[lttng-dev] Community input: Feedback on use of enumeration type within LTTng-UST

[Mathieu Desnoyers]

Hi,

We are currently working on the CTF2 specification [1] at EfficiOS, and there is
a user metric we would need to help us with a design decision with respect
to enumerations. This is where we would need community input.

The usage metrics in question are with respect to LTTng-UST enumeration types
(TRACEPOINT_ENUM) [2] used by tracepoint events within instrumented applications.

A tracepoint enumeration can be used by many events. What we are looking for is to get
an idea of the common use, and extreme cases as well.

Ideally, what we would need is:

- For each enumeration within your application instrumentation (TRACEPOINT_ENUM), how
  many events refer to each enumeration ? (average, mode, and maximum)
- For each TRACEPOINT_ENUM, how many labels do they possess ? (sum number of
  ctf_enum_value/ctf_enum_range/ctf_enum_auto entries) (average, mode, and maximum)
- For each TRACEPOINT_ENUM, for each label, what is the string length ? (average, mode, and maximum)

Based on this information, we can estimate the data overhead generated by repeating enumeration
labels in the LTTng-UST metadata. We need to decide whether we allow references to a single
enumeration description when it is used by many events, or if we require repeatedly serializing
the entire enumeration description for each event field using the enumeration.

Thanks,

Mathieu

[1] https://lists.lttng.org/pipermail/lttng-dev/2020-November/029777.html
[2] https://lttng.org/man/3/lttng-ust/v2.12/#doc-tracepoint-enum

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com