In Shadow's implementation of the CoDel control law, the order of operations is incorrect.

The CoDel RFC defines the control law as the following function:

time_t codel_queue_t::control_law(time_t t, uint32_t count)
   {
       return t + INTERVAL / sqrt(count);
   }

The following expression is equivalent but makes the correct order of operations explicit:

       return t + (INTERVAL / sqrt(count));

Shadow's implementation orders things the wrong way: it does the addition operation first, and then the division:

The result is that the schedule for dropping packets will be incorrect and unexpected.