This question came up in a practice exam. I do not really understand how they got to the answers that they said are correct.
I was hoping for some help understanding the question and how to answer it?
Thank you!
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Do you understand what P and V do ? https://en.wikipedia.org/wiki/Semaphore_%28programming%29#Operation_names http://stackoverflow.com/questions/29606162/what-is-the-original-meaning-of-p-and-v-operations-in-a-context-of-a-semaphore – Thilo May 10 '16 at 15:08
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I do now i have read the links you have provided – imconnor May 10 '16 at 15:18
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Good. Do you then understand what the correct answers are and why? – Thilo May 10 '16 at 15:24
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No i still do not understand – imconnor May 10 '16 at 15:29
1 Answers
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I think it all comes to this:
wait (P): If the value of semaphore variable is not negative, decrements it by 1. If the semaphore variable is now negative, the process executing wait is blocked (i.e., added to the semaphore's queue) until the value is greater or equal to 1. Otherwise, the process continues execution, having used a unit of the resource.
For the correct answers:
1) Since the semaphore is initially equal 0, thread 3 will block on P(S) waiting for another thread to do V(S). This will only happen on thread 1, and after A is completed. So no matter how long statement A takes to execute, thread 3 will wait until the instruction V(S) is executed. So A will always be executed before F.
2) The same concept applies with B and G. Before G you have to execute P(T) and this will wait for an instruction V(T). This only happens after B has been executed.
3) Since A is executed before F as demonstrated in (1), and F is always executed before G, A is always executed before G.
As for the incorrect answers:
a) A is executed before E? Maybe, but not always. Because thread 1 and thread 2 have to wait for a semaphore, thread 2 could execute B, V(T) and E faster than A, so in this case the sentence (a) is false.
b) B is executed before F? Maybe, but not always. Why? To execute F, thread 3 only depends on thread 1 (the semaphore S) so C and A can execute very fast and then go to F while B can be still executing because it is very slow.
d) C is executed before D? Maybe, but not always. Again, C might take a long time to execute and thread 1, since it has not to wait for any semaphore, could execute all its instructions very fast, before C is completed.
user1156544
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Is everything clear or do you want me to elaborate a bit further? Don't carry any questions : ) better clarify them now – user1156544 May 10 '16 at 15:49