Also, note that the actual code that's starting and setting the
interval is here:
/usr/lib/python2.7/site-packages/cinder/service.py
~~~
from oslo_service import loopingcall
(...)
        if self.periodic_interval:
            if self.periodic_fuzzy_delay:
                initial_delay = random.randint(0, self.periodic_fuzzy_delay)
            else:
                initial_delay = None

            periodic = loopingcall.FixedIntervalLoopingCall(
                self.periodic_tasks)
            periodic.start(interval=self.periodic_interval,
                           initial_delay=initial_delay)
            self.timers.append(periodic)
~~~

/usr/lib/python2.7/site-packages/oslo_service/loopingcall.py:class
FixedIntervalLoopingCall(LoopingCallBase):
~~~
class FixedIntervalLoopingCall(LoopingCallBase):
    """A fixed interval looping call."""

    _RUN_ONLY_ONE_MESSAGE = _("A fixed interval looping call can only run"
                              " one function at a time")

    _KIND = _('Fixed interval looping call')

    def start(self, interval, initial_delay=None, stop_on_exception=True):
        def _idle_for(result, elapsed):
            delay = round(elapsed - interval, 2)
            if delay > 0:
                func_name = reflection.get_callable_name(self.f)
                LOG.warning(_LW('Function %(func_name)r run outlasted '
                                'interval by %(delay).2f sec'),
                            {'func_name': func_name, 'delay': delay})
            return -delay if delay < 0 else 0
        return self._start(_idle_for, initial_delay=initial_delay,
                           stop_on_exception=stop_on_exception)
~~~

And from LoopingCallBase
~~~
    def _start(self, idle_for, initial_delay=None, stop_on_exception=True):
        """Start the looping

        :param idle_for: Callable that takes two positional arguments, returns
                         how long to idle for. The first positional argument is
                         the last result from the function being looped and the
                         second positional argument is the time it took to
                         calculate that result.
        :param initial_delay: How long to delay before starting the looping.
                              Value is in seconds.
        :param stop_on_exception: Whether to stop if an exception occurs.
        :returns: eventlet event instance
        """
        if self._thread is not None:
            raise RuntimeError(self._RUN_ONLY_ONE_MESSAGE)
        self._running = True
        self.done = event.Event()
        self._thread = greenthread.spawn(
            self._run_loop, idle_for,
            initial_delay=initial_delay, stop_on_exception=stop_on_exception)
        self._thread.link(self._on_done)
        return self.done
(...)
   def _run_loop(self, idle_for_func,
                  initial_delay=None, stop_on_exception=True):
        kind = self._KIND
        func_name = reflection.get_callable_name(self.f)
        func = self.f if stop_on_exception else _safe_wrapper(self.f, kind,
                                                              func_name)
        if initial_delay:
            greenthread.sleep(initial_delay)
        try:
            watch = timeutils.StopWatch()
            while self._running:
                watch.restart()
                result = func(*self.args, **self.kw)
                watch.stop()
                if not self._running:
                    break
                idle = idle_for_func(result, watch.elapsed())
                LOG.trace('%(kind)s %(func_name)r sleeping '
                          'for %(idle).02f seconds',
                          {'func_name': func_name, 'idle': idle,
                           'kind': kind})
                greenthread.sleep(idle)
        except LoopingCallDone as e:
            self.done.send(e.retvalue)
        except Exception:
            exc_info = sys.exc_info()
            try:
                LOG.error(_LE('%(kind)s %(func_name)r failed'),
                          {'kind': kind, 'func_name': func_name},
                          exc_info=exc_info)
                self.done.send_exception(*exc_info)
            finally:
                del exc_info
            return
        else:
            self.done.send(True)
(...)
~~~