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+15 votes
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Consider three CPU-intensive processes, which require 10, 20 and 30 time units and arrive at times 0, 2 and 6, respectively. How many context switches are needed if the operating system implements a shortest remaining time first scheduling algorithm? Do not count the context switches at time zero and at the end.

  1. 1
  2. 2
  3. 3
  4. 4
asked in Operating System by Active (3.7k points) | 3k views

2 Answers

+40 votes
Best answer

process execute in this way ...

0 ---- p1---- 10 (switching)--------p2-----30(switching)-----p3-----60 

so here only 2 switching possible (when we did not consider the starting and ending switching )

now here might be confusion that at t= 2 p1 is preempted and check that available process have shortest job time or not ...but he did not get anyone so it should not be consider as context switching ..(same happened at t=6) 

 ref :http://stackoverflow.com/questions/8997616/does-a-context-switch-occur-in-a-system-whose-ready-queue-has-only-one-process-a(thanks to anurag_s)

answer is B)

 

answered by Boss (14.8k points)
selected by
–3

what about OS Scheduler Process

       P1------>OS Scheduler--------->p2--------->OS scheduler--------->p3

and hence, Shouldn't there be 4 context switches?

0
4 in case of if we count both first and last context switching
0
Please just clear me... current process execution is finished and processor is going for another process is it a context switch? Is the cpu stores any context about the finished process?
+6 votes
Only two switches wil be required after process 1 and 2 complete.Option B.
answered by Active (3.3k points)
Answer:

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