LARGE-SCALE PARALLEL GRAPH-BASED SIMULATIONS - MATSim

3.6 Conclusions and Discussion
Computationally fast programs can be achieved by not only introducing parallel programming
techniques into an implementation, but also accelerating the sequential parts of the implementation.
When a data structure is accessed for its entities frequently, the type of data structure has a
prominent effect on performance. The sequential implementation of the traffic flow simulation
is improved such that:
Storage for graph data is modified in a way that the STL-map is replaced by STLvector
and the searching algorithm is modified to use binary search. The performance
gets better by 15-18% compared to the STL-map version.
Storage for parking and waiting queues, on which vehicles at the beginning, are often
accessed and removed, are advanced from the STL-multimap to a user-implemented
singly linked list structure that results in a 9-18% performance increase.
Representation of link cells as an STL-list degrades performance by 13-67% compared
to using the STL-deque. Even better speed-up (40-71% relative to the STLlist)
is achieved when using a user-defined data structure called Ring, which is nothing
but a circular STL-vector composed of pointers to vehicles.
Operations on files depend on the format of data stored in them. The XML-type files
are flexible in terms of management and are elegant, but they usually give worse performance.
The inherent simplicity of structured text files offer better performance but a lack
of flexibility limits their applicability.
The input stream operator >> of C++ promotes easy usage by letting users to not worry
about types of input to be read, but suffers from low performance.
Therefore, the following conclusions are drawn for the best design of MATSIM:
As the graph data, i.e. the links and nodes, being the most frequently accessed data, it is
best represented by the STL-vector. The binary search algorithm of STL should be
used finding an element in the vector structure.
The parking and waiting queues of the links are the data structures which hold the vehicles.
The vehicles on these queues have not started simulating yet either because of full
links or because of travelers performing activities at the location. Removing all the eligible
vehicles at the beginning of these queues prior to inserting them into other queues,
and inserting new vehicles into these queues are best performed on a data structure defined
as a singly linked list so that each vehicle points to another vehicle.
The spatial queues and buffers of the links are best implemented by using a fixed size
vector, elements of which are pointers to vehicles. The movement operations such as
insertion and deletion should be based on pointers to vehicles.
For the input and output data, XML files should be preferred to structured text files since
XML allows constructing user defined complex structures in a more efficient way.
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