Caml_thread_tick and signal SIGPREEMPTION

I notice multi-threaded ocaml programs spin up a dedicated thread that runs caml_thread_tick.

/* The tick thread: posts a SIGPREEMPTION signal periodically */

static void * caml_thread_tick(void * arg)
{
  struct timeval timeout;
  sigset_t mask;

  /* Block all signals so that we don't try to execute an OCaml signal handler*/
  sigfillset(&mask);
  pthread_sigmask(SIG_BLOCK, &mask, NULL);
  while(! caml_tick_thread_stop) {
    /* select() seems to be the most efficient way to suspend the
       thread for sub-second intervals */
    timeout.tv_sec = 0;
    timeout.tv_usec = Thread_timeout * 1000;
    select(0, NULL, NULL, NULL, &timeout);
    /* The preemption signal should never cause a callback, so don't
     go through caml_handle_signal(), just record signal delivery via
     caml_record_signal(). */
    caml_record_signal(SIGPREEMPTION);
  }
  return NULL;
}

Is there more information about what this is and why it’s necessary?

You can follow along with what is happening here: https://github.com/ocaml-multicore/ocaml-multicore/pull/381