[lttng-dev] LTTNG-ust detailed architecture?

[Scheler, Fabian (CT RDA IOT SES-DE)]

Hi,

we would like to trace real-time applications and we consider LTTNG-ust 
as one possible way to do this. But before we go for LTTNG-ust, we need 
to estimate the impact LTTNG-ust could have on the instrumented 
application. While I consider the user-manual of LTTNG-ust to be really 
great, I am missing a more detailed architecture documentation, that 
would be really helpful to answer this question.

Having a look at the picture here

https://lttng.org/docs/v2.10/#doc-plumbing

I want to review these component interactions more closely:

1. Session daemon -> liblttng-ust

This library seems to create a new thread in the context of the 
instrumented application. How could this thread impact the run-time 
behavior of the instrumented application, when it comes to

- submitting events to the buffer

   * Which component controls/instantiates/maintains the buffer?
   * How is the buffer synchronized?

- setting event-rules (i.e. enabling/disabling events)
- starting tracing sessions
- ...

   * What does happen along the red dashed arrow
     "Session Daemon -> liblttng-ust"?

2. liblttng-ust -> Consumer daemon

At some point the consumer daemon reads the events stored in the buffer 
and writes them to some output-file.

   * What does happen along the violet arrow
     "liblttng-ust -> Consumer daemon"?
   * Could the instrumented application be affected here somehow?

Is this documented somewhere - maybe I simply missed it.

Ciao
Fabian
-- 
With best regards,
Dr. Fabian Scheler

Siemens AG
Corporate Technology
Research and Development for Digitalization and Automation
Smart Embedded Systems
CT RDA IOT SES-DE
Guenther-Scharowsky-Str. 1
91058 Erlangen, Germany
Tel.: +49 1522 1702973
Mobile: +49 1522 1702973
mailto:fabian.scheler at siemens.com

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