[lttng-dev] LTTNG-ust detailed architecture?

[Michel Dagenais]

We have been using LTTng-UST for hard real-time systems for a while. Once tracing is setup, there are few interactions between the traced application and the rest. The tracing code simply fills the sub-buffers and switches to another sub-buffer when the current one is full. In one mode, the consumer daemon polls the state of tracing sub-buffers to know when they are full and can be copied and emptied; the traced application does not have to issue any potentially blocking system call. In the default mode, the traced application sends a short message on a pipe to the consumer daemon to signal when a sub-buffer is ready to be consumed.

----- Le 24 Oct 18, à 6:53, Scheler, Fabian (CT RDA IOT SES-DE) Fabian.Scheler at siemens.com a écrit :

> 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
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> Research and Development for Digitalization and Automation
> Smart Embedded Systems
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