[lttng-dev] efficient use of LTTng in legacy codebase

[Máté Ferenczy]

Hello Mathieu,

Our team was asked to support LTTng UST solution in a legacy codebase. The existing codebase is fairly large, and the requirement is that we provide all the existing logs and debugs in the system as LTTng traces.

The planned solution so far was just to pass everything we have as text (strings) to LTTng. For that we obviously have to convert the printf-like format string and parameter list to a string by calling snprintf before giving the string to the LTTng API. That would however mean, that these snprintfs are getting called every time even if there is no listener/active trace session for the given trace. In this case, all those unnecessary snprintf calls may end up significantly impacting the performance of our applications.

In order to work around this, we were thinking that if there was a way to query the LTTng library from our application code whether there is an active tracing session for the given trace level, we could avoid calling those snprintfs in case they are not needed. Do you think it would be feasible to easily get this data from LTTng lib? If yes, can you provide us some pointers where to look at in order to do those changes ourselves?

Any suggestions are welcome.

Thank you,
Mate Ferenczy
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