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When using a non-realtime scheduler (the default in ns-3), the simulator advances the simulation time to the next scheduled event. During event execution, simulation time is frozen. With the realtime scheduler, the behavior is similar from the perspective of simulation models (i.e., simulation time is frozen during event execution), but between events, the simulator will attempt to keep the simulation clock aligned with the machine clock.
When an event is finished executing, and the scheduler moves to the next event, the scheduler compares the next event execution time with the machine clock. If the next event is scheduled for a future time, the simulator sleeps until that realtime is reached and then executes the next event.
It may happen that, due to the processing inherent in the execution
of simulation events, that the simulator cannot keep up with realtime.
In such a case, it is up to the user configuration what to do. There
are two ns-3 attributes that govern the behavior. The first is
ns3::RealTimeSimulatorImpl::SynchronizationMode. The two
entries possible for this attribute are BestEffort (the default)
or HardLimit. In "BestEffort" mode, the simulator will just
try to catch up to realtime by executing events until it reaches
a point where the next event is in the (realtime) future, or else
the simulation ends. In BestEffort mode, then, it is possible for
the simulation to consume more time than the wall clock time. The
other option "HardLimit" will cause the simulation to abort if the tolerance
threshold is exceeded. This attribute is
ns3::RealTimeSimulatorImpl::HardLimit and the default is 0.1 seconds.
A different mode of operation is one in which simulated time is not
frozen during an event execution. This mode of realtime simulation was
implemented but removed from the ns-3 tree because of questions of whether
it would be useful. If users are interested in a realtime simulator
for which simulation time does not freeze during event execution (i.e.,
every call to Simulator::Now() returns the current wall clock time,
not the time at which the event started executing), please contact the
ns-developers mailing list.
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