3.1 MB - Evernote

34
Parallel Transaction-oriented Simulation
states can be reclaimed through fossil collection. Another important purpose is to
determine the overall progress of the simulation, which includes the detection of the
simulation end. Besides these reasons optimistic parallel simulations can often run
without any additional GVT calculations for long time periods or even until they reach
the simulation end if enough memory for the required state saving is available. In
environments with a relatively low communication performance like Computing Grids
it is desirable to minimise the need for GVT calculations because the GVT calculation
process is based on the exchange of messages and adds a communication overhead.
The best-known GVT calculation algorithm was suggested by Jefferson [16]. It defines
the GVT as the minimum of all local simulation times and the time stamps of all events
sent but not yet acknowledged as being handled by the receiving LP. The planned
parallel simulator will use this algorithm for the GVT calculation because it is relatively
easy to implement and well studied. Future work could also look at alternative GVT
algorithms that might be suitable for Grid environments, like the one suggested in [20].
The movement of a Transaction in transaction-oriented simulation can be seen as
equivalent to an event being executed in discrete event simulation as concluded in 4.1.1.
But in transaction-oriented simulation the causal order is not only determined by the
movement time of a Transaction but also by its priority because if several Transactions
exist that have the same move time then they are moved through the system in order of
their priority, i.e. Transactions with higher priority first. As a result the priority had to be
included in the GVT calculation in [19] because the Breathing Time Buckets algorithm
(SPEEDES algorithm) used there needs the GVT to guarantee outgoing Transactions.
For a parallel transaction-oriented simulator based on the Time Warp algorithm or the
Shock Resistant Time Warp algorithm it is not necessary to include the Transaction
priority in the GVT calculation because the GVT is only used to determine the progress
of the overall simulation and to regain memory through fossil collection. For the Shock
Resistant Time Warp algorithm one additional use of the GVT is to determine realistic
values for the CommittedEvents sensor. Events are committed when receiving a GVT
that is greater than the event’s time. As a result the number of committed events during
a certain period of time is only known if GVT calculations have been performed. The
suggested parallel simulator based on the Shock Resistant Time Warp algorithm will
therefore synchronise the processing of its LPCC with GVT calculations.