[lttng-dev] On lost events

[Matthew Khouzam]

On 13-07-05 03:40 PM, Yannick Brosseau wrote:
> 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.

To quote the green lantern, "I KNOW RIGHT?!!?!?!". jokes aside, that
implies that all packets should always overlap and if they don't there
should be anti-time. I think the safest way to go about it is "last
event of the last packet to first event of the next one".
>
>> 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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