On improving efficiency of model checking through systematically ...

Our method to get out of the problem is to restrict the number of iterations
that a system can loop around the good location. However, this
method is not a complete solution since it means that only a part of the
state space is verified.
In conclusion, experiment results on a few case studies are negative at
the time when this thesis is written. It remains the problem of state space
explosion and thus we have not reached our goal of efficiently obtaining a
reasonable combination of two tools. Anyhow, our work sets up a foundation
for the evaluation of the possibility of combining Nbac and PMC/Uppaal and
for further improvements to solve the problem of combining these tools. This
work also helps us to gain more insight on approaches implemented in these
tools and the general problem of model checking. We found that abstract
models generated by Nbac are not very well-fitted to be verified by Uppaal.
An Nbac abstract model helps Nbac to accelerate the verification but not
Uppaal for the reason that the Nbac abstract model is defined to work with
the verification approach in Nbac only. We believe that this problem is
dependent on specific tools employed in our combination approach.
Related work. There have been studies to employ abstraction techniques
to model checking to overcome the state space explosion problem,
for example [14, 17]. The main difference is that the abstraction technique
is “embedded” into the model checking algorithm in those works. That
means the data structures and algorithms are tailored for the abstraction
technique used. Our approach works at a higher level, between existing
tools and models of the system being verified. On the one hand, integrating
abstraction technique into the model checking algorithm gives us more control
and flexibility in the development of a specific model checker. On the
other hand, combining existing tools provides the possibility to reuse the
solution and part of the implementation work if we are successful with the
approach. However, to our knowledge, there has been no successful story of
this direction reported until now.
A successful work to develop an independent abstraction tool for explicit
model checking approach in Spin is reported in [15]. This work leads to
the idea of developing a general abstraction tool intentionally for a class of
symbolic model checking approach in real-time model checkers such as PMC,
Uppaal, Hytech, etc. Such an abstraction tool will be more useful for these
model checkers than Nbac since Nbac is not developed to serve as a general
abstraction tool for them.
Future work. This thesis work started from the combination between
Nbac and PMC but ended with the combination between Nbac and Uppaal.
We conclude that it is impossible to proceed the work of combining Nbac
and PMC with the current status of PMC. However, it should be possible to
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