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OS CPU Scheduling

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Question 1

Consider three processes (process id 0, 1, 2 respectively) with compute time bursts 2, 4 and 8 time units. All processes arrive at time zero. Consider the longest remaining time first (LRTF) scheduling algorithm. In LRTF ties are broken by giving priority to the process with the lowest process id. The average turn around time is:

13 units
14 units
15 units
16 units

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Question 2

Consider three processes, all arriving at time zero, with total execution time of 10, 20 and 30 units, respectively. Each process spends the first 20% of execution time doing I/O, the next 70% of time doing computation, and the last 10% of time doing I/O again. The operating system uses a shortest remaining compute time first scheduling algorithm and schedules a new process either when the running process gets blocked on I/O or when the running process finishes its compute burst. Assume that all I/O operations can be overlapped as much as possible. For what percentage of time does the CPU remain idle?

0%
10.6%
30.0%
89.4%

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Question 3

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.

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Question 4

Which of the following process scheduling algorithm may lead to starvation

FIFO
Round Robin
Shortest Job Next
None of the above

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Question 5

If the quantum time of round robin algorithm is very large, then it is equivalent to:

First in first out
Shortest Job Next
Lottery scheduling
None of the above

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Question 6

Which of the following is FALSE about SJF (Shortest Job First Scheduling)?

S1: It causes minimum average waiting time
S2: It can cause starvation

Only S1
Only S2
Both S1 and S2
Neither S1 nor S2

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Question 7

The most optimal scheduling algorithm is :

First come first serve (FCFS)
Shortest Job First (SJF)
Round Robin (RR)
None of the above

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Question 8

A scheduling algorithm assigns priority proportional to the waiting time of a process. Every process starts with priority zero (the lowest priority). The scheduler re-evaluates the process priorities every T time units and decides the next process to schedule. Which one of the following is TRUE if the processes have no I/O operations and all arrive at time zero?

This algorithm is equivalent to the first-come-first-serve algorithm
This algorithm is equivalent to the round-robin algorithm.
This algorithm is equivalent to the shortest-job-first algorithm..
This algorithm is equivalent to the shortest-remaining-time-first algorithm

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Question 9

Consider the 3 processes, P1, P2 and P3 shown in the table.

Process           Arrival time         Time Units Required
   P1                0                         5
   P2                1                         7
   P3                3                         4

The completion order of the 3 processes under the policies FCFS and RR2 (round robin scheduling with CPU quantum of 2 time units) are

```
FCFS: P1, P2, P3
 RR2: P1, P2, P3
```
```
 FCFS: P1, P3, P2
 RR2: P1, P3, P2
```
```
FCFS: P1, P2, P3
 RR2: P1, P3, P2
```
```
FCFS: P1, P3, P2 
RR2: P1, P2, P3
```

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Question 10

Consider the following table of arrival time and burst time for three processes P0, P1 and P2.

Process   Arrival time   Burst Time
P0            0 ms          9 ms
P1            1 ms          4 ms
P2            2 ms          9 ms

The pre-emptive shortest job first scheduling algorithm is used. Scheduling is carried out only at arrival or completion of processes. What is the average waiting time for the three processes?

5.0 ms
4.33 ms
6.33
7.33

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Operating Systems

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