matthew
>>>
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>
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