On improving efficiency of model checking through systematically ...

the size of the state space based on the amount of memory usage is shown
hereafter.
State space explosion has been a well-known problem of model checking
in practice. Therefore, it is not a surprise that we could not successfully
verify a model. However, experimental results from verifying models of
the Fischer’s protocol and Subway system are still disappointing since Nbac
could prove the same models in a short time. We then figure out the reason
for such large state space to answer the second question posed in Section
1.3 of this thesis.
Wetriedtogetaviewonthesizeandgrowthofthestatespaceusinga
practical method. We used the simulation utility of Uppaal to see how the
system worked. Then we found the execution trace looped around the good
location. First, we measured the memory usage with respect to the number
of iterations around the good location. This measurement was to estimate
the size of state space of the model and to see how it grew after every
iteration. To measure the memory usage for that purpose, we modified the
resulting Uppaal model manually by adding a counter so that if the number
of iterations around the good location exceeded the constant specifying the
maximum number of iterations allowed, the system would stop and thus
the verification could finish. The implementation of the abovementioned
mechanism is illustrated in Figure 5.1.
counter < N
counter := counter + 1,
x:=x+1
init good bad
success
counter >= N
Figure 5.1: Adding a counter to restrict the number of iterations
In Figure 5.1, N is a constant specifying the maximum number of iterations
allowed. From the measurements on the verification of the modified
model, we found that the memory usage grew quickly as we increased constant
N. Figure 5.2 presents the memory usage of the experiment for the
subway model. The maximum N for which the verification could be finished
is 19.
The measurement showed that the state space created from the abstract
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