CPU Scheduling

Question

Q1)
An OS contains 10 identical processes that were initiated at the same time. Each process contains 15 identical requests, and each request consumes 20msec of CPU time. A request is followed by an I/O operation that consumes 10 msec. The system consumes 2 msec in CPU scheduling. For time quantum of 20 msec , the response times of the first request of the last process is

 
A) 26 msec B) 220 msec C) 200 msec D) none

Q 2)
In the above problem , the scheduler length is
A) 3300 msec B) 3600 msec C) 6300 msec D) 6600 msec

Answer 1

Take the last process -> it will get its first request processed after 10 schedules. Because after the first time quantum, the first process does an I/O operation (as it would have finished its request which also takes 20ms and hence CPU given to next process) and hence the second process gets its chance. Thus, after 9 time quantum, the final process gets its request served (assuming a previously given process is never given CPU time again until all other processes in wait are given). So, this means we need 10 schedules (including one at the beginning) and given in question that each scheduling takes 2ms. So, response time for the first request of the last process 

$= 9\times 20 ms + 10 \times 2$

$= 180 + 20 = 200 ms$.

 

(2) The scheduling goes like this:

P1 P2 P3 ... P10 P1 P2 P3 ... P10 .... P10 P1 P2 ... P10 (P1-P10 repeated 15 times)

After every time quantum and at the beginning we need 2ms for scheduling the next process. So, total scheduling length

$= (20ms + 2ms)\times 10 \times 15$

$=220 \times 15 = 3300 ms$

 

Comments

@arjun sir in part 2 ,i think we have to consider I/O time of last request of last process..ie. 10ms as other are over in cpu time but last request I/O one get only completed after 3300ms..so schedule length must be 3310ms

---

When the last process is doing IO it goes to blocked state- why should we count this for scheduler length?

---

But sir no process can go to terminate state from blocked state so the last process must take cpu to go termination state as there is no path from block to terminate state

Answer 2

180 + 20 + 20(10th process will respond after 20ms) = 220

I m thinking like this, please suggest!

Comments

But sir for atleast one instruction to be executed the process should be scheduled, then cpu scheduling overhead has to be taken into account. Then response time will be 2ms for the first process?

---

Yes you are right. In this question, response time for first process will be 2ms and not 0. I was thinking just about context switch time and not scheduling time.

---

Thanks sir for clearing the doubt. :)