[lttng-dev] lttng live event loss with babeltrace2

Jonathan Rajotte-Julien jonathan.rajotte-julien at efficios.com
Fri May 7 14:05:32 EDT 2021


See https://github.com/lttng/lttng-tools/commit/c876d657fb08a91ca7c907b92f1b7604aee664ee . 

We would appreciate your feedback on this if possible based on your use case.


On Mon, May 03, 2021 at 05:05:10PM -0700, Eqbal via lttng-dev wrote:
> Hi,
> I have a lttng live session trace consumer application using libbabeltrace2
> where I create a graph to consume lttng live session traces and output to
> another sink. I am running the graph in a loop at some polling interval as
> long as I get BT_GRAPH_RUN_STATUS_AGAIN status. What I am noticing is that
> if my polling interval is large enough I tend to lose either all or some of
> the events. I experimented with various polling intervals and it seems if
> the polling interval is less than *DELAYUS *from "lttng-create
> --live=DELAYUS" option then I am able to get all the events, otherwise I
> tend to lose events.
> Here are the steps I follow:
> 1. start session daemon and relay daemon
> 2. create a live session (with default delay of 1s), enable events and start
> 3. Start my application (hello world example from lttng docs)
> 4. Start the consumer application built using libbabeltrace that connects
> to the live session
> I noticed that the events are actually persisted in the ~/lttng-traces by
> the relay daemon, but it does not reach babeltrace consumer application. I
> have noticed the same behavior with babeltrace2 cli.
> I would like to understand what is the reason for such behavior and if
> playing with the polling interval in relation to the DELAYUS value is the
> right thing to do.
> Thanks,
> Eqbal

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Jonathan Rajotte-Julien

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