Question

5.25. The following pseudocode (next page) is a correct implementation of the producer/consumer problem with a bounded buffer:

Labels p1, p2, p3 and c1, c2, c3 refer to the lines of code shown above (p2 and c2 each cover three lines of code). Semaphores empty and full are linear semaphores that can take unbounded negative and positive values. There are multiple producer processes, referred to as Pa, Pb, Pc, etc., and multiple consumer processes, referred to as Ca, Cb, Cc, etc. Each semaphore maintains a FIFO (first-in-first-out) queue of blocked processes. In the scheduling chart below, each line represents the state of the buffer and semaphores after the scheduled execution has occurred. To simplify, we assume that scheduling is such that processes are never interrupted while executing a given portion of code p1, or p2, . . . , or c3. Your task is to complete the following chart.

item[3] buffer; // initially empty semaphore empty; // initialized to +3 semaphore full; // initialized to 0 binary_semaphore mutex; // initialized to 1
void producer()	void consumer()
{	{
...	...
while (true) {	while (true) {
item = produce();	c1:     wait(full);
p1:   wait(empty);	/    wait(mutex);
/         wait(mutex);	c2:     item = take();
p2:   append(item);	\    signal(mutex);
\      signal(mutex);	c3:     signal(empty);
p3:   signal(full);	consume(item);
}	}
}	}

Scheduled Step of Execution	full’s State and Queue	Buffer	empty’s State and Queue
Initialization	full=0full=0	OOO	empty=+3empty= +3
Ca executes c1	full=−1full= −1 (Ca)	OOO	empty=+3empty= +3
Cb executes c1	full=−2full= −2 (Ca, Cb)	OOO	empty=+3empty= +3
Pa executes p1	full=−2full= −2 (Ca, Cb)	OOO	empty=+2empty= +2
Pa executes p2	full=−2full= −2 (Ca, Cb)	X   OO	empty=+2empty= +2
Pa executes p3	full=−1full= −1 (Cb) Ca	X   OO	empty=+2empty= +2
Ca executes c2	full=−1full= −1 (Cb)	OOO	empty=+2empty= +2
Ca executes c3	full=−1full= −1 (Cb)	OOO	empty=+3empty= +3
Pb executes p1	full= full=		empty= empty=
Pa executes p1	full= full=		empty= empty=
Pa executes __	full= full=		empty= empty=
Pb executes __	full= full=		empty= empty=
Pb executes __	full= full=		empty= empty=
Pc executes p1	full= full=		empty= empty=
Cb executes __	full= full=		empty= empty=
Pc executes __	full= full=		empty= empty=
Cb executes __	full= full=		empty= empty=
Pa executes __	full= full=		empty= empty=
Pb executes p1-p3	full= full=		empty= empty=
Pc executes __	full= full=		empty= empty=
Pa executes p1	full= full=		empty= empty=
Pd executes p1	full= full=		empty= empty=
Ca executes c1-c3	full= full=		empty= empty=
Pa executes __	full= full=		empty= empty=
Cc executes c1-c2	full= full=		empty= empty=
Pa executes __	full= full=		empty= empty=
Cc executes c3	full= full=		empty= empty=
Pd executes p2-p3	full= full=		empty=

Answer 1

Solution:

The completed chart for scheduling processes with corresponding values for full and =pc> semaphores are shown below.

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