Performance Improvements for Large-Scale Traffic Simulation in MATSim - Computational Approaches for Urban Environments - page 209

9.2.4 DEQSim

A major performance breakthrough, within the MATSim context, is achieved
with a more recent micro-simulation called Deterministic Event-Driven Queue-
Based Traffic Flow Micro-Simulation (DEQSim, Charypar et al. 2007a ), which
is implemented in C CC . Instead of performing the simulation along fixed time
steps, an event-based model is used performing only discrete actions which are
relevant to the model, i.e., entering and leaving roads. Furthermore, DEQSim has
been parallelized making it one of the fastest large-scale transport micro-simulations
currently available (Charypar et al. 2007b ).
A major drawback of DEQSim within the MATSim context is that it is imple-
mented in C CC , while the other modules of MATSim are implemented in Java.
This means that the communication of DEQSim with the other MATSim modules
is bridged by a slow file input/output (I/O) interface.

9.2.5 Graphical Processing Units

Yet a different approach to accelerate the micro-simulation was tried using graphical
processing units (GPUs) on computer graphic cards. These GPUs perform many
more operations in the same amount of time than central processing units (CPUs) on
computer boards. A first successful implementation of QueueSim on GPUs rendered
a speedup of 67 times (Strippgen and Nagel 2009 ) compared to JQueueSim. The
main drawback of GPUs is similar to that of DEQSim, as the interface between
the graphic card and the rest of MATSim modules poses a bottleneck. Furthermore,
current GPUs have a limited amount of memory. For example, the traffic simulation
of Switzerland, with 7.3 million agents, requires around 60 GB of memory. Graphic
cards today have often less than 4 GB of memory. Also, maintainability of the
implementation is an issue, as the program code is not written in Java.
In order to provide a faster micro-simulation in Java than QueueSim, here a
redesign and re-implementation of DEQSim in Java is proposed, called JDEQSim .
A second feature, for which an improvement is also proposed, is the event-handling
module in MATSim, which is described in the next section.

9.2.6 Event Handling

The output of the traffic simulation contains detailed information about the course
of the simulation. It describes, e.g., when an agent's vehicle enters a road or arrives
at an activity location. This information is embedded within a data structure called
“events,” which contains information such as the identification of the agent and the
link where the event occurred and its time. These events are used to communicate
the output of the micro-simulation to other modules in MATSim, such as scoring