MobDev.OtpTrace (mob_dev v0.3.37)

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Runtime call-tracing utility. Wraps :erlang.trace_pattern/3 and :erlang.trace/3 to capture the set of {module, function, arity} actually called during a function's execution.

Used by the characterization harness (priv/trace/harness.exs) to empirically map what Elixir's runtime calls under the hood — the baseline that complements MobDev.OtpAudit's static analysis.

Why this exists

Static reachability misses dynamic dispatch (apply/3 with a computed atom, Code.ensure_loaded/1, NIF lazy load via :erlang.load_nif/2). Tracing catches everything that actually ran during the trace window — ground truth, no inference.

Combined with static analysis: static ∪ trace gives a high-confidence reachable set; static ∩ trace is the "definitely called and statically reachable" core.

Usage

result = MobDev.OtpTrace.capture(fn ->
  # any Elixir code — exercise features you want to measure
  Enum.map(1..10, &(&1 * 2))
end)

result.mfas      # MapSet of {module, function, arity}
result.modules   # MapSet of modules called
result.elapsed_us # How long the wrapped fn took (incl. trace overhead)

Trace overhead is real (~10x slowdown for tight loops) — only enable for measurement runs, never in production.

Summary

Types

mfa_set()
module_set()
result()

Functions

capture(fun, opts \\ [])

Run fun with full call tracing enabled on the calling process and any processes it spawns during execution. Returns a result/0 map.

Types

mfa_set()

@type mfa_set() :: MapSet.t({module(), atom(), arity()})

module_set()

@type module_set() :: MapSet.t(module())

result()

@type result() :: %{
  mfas: mfa_set(),
  modules: module_set(),
  elapsed_us: non_neg_integer()
}

Functions

capture(fun, opts \\ [])

@spec capture(
  (-> any()),
  keyword()
) :: result()

Run fun with full call tracing enabled on the calling process and any processes it spawns during execution. Returns a result/0 map.

Options

  • :exclude_modules — modules whose calls should NOT be recorded. Defaults to [__MODULE__, MobDev.OtpTrace.Collector, Agent, Task] so we don't pollute the trace with our own machinery.