Process Synchronisation

Question

Consider the below Algorithm:

P0: Process= 0                                                    P1: Process= 1
other= 1-Process                                                 other= 1-Process
while (flag!= Process);                                         while (flag!= Process);
Enter CS                                                              Enter CS
………                                                                  ………
………                                                                  ………
flag= other.                                                           flag= other.

(a) The algorithm guarantees Mutual Exclusion
(b) The algorithm is busy waiting solution
(c) The algorithm does not guarantee progress
(d) The algorithm guarantee Bounded waiting

Comments

i think option c is also correct. it doesnot guarantee progress. if p0 does not want to enter critical section and flag points to p0 then it will block p1 to enter cs.

please comment if i am wrong

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yes I am getting the same ,

lets say initially , FLAG=0, and following is the numbering of instructions

1, P0: Process= 0                                                    5. P1: Process= 1
2. other= 1-Process                                                 6. other= 1-Process
3. while (flag!= Process);                                          7. while (flag!= Process);
    Enter CS                                                                Enter CS
     ………                                                                  ………
     ………                                                                  ………
4.flag= other.                                                           8. flag= other.

then the following order of execution will lead in NO PROGRESS

Po:1,2,3,CS,4 || Po:1,2,3(block)

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if process =1 and flag = 0 then then there is deadlock in system this implies

Mutual exclusion and no progress also if P0 or P1 want to enter C.S then they can enter again and again so there is no bounded waiting

Answer 1

If the question was which of these is/are True then option a,b,c and d all are true...

i) Used Loop Controls =====> Busy Waiting Solution...

ii) After Updating local variable other only both processes are going into loop ===> only one can enter ==> Mutual Exclusion Guarantee

iii) the sequence of execution is

           p0,p1,p0,p1,p0,p1,p0,..... ( if Flag=0 initially )

           p1,p0,p1,p0,p1,p0,p1,..... ( if Flag=1 initially )

      in both cases no long waiting ====> Bounded waiting ( waiting time = 1 ===> strict alternation )

iv) if the p0 enter into CS and exit flag updated then it only allows p1 to enter.. if P1 doesn't want to enter... then even p0 wants to enter it can't ====> not guarantee progress

Answer 2

Option A

As the code only allows only one process to be in critical section.

Comments

brother , I know the answer , can u please elaborate ,in a diagram.

I would be really glad

Answer 3

(b) The algorithm is busy waiting solution

This option is also true along with option A.

option b. is true because while loop create busy waiting it will continusly checking.

Answer 4

Answer: A,C,D

Option (B) is not included as we can see if process 0 is using critical section then process 1 is in waiting state which mean it continuously iterating the while to check condition till the flag become 1, so this is note the solution for busy waiting, for busy waiting solution we use semaphore,

Comments

It’s busy waiting because if Process 0 does not update the flag then Process 1 will stuck in while loop busy waiting for Process 0 to update the flag value. All options are correct.

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yes it is busy waiting but its not the solution for busy waiting