Simulator Protocol

coordinator

simulator

Component

tN

tN. tL

After each transition tN = t + ta(), tL = t

simulator

Component

tN

tN. tL

simulator

Component

tN

tN. tL

Coupled Model

1 nextTN

2. outTN

3 getOut

4 sendOut

5 applyDelt

Simulation of coupled models-- Basic DEVS Simulation Cycle

Each simulator reacts to the incoming message as follows:

•If it is imminent and its input message is empty, then it invokes its model’s inernal transition function•If it is imminent and its input message is not empty, it invokes its model’s confluence transition function•If is not imminent and its input message is not empty, it invokes its model’s external transition function•If is not imminent and its input message is empty then nothing happens.

Coupled Model Coordinator:

1. Coordinator sends nextTN to request tN from each of the simulators.

2. All the simulators reply with their tNs in the outTN message to the coordinator

3. Coordinator sends to each simulator a getOut message containing the global tN (the minimum of the tNs)

4. Each simulator checks if it is imminent (its tN = global tN) and if so, returns the output of its model in a message to the coordinator in a sendOut message.

5. Coordinator uses the coupling specification to distribute the outputs as accumulated messages back to the simulators in an applyDelt message to the simulators – for those simulators not receiving any input, the messages sent are empty.

For a coupled model with atomic model components, a coordinator is assigned to it and coupledSimulators are assigned to its components. In the basic DEVS Simulation Protocol, the coordinator is responsible for stepping simulators through the cycle of activities shown.

coordinator

Atomic

ModelAtoimc3

coupledSimulator

simulators.tellAll("initialize“)

simulators.AskAll(“nextTN”)

simulators.tellAll("computeInputOutput“)

simulators.tellAll("sendMessages")

simulators.tellAll("DeltFunc“)

messagemessage

coupledSimulator coupledSimulator

Atomic

Model

Atomic

Model

simulators.tellAll("initialize“) --- at start of simulation

simulators.AskAll(“nextTN”)

simulators.tellAll("computeInputOutput“)

simulators.tellAll("sendMessages")

simulators.tellAll("DeltFunc“)DEVS cycle

DEVS Simulation Protocol as Implemented in DEVSJAVA

coupling

The DEVS Simulation Protocol implemented in DEVSJAVA is basically the same as the one just discussed. However, instead of having simulators send their output messages to the coordinator, they exchange messages when told to do so by the coordinator. To be able to do so, they need the coupling that is relevant to their attached models – this is transmitted to them in a a step, called informCoupling, that is performed by the coordinator at start up.

genDevs.simulation.coordinator

genDevs.simulation.

coupledSimulator

For example, the simulator for generator, g in the coupled model gpt, knows that an output on port “out” should be paired with port “in” in a message to the simulator for processor p.

Simulating Large-Scale Models

Considering a coupled model that includes thousands of atomic models

Two Situations:

1. In every cycle, almost every atomic model are imminent

2. In every cycle, only one or a few atomic models are imminent

Question: How to revise the standard protocol to improve the performance of the simulation for these two special situations?

DTSSCoord

Atomic

ModelAtoimc3

DTSSSim

simulators.tellAll("initialize“)

simulators.tellAll("computeInputOutput“)

simulators.tellAll("sendMessages")

simulators.tellAll("DeltFunc“)

messagemessage

DTSSSim DTSSSim

Atomic

Model

Atomic

Model

simulators.tellAll("initialize“) --- at start of simulation

simulators.tellAll("computeInputOutput“)

simulators.tellAll("sendMessages")

simulators.tellAll("DeltFunc“)

Situation one: DEVS Simulation Protocol Specialized to Discrete Time

coupling

For an isolated discrete time coupled model the DEVS simulation protocol can be simplified. Assuming the time advance of each atomic model is always the same, the global tN need not be computed. Further, all components are always imminent, leading to the cycle shown below.Note: This alternative only is more efficient that the standard DEVS protocol under the conditions of very high activity.

genDevs.simulation.

DTSSCoord.java

genDevs.simulation.

DTSSSim.java

Situation two

Simulators.tellAll() or AskAll() is not efficient if only one or a few models are imminent in every cycle

So, the idea is to get the imminents in every simulation cycle and then only tell those imminents to go through the cycle, instead of telling all simulators to go through the cycle

A new coordinator -- oneDCoord

oneDCoord

tN = minSelTree.getMin()

imminents = minSelTree.getImms()

imminents.tellAll("computeOut“,tN)

imminents.tellAll("sendOut")

imminents = imminents.addAll(influencees)

imminents.tellAll("ApplyDelt“,tN)

imminents.tellAll(“sendTNUp”)

coordinatorsimulators.AskAll(“nextTN”)tN = compareAndFindTN();simulators.tellAll("computeOut“,tN)simulators.tellAll("sendOut")simulators.tellAll("ApplyDelt“,tN)

minSelTree is a data structure which keeps track of the smallest tN and the associated simulators. The last step of the oneDCoord cycle: imminents.tellAll(“sendTNUp”) updates the minSelTree data structure.

The variable imminents are the simulators which has smallest tN in this cycle. The variable influencees are the simulators which are receive messages from the imminent models. Step imminents = imminents.addAll(influencees) updates the imminents to include influencees --- these are the simulators which need to execute ApplyDelt.

Simulate hierarchical coupled model in fast mode

Coordinator

CoupledCoordinator CoupledSimulator4

Atomic4

CoupledSimulator3

Atomic3

simulators.tellAll("initialize“)

simulators.AskAll(“nextTN”)

simulators.tellAll("computeInputOutput“)

simulators.tellAll("sendMessages")

simulators.tellAll("DeltFunc“)

putMessage putMessage

putMessage

CoupledSimulator1 CoupledSimulator2

Atomic1 Atomic2

putMessage

putMyMessageCoupled1

sendDownMessage

Real-time Simulation

Time in Simulation

Time concepts in simulation:

Simulation time

Wall clock time

Two simulation types

Fast simulation – run simulation as fast as possible

Real-time simulation -- time are synchronized with the wall clock time

Real-time simulationWhy need real-time simulation?

Hardware-in-the-loop simulation

Human-in-the-loop simulation

PC

Consider testing a hardware, saying a robot, using simulation methods. The simulation running on PC provides stimuli to the robot to test if the robot moves as expected. Because it takes time for the robot to move, the simulation running on PC has to synchronize with this time. In this case, the simulation running on PC need to be real-time simulation

Real-Time Simulator/Executor

• When first entering the state, s, the simulator starts a thread to sleep for a time ta(s)

• If an external event is injected to the model at some time, text (no earlier than the current clock and no later than tN), the sleeping thread is interrupted and

•If text == tN the output is generated.•Then the input is processed by the confluent or external event transition function, depending on whether text coincides with tN or not.

genDevs.simulation.realTi

meatomicRTSimula

tor

tL =: t

tN =: t + ta(s)

When receive m

if m!= null and t < tN,

s := ext (s,t-tN,m)

if m!= null and t == tN,

s := con (s,t-tN,m)

if m= null and t == tN,

s =: int (s)

s =: s init

tL =: 0

tN =: ta(sinit)

mtimeline

inject at time t

tNtL

Legend:m = messages = statet = clock timetL = time of last eventtN = time of next event

If t == tN

generate output (s)sleep for ta(s)

Execution in real time of an atomic model is similar to its simulation in logical or abstract time. The difference is that rather than being controlled by an events list within the coordinator, each simulator governs itself according to the host processor system clock. Thus passage of time in the model is identical to that of a wall clock.

This difference is seen in the following alteration of the simulator:

real

Real time execution of DEVS models is facilitated by the separation of model from simulator which allows the simulator to be “changed out” by one that adheres to a real clock. The ease of migration from simulation to execution enables a design process called model-continuity discussed later.

Simulate coupled model in real time mode-- centralized way

RTCentralCoord

CoupledSimulator1

Atoimc1

CoupledSimulator3

Atoimc3

CoupledSimulator2

Atoimc2

simulators.tellAll("initialize“)

simulators.AskAll(“nextTN”)

myThread.sleep( nextTN – currentTime)

simulators.tellAll("computeInputOutput“)

simulators.tellAll("sendMessages")

simulators.tellAll("DeltFunc“)

putMessageputMessage

putMessage

RTcoordinator

Time

synchronization

Atomic

ModelAtoimc3

RTSimulator

sideMessagesideMessage

RTSimulator RTSimulator

Atomic

Model

Atomic

Model

sideMessage

Flat Decentralized Distributed Real-Time Simulation with Activities

Real World

DEVS Activity

outputFromActivity

Simulate hierarchical coupled model in real time mode-- Centralized way

RTCentraCoord

CoupledCoordinator CoupledSimulator4

Atomic4

CoupledSimulator3

Atomic3

putMessage putMessage

putMessage

simulators.tellAll("initialize“)

simulators.AskAll(“nextTN”)

myThread.sleep( nextTN – currentTime)

simulators.tellAll("computeInputOutput“)

simulators.tellAll("sendMessages")

simulators.tellAll("DeltFunc“)

CoupledSimulator1 CoupledSimulator2

Atomic1 Atomic2

putMessage

putMyMessageCoupled1

sendDownMessage

Simulate hierarchical coupled model in real time mode-- decentralized way

RTCoordinator

RTCoupledCoordinator CoupledRTSimulator4

Atomic4

CoupledRTSimulator3

Atomic3

putMessage putMessage

putMessage

CoupledRTSimulator1 CoupledRTSimulator2

Atomic1 Atomic2

putMessage

putMyMessageCoupled1

sendDownMessage

simulators.tellAll("initialize“)

simulators.tellAll("simulate“)

simulators.tellAll("stopSimulate")

Running a digraph model in Fast-mode

To run any digraph model in fast mode, provide a main function in the class that uses a coordinator, explicitly. Below is the code for the digraph model “HierarModel” in package GenDevsTest.

public static void main(String[] args){ HierarModel hm = new HierarModel();

coordinator c = new coordinator(hm); //create a coordinator and pass the digraph model c.initialize(); //initialize the coordinator c.simulate(600); //provide the number of iterations to execute }