That's the Chrome tracing tool: https://www.chromium.org/developers/how-tos/trace-event-prof...

It uses the event logging infrastructure in multicore which outputs json.

Normal OCaml has integrated this since 4.11: https://ocaml.org/releases/4.11/htmlman/instrumented-runtime... but switches to the Common Tracing Format (CTF) so you'll need an extra step to convert it to a format Chrome tracing can ingest.