[lttng-dev] LTTNG-ust detailed architecture?

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

Hi,

thanks for your answer!

> 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.

In principle, this sounds quite well, but there are still plenty of 
other thinks we would like to know in more detail:

- How does the ust_listener_thread interact with the instrumented 
application?
- How does the ust_listener_thread interact with the consumderd (I guess 
your answer targeted this part, right?) and/or the sessiond?
- What is the purpose of the shared memory in the ringbuffer (shm.{c,h})?
- How does the consumerd poll the buffers?
- ...

So, it would be really great if there is some document where this 
architecture is noted down. Maybe, there are also helpful comments in 
the code - if you can point me there, this would also be helpful.

Ciao
Fabian

> 
> ----- 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 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
>> 
>> www.siemens.com/ingenuityforlife
>> 
>> Siemens Aktiengesellschaft: Chairman of the Supervisory Board: Jim 
>> Hagemann Snabe; Managing Board: Joe Kaeser, Chairman, President
>> and Chief Executive Officer; Roland Busch, Lisa Davis, Klaus
>> Helmrich, Janina Kugel, Cedrik Neike, Michael Sen, Ralf P. Thomas;
>> Registered offices: Berlin and Munich, Germany; Commercial
>> registries: Berlin Charlottenburg, HRB 12300, Munich, HRB 6684;
>> WEEE-Reg.-No. DE 23691322
>> 
>> 
>> _______________________________________________ lttng-dev mailing
>> list lttng-dev at lists.lttng.org 
>> https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev

-- 
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

www.siemens.com/ingenuityforlife

Siemens Aktiengesellschaft: Chairman of the Supervisory Board: Jim
Hagemann Snabe; Managing Board: Joe Kaeser, Chairman, President and
Chief Executive Officer; Roland Busch, Lisa Davis, Klaus Helmrich,
Janina Kugel, Cedrik Neike, Michael Sen, Ralf P. Thomas; Registered
offices: Berlin and Munich, Germany; Commercial registries: Berlin
Charlottenburg, HRB 12300, Munich, HRB 6684; WEEE-Reg.-No. DE 23691322

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