Parallel and Distributed Simulation

1
Parallel and Distributed Simulation

• Zero Lookahead, Simultaneous Events and
  Repeatability

2
Outline

• Zero Lookahead, Simultaneous Events, and
  Repeatability
• Problem Scenarios
• Wide Virtual Time

3
Issues

• Zero lookahead An LP has zero lookahead if it
  can schedule an event with time stamp equal to
  the current simulation time of the LP
• Simultaneous events events containing the same
  time stamp in what order should they be
  processed?
• Repeatability An execution mechanism (e.g., Time
  Warp) is repeatable if repeated executions
  produce exactly the same results
• Often a requirement
• Simplifies debugging

4
Simultaneous Events

• Ordering of events with the same time stamp may
  be important in some applications
• Two missiles detonate at a target which system
  gets credit for the kill?
• Two packets arrive at a switch at the same time
  which one is given service?
• Observations
• Concern often is biased simulation results, e.g.,
  if ordering decisions are systematic
• Proper ordering depends on the intended
  application of the simulation results

5
Zero Lookahead and Simultaneous Events

• User specified orderings of events are subject to
  certain constraints
• Example
• For any two simultaneous events, type A events
  must always be processed before type B events

The above rule cannot be realized, not even for
sequential simulations!
6
Zero Lookahead and Simultaneous Events

• Time Warp Do simultaneous event cause rollback?
• A possible rule
• If an LP processes an event at simulation time T
  and then receives a new event with time stamp T,
  roll back the event that has already been
  processed.

If an event can roll back another event on which
it depends, unending rollback cycles may occur.
12
Rollback!
12
12
7
Repeatability

• Assuming event computations in each LP are
  repeatable and ignoring simultaneous events, is
  the execution of the parallel simulation
  repeatable?
• Yes Produces same results as sequential
  execution where all events are processed
  sequentially in time stamp order
• To ensure repeatable executions, it is sufficient
  to ensure simultaneous events are processed in
  the same order with each execution (provided
  event computations are deterministic)
• Simultaneous events may arrive at an LP in a
  different order from one execution to the next

8
Wide Virtual Time (WVT)

• Approach
• Application uses time value field to indicate
  time when the event occurs
• Tie breaking field used to order simultaneous
  events (events with same time value)

• Tie breaking field can be viewed as low precision
  bits of time stamp
• Time definition applies to all simulation time
  values (e.g., current time of an LP)

9
An Approach Using WVT
time value
Time stamp

• Application specified ordering of events

Constraint on zero lookahead events
Avoid rollback cycles
Non-zero lookahead events Age1 Zero lookahead
events Age Current Age 1
Repeatable execution
10
WVT Example

• Avoid rollback cycles despite zero lookahead
  events

12.2
No Rollback!
12.1
11
Summary

• Zero lookahead, simultaneous events,
  repeatability
• Complex modeling and implementation issues
• Complex interaction between users ability to
  specify order, implementation correctness, and
  ensuring repeatability
• Wide Virtual Time provides one solution