LARGE-SCALE PARALLEL GRAPH-BASED SIMULATIONS - MATSim

Abstract
When systems are modeled, different techniques are used. Computer simulation is one of these
techniques. It draws attention since it makes it possible to model a system that might be real
or theoretical, to execute the model on a computer, and to analyze the output of the execution
of the model. Execution of a model on a computer develops through time, i.e. the states of
the different parts of a system, such as variables and environment, are updated through time
according to the rules defined in the model.
Computer simulations come into prominence since they allow models to have complex
objects/variables, allow objects to have complex relationships, allow users to model artificial
worlds, etc. This thesis focuses on different parts of a transportation planning system, MATSIM
(Multi-Agent Transportation SIMulation), which is a computer simulation.
In MATSIM, similar to other multi-agent simulations, all entities are treated at the individual
level. Their behavior and interactions, both with each other and with the environment, are
defined by their internal rules.
There are two layers in a transportation planning system: the physical layer that includes
a traffic flow simulator, and the strategic layer. In the traffic flow simulator, the agents are
interacts with each other and with the environment based on the rules defined in the model. In
the strategic layer, the agents make their strategies. The relationship between these two layers
is best understood in an implementation called a framework.
A framework couples the modules such as traffic flow simulator, router, agent database,
activity generator, etc. A traffic flow simulator defines the rules of interactions of the entities in
the system. The traffic flow simulator used in MATSIM is based on a queue model developed
by Gawron. It reads the street network of the area to be simulated and the plans of the agents,
then it executes these plans according to the rules of the queue model. The output of the traffic
flow simulation, the events, are used to evaluate the performance of the plans. The evaluation
is achieved by the modules of the strategic layer. The evaluated plans are fed to traffic flow
simulator simulator by starting a new iteration.
Parallel computing issues are applied to the traffic simulator to handle the large-scale scenarios
detailed at microscopic level. Different communication media and different communication
libraries are used during this process.
The coupling of modules by framework is via files. From the viewpoint of a traffic flow
simulator, this means two files: plans as input and events as output. To avoid the inefficiencies
of file I/O, a message passing approach is developed for plans and events. Different methods
for creating and transferring different types of messages are investigated.
The traffic flow simulator based on the queue model can be used for simulating other types
of entities such as Internet data packet traffic. As Internet grows more, analyzing the data
flowing through Internet becomes more interesting between researchers.
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