The Saga of Multicore OCaml

Engineers from Jane Street share the challenges and lessons learned from migrating their massive codebase to the multicore OCaml 5 runtime.

Jane Street is an electronic trading firm that uses low latency trading systems built in OCaml to provide liquidity to financial markets worldwide. In December 2022, after nearly a decade of development, OCaml 5.0 was released with OCaml’s first multi-core capable runtime. This was an exciting milestone, finally making it possible to write shared-memory parallel programs in OCaml. The new runtime was designed to be easy to adopt: it didn’t disturb OCaml’s FFI, and performance was meant to be only a few percentage points slower in single-core mode. Despite those promising beginnings, switching to runtime-5 within Jane Street was harder than we expected. Indeed, we've only just switched to it this year, after 2.5 years of research and engineering effort. This talk will give an overview of the problems we ran into, and what we learned from the process, including some new ideas that solved some very old problems in the design of OCaml's GC. Read the full transcript here: janestreet.com/tech-talks/the-saga-of-multicore-ocaml/