2. Recap
In the last class, you have learnt
• Priority Scheduling algorithms
• Round Robin Scheduling algorithms
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4. Objectives
On completion of this period, you would be able to
know
• Multilevel Queue Scheduling
• Multilevel Feedback Queue Scheduling
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5. Multilevel Queue Scheduling
• This algorithm is used where processes are classified into
different groups
• Example :
• A division is made between fore ground (interactive)
process & back ground ( batch ) processes
• Foreground processes have higher priority than
background
• A multilevel queue scheduling algorithm partitions the
ready queue into several separate queues as shown in Fig.1
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6. Multilevel Queue Scheduling
• Separate queues might be used for fore ground & back
ground process
• Foreground queue is scheduled by RR
• Back ground queue is scheduled FCFS
• In addition there must be scheduling among queues
• The scheduling used is fixed priority preemptive
scheduling
• Fore ground queue has priority over background queue
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7. Multi Level Queue Scheduling
•It separates the CPU processes with different CPU
burst characteristics
•The processes are permanently assigned to one
queue based on following properties
-Memory size
-Process priority
-Process type
• Each queue has its own scheduling algorithm
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9. Multi Level Queue Scheduling
• Multi level queue Scheduling algorithm with five queues:
1. System Processes
2. Interactive Processes
3. Interactive Editing Processes
4. Batch Processes
5. Student Processes
• Each queue has absolute priority over lower priority
queues Example:
– no process in batch queue could run unless the queues for
System, Interactive & Interactive Editing are empty
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10. Multi Level Feedback Queue Scheduling
• The idea behind is to separate processes with different
CPU burst characteristics
• If a process uses too much CPU time it will be removed
to lower priority queue
• This scheme leaves I/O bound & interactive processes in
the higher priority queues
• A process that waits too long in a lower priority queue is
mounted to higher priority queue
• Prevents star variations
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11. Multilevel Feedback Queue Scheduling
•Consider a multilevel feed back queue scheduler with
three queues
q0 quantum=8
q1 quantum=16
FCFS
q2
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12. Multilevel Feedback Queue Scheduling
• Numbered from q0 to q2 as shown in the above fig
• The scheduler first executes all processes in queue 0
• When empty it executes all processes in queue 1 and
so on
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13. Multilevel Feedback Queue Scheduling
• This scheduling algorithm gives the highest priority to
any process with a CPU burst of 8 milliseconds or less
• In general multilevel feed back queue scheduler is
defined by the following parameters:
– The number of queues
– The scheduling algorithm for each queue
– The method used to determine when to demote a process to a
lower priority queue
– The method used to determine which queue a process will
enter when that process need service
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14. Summary
In this class, you have learnt
• Multilevel queue scheduling algorithm
• Various queues in multilevel queue
• Multilevel feedback queue scheduling
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15. Frequently Asked Questions
1. Explain multilevel queue scheduling algorithm
2. List the no of queues in the multilevel queue scheduling
algorithm
3. Explain multilevel feedback queue scheduling
algorithm
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16. Quiz
1. Foreground in multilevel queue scheduling algorithm is
scheduled as
a) FCFS
b) RR
c) SJF
d) None
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17. Quiz
2. Background queue in multilevel queue scheduling algorithm is
scheduled as
a) FCFS
b) RR
c) SJF
d) None
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18. Quiz
3.A process that waits too long in a lower priority queue is
moved to higher priority queue in _________ algorithm..
a) Multilevel queue
b) Multilevel feedback queue
c) FCFS
d) None
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19. Quiz
4.The processes in multilevel queue algorithm are permanently
assigned to a queue based on
a) Memory size
b) Process priority
c) Process type
d) All the above
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