Eio's Ctf module provides a ring-buffer for storing trace events. Currently, this only works for a single domain. OCaml has its own CTF-based eventlog system (https://ocaml.org/manual/instrumented-runtime.html) and we should probably just use that somehow. It might not currently support user-defined events. There's also a proposal to replace it with eventring (ocaml-multicore/ocaml-multicore#793), which says:

This implementation is designed to be open for extension to user tracing events in the future, as well as integration with statmemprof (e.g continuous allocation profiling).

/cc @sadiqj