[lttng-dev] lost trace events

[Alok Priyadarshi]

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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