Question

Exercise 4.4.1: Tracing a monitor execution. About While a process is executing inside the function A of the monitor m, the following calls are issued by different processes: m.A0; m.B0; m.B0; m.B0; m.B(; m.A); m.A(); monitor m { int x = 10, у %3D 2 condition c A ( ) { (1) (2) (3) X++ c.signal у 3 х — 2 } B( ) { if (x > 10) (4) (5) (6) (7) х-- else {c.wait x--} } (a) Using the line numbers in the code, trace the sequence of instruction execution. Show any changes of x and y at the end of each instruction. Feedback?

Answer 1

Let us first understand wait and signal operation.

wait(): process which performs wait operation on condition variable c will get suspended and will be placed in block queue of c.
signal(): Wherever a process performs a signal operation on condition variable c, one process from blocked queue of c will be executed. If block queue is empty, this will be ignored.

Let us represent block queue by q.

(a)    Initially, x = 10, y = 2, q is empty.

          m.A() : line 1 : x++; x = 11
                       line 2 : c.signal : q empty : ignore signal
                       line 3 : y = x- 2; y = 11 - 2 = 9

           Now, x = 11, y = 9, q empty
           m.B() : line 4 : if (x > 10) : true as x = 11
                         line 5 : x--; x = 10

           Now, x = 10, y = 9, q empty
           m.B() : line 4 : if (x > 10) : false as x = 10
                         line 6 : c.signal : this process will be placed in q

           Now, x = 10, y = 9, q = (m.B())
           m.B() : line 4 : if (x > 10) : false as x = 10
                         line 6 : c.signal : this process will be placed in q

           Now, x = 10, y = 9, q = (m.B(), m.B())
           m.B() : line 4 : if (x > 10) : false as x = 10
                         line 6 : c.signal : this process will be placed in q

           Now, x = 10, y = 9, q = (m.B(), m.B(), m.B())
           m.A() : line 1 : x++; x = 11
                         line 2 : c.signal : run first m.B() from q : line 7 : x-- : x = 10

           Now, x = 10, y = 9, q = (m.B(), m.B())
           m.A() : line 1 : x++; x = 11
                         line 2 : c.signal : run first m.B() from q : line 7 : x-- : x = 10

            Final values will be x = 10, y = 9, q = (m.B())