[lttng-dev] lttng live event loss with babeltrace2

[Eqbal]

Hi,

I ran my test with the new changes and it seems to improve the lossiness
but not eliminate it. For example it seems in the hello world example that
if the application exits too quickly after emitting the tracepoint, we
might still lose some events in the consumer even though they are present
in the relayd output folder. I don't quite understand why that is. Would
love some explanations on the various timing windows where we have a
potential to lose the events on the consumer side.

Thanks,
Eqbal

On Fri, May 7, 2021 at 11:05 AM Jonathan Rajotte-Julien <
jonathan.rajotte-julien at efficios.com> wrote:

> Hi,
>
> See
> https://github.com/lttng/lttng-tools/commit/c876d657fb08a91ca7c907b92f1b7604aee664ee
> .
>
> We would appreciate your feedback on this if possible based on your use
> case.
>
> Cheers
>
> 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
>
> > _______________________________________________
> > lttng-dev mailing list
> > lttng-dev at lists.lttng.org
> > https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
>
>
> --
> Jonathan Rajotte-Julien
> EfficiOS
>
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