Scheduling Algorithm

Question

 

Answer 1

Why complicate this one . first of all one tip. never panic seeing a question. whenever u see such a question the answer is very easy just rewind the concept . 

so here it is asking cpu efficincy . simply cpu efficency = usefull work done by cpu/ total time .

now there are n processes which will be running and after every process context switch willl occur.

total time=n*time qunatun(q)+ n*context switch time (s)=nq+ns .
what is the usefull work done by cpu . we know context switching the wastage time . the usefull time will be only nq. because in that time cpu was doing usefull work . 

A-

cpu efficiency =nq/nq+ns=q/q+s

b- now it is giving s=q. just replace s with q then it is q/q+q=q/2q=1/2
 

Answer 2

cpu efficiency = useful time of cpu / total cpu time

given r is avg running time .

q is quantum time

now we have to calculate how many time processes are switched ,so no of switched = r/q

so total time in switching is s.*r/q

total cpu time is = avg running time + swiching overhead = r +r/q*s

and usefull time is only r .

so efficiency is r/(r(q+s)/q)= q/(q+s)

2) q=s so cpu efficiency = 1/2

Comments

Can you please explain how the no of switches is r/q?

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given here q<r so run of r requied r/q switch .. let if avg time is 4 and q is 2 then run of 4 required 2 switch ..that y no of switches is r/q .  now i think ,its clear

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@sonam

Suppose this be the scenario.n=3 processes.p1 executes then comes again after p2 p3  gets turn.

p1 |p2 |p3 |p1

tq=5

so as per question r=10(note it says average run time of A process)

now r/tq=2 which is no of context switch involving p1 no total no of cs which is 3

unless we have idea of  total no of processes i cnt find total context switch time

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here avg running time given for all process , so no need to know how many process are there.. yes if given only for one process then we have to know how many processes are there and whats there running time ,so how many time they switched ...

Answer 3

for question1 . total no.of context switching =r/q; so time spend in context switching = s*r/q; total time in process completion=r+s*r/q; efficiency= useful time/total time; then efficiency=r/(r+s*r/q) =r*q/r*(q+s) q/(q+s). answer (A) is right. **FOR QUESTION 2** put q=s in above answer then we get efficiency=q/(q+q); =1/2 answer (c) is right.