[lttng-dev] lost trace events

[Alok Priyadarshi]

Any advice on how to debug lost events? Thanks in advance!


On Wed, Oct 31, 2018 at 2:58 PM Alok Priyadarshi <alokpr at gmail.com> wrote:

> We are tracing a multi-process application that generates high-frequency
> user-space trace events.
>
> Environment:
> - Ubuntu 16.04 LTS
> - Lttng 2.10.4 - KeKriek
>
> Session:
> - Snapshot mode
> - One channel with default config:
> overwrite=1, subbuf_size=524288, num_subbuf=4
>
> We have noticed that a few events are missing. I understand that in
> overwrite mode we can lose entire sub-buffers but IIUC we would only lose
> events at the beginning, not in-between events in a sub-buffer.
>
> Is it not true that only the oldest sub-buffer is overwritten and when
> overwrite happens, none of the latest events are discarded?
>
> Few more things to note:
> - Before lttng-snapshot-record, we stop tracing as recommended by the
> documentation
> - Packet context field events_discarded is always set to 0
> - Event loss seems to happen around packet transition. I have noticed this
> in at least two traces
>
> The snapshot mode seems perfect for our use case. It is OK to lose events
> from either ends of the snapshot, but it is essential to not lose any
> in-between events. I was under the impression that overwrite mode
> guarantees that. If not, is there anything else we can try?
>
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