[lttng-dev] [documentation] Drawing: LTTng architecture in relation to the commandline / buffer options

Thibault, Daniel Daniel.Thibault at drdc-rddc.gc.ca
Tue May 27 09:18:41 EDT 2014

De : Randy Wijnants [mailto:randy at tty32.org]
Envoyé : 27 mai 2014 06:34

> Changed the dashed arrows into dotted arrows.
> Is the arrow running from the lttng session daemon to liblttng-ust valid?
> Does the liblttng-ust always write to the shared memory buffer or only if the session daemon instructs it to (via which interface?)
> And what is the interface used between the session and consumer daemon?
> New version:
> http://tty32.org/Logging%20configuration2.png<--ESFSECEV-TY3011---------------------------->
> Randy Wijnants.

   Unless I’m mistaken, when a session daemon disables a channel, for instance, it asks liblttng-ust to turn off the immediate value test in the registered applications (each tracepoint entry is guarded by an immediate value test which is controlled by changing a single byte).  That’s the principle, anyway.  I haven’t actually followed this through the tangle of LTTng’s code.
   Rest assured that if liblttng-ust writes to a channel’s buffer, it’s because the event was enabled at the time.
   I know the consumer daemon(s) read(s) /run/lttng/lttng-session.pid (or $HOME/.lttng/lttng-session.pid) to reach the session daemon.  I think the session daemon uses the /run/lttng/ustconsumerd64 (or $HOME/.lttng/ustconsumerd64; 32-bit versions may also exist) socket to send commands to the consumer daemon(s).
   About the only things the diagram is missing now are:
* the lttng-relayd daemon (which completes the “Network destination / streaming” box);
* the add-context command (easily put beside enable-channel and enable-event); and
* the buffers-global / buffers-pid / buffers-uid alternative (could be put opposite the output TYPE at the Buffer’s top right).
* “LTTv” into “LTTV”
* “Eclipse” into “Eclipse TMF”
* “tracefilesize” into “tracefile-size”
* “commandline” into “command line”
Daniel U. Thibault
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