34.2 Real-Time Scheduler

The real-time scheduler implements a soft real-time scheduler which ties event execution within the simulator to real time. Provided sufficient CPU horsepower is available to keep up with arriving packets, the simulator virtual time should closely track real-time. If the simulator becomes too slow to keep up with elapsing real time, a warning is continually produced if the skew exceeds a pre-specified constant ``slop factor'' (currently 10ms).

The main dispatch loop is found in the routine RealTimeScheduler::run(), in the file scheduler.cc. It follows essentially the following algorithm:

• While simulator is not halted
  • get current real time (``now'')
  • dispatch all pending simulator events prior to now
  • fetch next (future) event if there is one
  • delay until the next simulator event is ready or a Tcl event occurs
  • if a tcl event occured, re-insert next event in simulator event queue and continue
  • otherwise, dispatch simulator event, continue
  • if there was no future even, check for Tcl events and continue

The real-time scheduler should always be used with the emulation facility. Failure to do so may easily result in the simulator running faster than real-time. In such cases, traffic passing through the simulated network will not be delayed by the proper amount of time. Enabling the real-time scheduler requires the following specification at the beginning of a simulation script:

    set ns [new Simulator]
    \$ns use-scheduler RealTime