Deterministically debugging concurrent GC bugs with rr

Multicore OCaml comes with a concurrent garbage collector , where the garbage collector and the mutator threads run concurrently. Debugging concurrent GC bugs has been the most frustrating / satisfying (when fixed) part of Multicore OCaml development. rr , a record and replay tool has made debugging concurrent GC bugs a sustainable exercise. In this short post, I’ll describe why. A particularly tricky concurrent GC bug is one which occurs once every 10 to 100 runs due to non-determinism and any attempt to i