On improving efficiency of model checking through systematically ...
On improving efficiency of model checking through systematically ...
On improving efficiency of model checking through systematically ...
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start<br />
L2==0<br />
start<br />
L1:=0<br />
l2<br />
l3<br />
L2:=1<br />
w1:=Latest<br />
l2<br />
L2:=0<br />
L2==1<br />
l4<br />
Prefer1:=w1<br />
l3<br />
l4<br />
l5<br />
L1==0<br />
L1:=1<br />
l6<br />
ridx:=Prefer2<br />
r1:=Latest<br />
Prefer2:=r1<br />
L1==1<br />
l7<br />
ridx:=Prefer1<br />
reading<br />
l9<br />
l5<br />
L1==0<br />
L1:=1<br />
l6<br />
L1==1<br />
l7<br />
w2:=Prefer1 w2:=Prefer2<br />
l8<br />
l11<br />
Precon:=w2<br />
l10<br />
j:=Latest<br />
w2:=Precon<br />
(w2==3 and j==2) or<br />
(w2==3 and j==3) or<br />
(w2==2 and j==3)<br />
widx:=1<br />
widx:=3<br />
writing<br />
(w2==1 and j==2) or<br />
(w2==2 and j==1) or<br />
(w2==2 and j==2)<br />
widx:=2<br />
(w2==1 and j==1) or<br />
(w2==1 and j==3) or<br />
(w2==3 and j==1)<br />
Figure 4.2: Reader and Writer graph for the algorithm<br />
bounded. Moreover, there is no real-time in this <strong>model</strong>. Since this <strong>model</strong><br />
contains many locations and transitions, the Nbac <strong>model</strong> is large in terms<br />
<strong>of</strong> physical size.<br />
We choose it as a case study because PMC proves that the algorithm does<br />
not satisfy the safety property. We are interested in <strong>checking</strong> whether we<br />
get the same result using our approach. However, Nbac and Uppaal can<br />
prove the safety property separately. Despite <strong>of</strong> that fact, this system still<br />
remains a good case study for its characteristic which is a finite system and<br />
large in physical size.<br />
4.3 Fischer’s protocol<br />
Fischer’s protocol is a mutual exclusion algorithm in which n processes uses<br />
timing constraints and a shared variable to ensure mutual exclusion. Figure<br />
4.3 shows two processes from Fischer’s protocol. Each process has a unique<br />
identifier, a clock x and 4 locations A, B, C and CS. They share a common<br />
variable namely lock. From location A each process can make a transition<br />
to location B if lock has value 0. It stays in B for k seconds before assigning<br />
its id to lock. After setting value for lock, it goes to location C. Then<br />
the process may enter the critical section (location CS) after it spends at<br />
40
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