1 by vanessa newey. 2 introduction background scalability in distributed simulation traditional...
Post on 21-Dec-2015
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Introduction
Background Scalability in Distributed Simulation Traditional Aggregation Techniques Problems with Traditional Methods Projection Aggregation Future Directions Summary
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Distributed Interactive Simulation Systems Include:
Multiplayer Video Games, Collaborative EngineeringMilitary and Industrial training
Systems are Growing Can be hundreds of thousands of dynamic
entities
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Scalability in Distributed Simulation Number of entities that may simultaneously
participate in the system. Entity = participating object that is separately
modeled. Scalability depends on
Network Capacity Processor capabilities Rendering speeds Speed of throughput of shared servers
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Limits to Scalability
Host has to:Receive updatesModel and render scene Other tasks, including collision detection
So…as the number of entities increases Increased Load on Network Resources. Increased Computational Load
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Limits to Scalability cont.
Even with dead-reckoning on a large simulation >125,000 packets/second
Approaching limit of interrupts for a general-purpose processor
Rapidly increasing Computational Load due to: Increasing number of entities Increasing detail of entity models More fine-grained graphical representations
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Aggregation
Used to reduce network and computational load.
“An aggregations is a simulation entity that represents a group of other entities.”
Previously used aggregation techniques are organisational and grid location
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Organisational Aggregation Groups entities by their organisational structure.
e.g. armies, brigades, battalions etc Easy to construct An organisation’s members may be dispersed
throughout regions of the VR world In military simulations up to half the entities are
destroyed Each host has to receive information from all
organisation represented within that region Does not sufficiently reduce network traffic or the
computational load
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Grid Aggregation Groups entities by their location within the virtual
world. Virtual world divided into regions, each is
associated with an aggregation that transmits information about entities in that region
Masks organisational structure For a host to send summary information about a
battalion of tanks, then it must subscribe to information from all regions that could potentially contain one of those tanks.
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Grid Aggregation cont. Does not allow remote hosts to receive different
fidelity information for different entity types. Does not allow hosts to access entities by their
organisation or type, so limited value for reducing network traffic or computational load.
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Projection Aggregation Unifies the organisational and grid
aggregation strategies. Each projection aggregation includes
entities from a single organisation located within a single grid of the virtual world.
Oraganisation is “projected” onto the virtual world.
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Projection Aggregation cont. Each periodically transmits summary information
about its members across the network. Hosts use projections to represent groups of
remote entities that do not merit local modeling at high detail because far from viewer or
Useful abstraction for describing all entities in a simulation
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Projection Aggregation Summary Protocol Each periodically summarises its member entities Transmits summary packets over an associated
multicast address. Enough information in packets for remote hosts
to generate a low fidelity model of those member entities.
Transmitted relatively infrequently (2-3 seconds) Regular entities transmit every second Errors not noticeable
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Projection Aggregation Summary Packets Contain
count of entities represented a single position point (average position) Radius of bounding sphere Distribution information (mean, standard deviation)
On remote host to place each entity the simulation generates a
distribution of locations within the bounding sphere, given the mean and standard deviation.
Projection aggregation’s center point and mean distance can be dead reckoned.
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All entities treated as Projection Aggregation
All entities can be treated as projection aggregations Only the rendering algorithm is different Remote hosts can dynamically change the fidelity of their
local entity model without impacting other hosts Projections are easily integrated into existing
simulations. Provides a natural mechanism for introducing more
detailed entity models into an existing simulation.
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Heirachical Projection Aggregation Projection Aggregations are arranged in a heirachy A projection aggregation is associated with a parent
aggregation (a broader organisation and a larger grid. Each organisation maintains links to all its descendent
entities. Grids maintain links to all projections in that grid Allows top-level filtering, reducing processing Collision detection algorithms can use projection
aggregations to quickly filter unlikely or uninteresting collisions.
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Experience with Projection Aggregations Used in PARADISE distributed simulation to
assess the effectiveness of projections 72% less packets. Host’s multicast subscriptions down by 40% Implemented and managed in roughly 4000 lines
of code
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Conclusion
Projections provide a single abstraction for all simulation entities.
Design allows hosts to view each entity in several ways: Part of an organisation Part of a world Grid Part of a projection aggregation summary
Projection aggregations reduce both network and computational requirements.
A promising technique for network and computational resources
support the evolution of more detailed entity models
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More recently
MASSIVE-3 uses abstraction as well, but more spatially focused
In Singhal’s book he describes a slightly different approach. Where objects subscribe information sets.