[lttng-dev] Questions about LTTNG_UST_REGISTER_TIMEOUT

[Martin Ünsal]

Hi all,

I am a new lttng-ust user. I would like to leave tracepoints compiled into
production code. I have verified that inactive tracepoints have extremely
low overhead, but I am concerned about startup time and
LTTNG_UST_REGISTER_TIMEOUT. Our typical use case is that we are not tracing
and there is no lttng-sessiond, we enable tracing only in development or QA.

The specific questions:

1) What scenarios might the timeout come into play? I have tested with and
without lttng-sessiond running and I have seen no more than 10ms overhead
during app startup caused by lttng. That's no problem, but if there is some
unforeseen scenario where I introduce 3s overhead I am in a world of hurt!

2) If I set LTTNG_UST_REGISTER_TIMEOUT=0, what scenario would this fail and
how would it fail? (Silently or detectably?)

Actually I think these are more or less the same question viewed from two
different angles.

Thanks much. BTW I should say, I've been doing coordinated userspace and
kernel tracing for performance issues in Linux for many, many years using a
variety of homegrown tools and LTTng is fantastic, just the right feature
set and excellent performance. It's what I wish I'd built. :)

Martin
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