Parallel and Distributed Simulation

1. Parallel and Distributed Simulation Distributed Virtual Environments (DVE) & Software Introduction

2. Outline • Intro: DVE example • Federated Simulations • Basic Services • Class Based Subscription • Attribute updates • Tick • Synchronized delivery: Time management

3. network visual display system Image Generator Other Vehicle State Table network interface terrain database control/ display interface own vehicle dynamics sound generator controls and panels A Typical DVE Node Simulator • Execute every 1/30th of a second: • receive incoming messages & user inputs, update state of remote vehicles • update local display • for each local vehicle • compute (integrate) new state over current time period • send messages indicating new state Reproduced from Miller, Thorpe (1995), “SIMNET: The Advent of Simulator Networking,” Proceedings of the IEEE, 83(8): 1114-1123.

4. visual display 3 6 7 4 8 1. Detect trigger press Image Generator Other Vehicle State Table network interface 2. Audio “fire” sound 3. Display muzzel flash terrain database control/ display interface own vehicle dynamics sound generator 4. Send fire message 5. Display muzzel flash Controls/panels 6. Compute trajectory, display tracer 1 2 7. Display shell impact visual display 5 9 4 8 11 8. Send detonation message Image Generator Other Vehicle State Table network interface 9. Display shell impact 10. Compute damage terrain database 11. Send Entity state message indicating damage control/ display interface own vehicle dynamics sound generator 10 Controls/panels Typical Sequence

5. Federated Simulations Interconnecting autonomous simulators Federation Simulation (federate) Simulation (federate) Simulation (federate) Interface Specification Interface Specification Runtime Infrastructure(RTI) • Services to create and manage the execution of the federation • Federation setup / tear down • Transmitting data among federates • Synchronization

6. Federates not designed for a specific federation • Internet gaming simulations • Plug ‘n play capability • Number, type of players change dynamically • Department of Defense High Level Architecture • Model reuse: Federates may be used in different federations during its lifetime • Legacy simulations: not originally designed for inclusion in federations

7. Message Passing Alternatives • Traditional message passing mechanisms: Sender explicitly identifies receivers • Destination process, port, etc. • Poorly suited for federated simulations • Broadcast • Receiver discards messages not relevant to it • Used in SIMNET, DIS (initially) • Doesn’t scale well to large federations • Publication / Subscription mechanisms • Analogous to newsgroups • Producer of information has a means of describing data it is producing • Receiver has a means of describing the data it is interested in receiving • Used in High Level Architecture (HLA)

8. Class-Based Data Distribution (simplified) • Federation Object Model (FOM) defines type of information transmitted among federates • Object classes (e.g., tank) • Attributes (e.g., position, orientation of turret) • Primitives (Federate/RTI interface) • Publish Object Class Attributes: Called by a federate to declare the object classes and attributes it is able to update • Subscribe Object Class Attributes: Declare the object classes and attributes that the federate is interested in receiving • Register Object Instance: Notify RTI an instance of an object has been created within the federate • Discover Object Instance: Notify federate an instance of an object of a subscribed class has been registered • Update Attribute Values: notify RTI one or more attributes of an object has been modified • Reflect Attribute Values: notify federate attributes to which it has subscribed have been modifed

9. SubscribeOCA (Tank, position) PublishOCA (Tank, position) DiscoverOI (Tank, instance) handle := RegisterOI (Tank) UpdateAV (handle, position, <30,89>) ReflectAV (instance, position, <30,89>) Example OCA = Object Class Attributes OI = Object Instance AV = Attribute Values Federate 1 Federate 2 RTI

10. Federated vs. RTI Initiated Services Some services are initiated by the federate, others by the RTI • Federate invoked services • Publish, subscribe, register, update • Not unlike calls to a library • Procedures defined in the RTI ambassador • RTI invoked services • Discover, reflect • Federate defined procedures, in Federate Ambassador Federate Federate ambassador Update Reflect RTI ambassador RTI

11. Single vs. Multi-threaded • Multi-threaded implementation • RTI software and federate code execute as separate threads of execution • Switching execution between RTI and federate largely transparent to the federate • Single-threaded implementation • RTI and federate share a single thread of execution (like libraries) • Federate must explicitly pass control to the RTI, e.g., to deliver messages • Tick() procedure is defined for this purpose • Callbacks (Discover, Reflect) made within Tick

12. Federate RTI Tick ReflectAttributeValues return (RAV) return (Tick) Example: Receiving a Message Boolean: Waiting4Message; { … Waiting4Message := TRUE; while (Waiting4Message) Tick(); … } /* Federate ambassador */ Proc ReflectAttributeValues (…) { save incoming message in buffer Waiting4Message := FALSE; }

13. Synchronizing Message Delivery Goal: process all events (local and incoming messages) in time stamp order; To support this, RTI will • Deliver messages in time stamp order (TSO) • Synchronize delivery with simulation time advances next TSO message RTI TSO messages next local event T’ federate local events logical time T current time Federate: next local event has time stamp T • If no TSO messages w/ time stamp < T, advance to T, process local event • If there is a TSO message w/ time stamp T’ ≤ T, advance to T’ and process TSO message

14. Typical execution sequences Federate RTI Federate RTI NER(T) NER(T) NER: Next Event Request TAG: Time Advance Grant RAV: Reflect Attribute Values Federate calls in black RTI callbacks in red RAV (T’) TAG(T) RAV (T’) TAG(T’) Wall clock time RTI delivers events no TSO events Next Event Request (NER) • Federate invokes Next Event Request (T) to request its logical time be advanced to time stamp of next TSO message, or T, which ever is smaller • If next TSO message has time stamp T’ ≤ T • RTI delivers next TSO message, and all others with time stamp T’ • RTI issues Time Advance Grant (T’) • Else • RTI advances federate’s time to T, invokes Time Advance Grant (T)

15. Code Example: Event Stepped Federate sequential simulator T = current simulation time PES = pending event set While (simulation not complete) T = time of next event in PES process next event in PES End-While federated simulator While (simulation not complete) T = time of next event in PES PendingNER = TRUE; NextEventRequest(T) while (PendingNER) Tick(); process next event in PES End-While /* the following federate-ambassador procedures are called by the RTI */ Procedure ReflectAttributeValues (…) place event in PES Procedure TimeAdvanceGrant (…) PendingNER = False;

16. Typical execution sequence Federate RTI TAR: Time Advance Request RAV: Reflect Attribute Values TAG: Time Advance Grant Federate calls in black RTI callbacks in red LT=10 TAR(20) RAV (14) RAV (18) TAG(20) LT=20 Wall clock time Time Advance Request (TAR) • Typically used by time stepped federates • Federate invokes Time Advance Request (T) to request its logical time (LT) be advanced to T • RTI delivers all TSO messages with time stamp ≤ T • RTI advances federate’s time to T, invokes Time Advance Grant (T) when it can guarantee all TSO messages with time stamp ≤ T have been delivered • Grant time always matches the requested time

17. Code Example: Time Stepped Federate sequential simulator T = current simulation time While (simulation not complete) update local simulation state T = T + ∆T; End-While federated simulator While (simulation not complete) update local simulation state UpdateAttributeValues (…) PendingTAR = TRUE; TimeAdvanceRequest(T+ ∆T) while (PendingTAR) Tick(…); T = T + ∆T; End-While /* the following federate-defined procedures are called by the RTI */ Procedure ReflectAttributeValues (…) update local state Procedure TimeAdvanceGrant (…) PendingTAR = False;

18. Summary • Distributed simulation: federates, RTI • Federates • Model some subset of entities in virtual world • Send messages to other federates concerning aspects of entities that are “visible” to entities in other federates • RTI services • Setting up and realizing communication among federates • Control and management of federation execution • Synchronizing message delivery (if needed)