1. Advanced Operating Systems
III
Scheduling
Prof. Muhammad Saeed

2. Advanced Operating Systems
The part of the operating
system that makes the choice of the process is called the scheduler, and the algorithm it uses is called the scheduling algorithm.
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Advanced Operating Systems

3. Advanced Operating Systems
The part of the operating
system that makes the choice of the process is called the scheduler, and the algorithm it uses is called the scheduling algorithm.
March 27, 2017
Advanced Operating Systems

4. Advanced Operating Systems
The separation of mechanism and policy is a design principle in computer science. It states that mechanisms (those parts of a system implementation that control the authorization of operations and the allocation of resources) should not dictate (or overly restrict) the policies according to which decisions are made about which operations to authorize, and which resources to allocate.
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Advanced Operating Systems

5. Advanced Operating Systems
Page No.
CPU-Scheduling
Scheduling the processor among all ready processes
The goal is to achieve:
High processor utilization
High throughput
number of processes completed per of unit time
Low response time
time elapsed from the submission of a request until the first response is produced
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Advanced Operating Systems

6. Advanced Operating Systems
Classification of Scheduling Activity
Long-term: which process to admit?
Medium-term: which process to swap in or out?
Short-term: which ready process to execute next?
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Advanced Operating Systems

7. Queuing Diagram for Scheduling
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Advanced Operating Systems

8. Medium-Term Scheduling
Long-Term Scheduling
Determines which programs are admitted to the system for processing
Controls the degree of multiprogramming
Attempts to keep a balanced mix of processor-bound and I/O-bound processes
CPU usage
System performance
Medium-Term Scheduling
Makes swapping decisions based on the current degree of multiprogramming
Controls which remains resident in memory and which jobs must be swapped out to reduce degree of multiprogramming
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Advanced Operating Systems

9. Short-Term Scheduling
Selects from among ready processes in memory which one is to execute next
The selected process is allocated the CPU
It is invoked on events that may lead to choose another process for execution:
Clock interrupts
I/O interrupts
Operating system calls and traps
Signals
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Advanced Operating Systems

10. Characterization of Scheduling Policies
The selection function determines which ready process is selected next for execution
The decision mode specifies the instants in time the selection function is exercised
Nonpreemptive
Once a process is in the running state, it will continue until it terminates or blocks for an I/O
Preemptive
Currently running process may be interrupted and moved to the Ready state by the OS
Prevents one process from monopolizing the processor
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Advanced Operating Systems

11. Short-Term Scheduler Dispatcher
The dispatcher is the module that gives control of the CPU to the process selected by the short-term scheduler
The functions of the dispatcher include:
Switching context
Switching to user mode
Jumping to the location in the user program to restart execution
The dispatch latency must be minimal
Context Switching: Saving and loading registers and memory maps, updating various tables and lists, flushing and reloading the memory cache, and so on.
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Advanced Operating Systems

12. Advanced Operating Systems
The CPU-I/O Cycle
Processes require alternate use of processor and I/O in a repetitive fashion
Each cycle consist of a CPU burst followed by an I/O burst
A process terminates on a CPU burst
CPU-bound processes have longer CPU bursts than I/O-bound processes
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Advanced Operating Systems

13. Short-Tem Scheduling Criteria
User-oriented criteria
Response Time: Elapsed time between the submission of a request and the receipt of a response
Turnaround Time: Elapsed time between the submission of a process to its completion
System-oriented criteria
Processor utilization
Throughput: number of process completed per unit time
fairness
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Advanced Operating Systems

14. Scheduling Algorithms
First-Come, First-Served Scheduling
Shortest-Job-First Scheduling
Also referred to as Shortest Process Next
Priority Scheduling
Round-Robin Scheduling
Multilevel Queue Scheduling
Multilevel Feedback Queue Scheduling
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Advanced Operating Systems

15. Advanced Operating Systems
Process Mix Example
Process
Arrival
Time
Service 1 2 3 4 5 6 8
Service time = total processor time needed in one (CPU-I/O) cycle Jobs with long service time are CPU-bound jobs and are referred to as “long jobs”
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Advanced Operating Systems

16. First Come First Served (FCFS)
Selection function: the process that has been waiting the longest in the ready queue (hence, FCFS)
Decision mode: non-preemptive
a process runs until it blocks for an I/O
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Advanced Operating Systems

17. Advanced Operating Systems
FCFS Drawbacks
Favors CPU-bound processes
A CPU-bound process monopolizes the processor
I/O-bound processes have to wait until completion of CPU-bound process
I/O-bound processes may have to wait even after their I/Os are completed (poor device utilization)
Better I/O device utilization could be achieved if I/O bound processes had higher priority
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Advanced Operating Systems

18. Advanced Operating Systems
SJF / SPN Critique
Possibility of starvation for longer processes
Lack of preemption is not suitable in a time sharing environment
SJF/SPN implicitly incorporates priorities
Shortest jobs are given preferences
CPU bound process have lower priority, but a process doing no I/O could still monopolize the CPU if it is the first to enter the system
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Advanced Operating Systems

19. Advanced Operating Systems
Is SJF/SPN Optimal?
If the metric is turnaround time (response time), is SJF or FCFS better?
For FCFS, resp_time=( )/5 = ?
Note that Rfcfs = 3+(3+6)+(3+6+4)+…. = ?
For SJF, resp_time=( )/5 = ?
Which one is smaller? Is this always the case?
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Advanced Operating Systems

20. Advanced Operating Systems
Is SJF/SPN Optimal?
Take each scheduling discipline, they both choose the same subset of jobs (first k jobs).
At some point, each discipline chooses a different job (FCFS chooses k1 SJF chooses k2)
Rfcfs=nR1+(n-1)R2+…+(n-k1)Rk1+….+(n-k2) Rk2+….+Rn
Rsjf=nR1+(n-1)R2+…+(n-k2)Rk2+….+(n-k1) Rk1+….+Rn
Which one is smaller? Rfcfs or Rsjf?
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Advanced Operating Systems

21. Advanced Operating Systems
Priorities
Implemented by having multiple ready queues to represent each level of priority
Scheduler the process of a higher priority over one of lower priority
Lower-priority may suffer starvation
To alleviate starvation allow dynamic priorities
The priority of a process changes based on its age or execution history
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Advanced Operating Systems

22. Advanced Operating Systems
Round-Robin
Clock interrupt is generated at periodic intervals
When an interrupt occurs, the currently running process is placed in the read queue
Next ready job is selected
Known as time slicing
Uses preemption based on a clock
An amount of time is determined that allows each process to use the processor for that length of time
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Advanced Operating Systems

23. Advanced Operating Systems
Round-Robin
Selection function: same as FCFS
Decision mode: preemptive
a process is allowed to run until the time slice period (quantum, typically from 10 to 100 ms) has expired
a clock interrupt occurs and the running process is put on the ready queue
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Advanced Operating Systems

24. Advanced Operating Systems
RR Time Quantum
Quantum must be substantially larger than the time required to handle the clock interrupt and dispatching
Quantum should be larger then the typical interaction
but not much larger, to avoid penalizing I/O bound processes
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Advanced Operating Systems

25. Advanced Operating Systems
RR Time Quantum
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Advanced Operating Systems

26. Advanced Operating Systems
Round Robin: critique
Still favors CPU-bound processes
An I/O bound process uses the CPU for a time less than the time quantum before it is blocked waiting for an I/O
A CPU-bound process runs for all its time slice and is put back into the ready queue
May unfairly get in front of blocked processes
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Advanced Operating Systems

27. Multilevel Feedback Scheduling
Preemptive scheduling with dynamic priorities
N ready to execute queues with decreasing priorities:
P(RQ0) > P(RQ1) > ... > P(RQN)
Dispatcher selects a process for execution from RQi only if RQi-1 to RQ0 are empty
New process are placed in RQ0
After the first quantum, they are moved to RQ1 after the first quantum, and to RQ2 after the second quantum, … and to RQN after the Nth quantum
I/O-bound processes remain in higher priority queues.
CPU-bound jobs drift downward.
Hence, long jobs may starve
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Advanced Operating Systems

28. Multiple Feedback Queues
Different RQs may have different quantum values
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Advanced Operating Systems

29. Time Quantum for feedback Scheduling
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Advanced Operating Systems

30. Advanced Operating Systems
Round Robin: critique
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Advanced Operating Systems

31. Advanced Operating Systems
Lottery Scheduling
Give processes lottery tickets for various system resources,
such as CPU time. Whenever a scheduling decision has to be made, a lottery ticket is chosen at random, and the process holding that ticket gets the resource.
When applied to CPU scheduling, the system might hold a lottery 50 times a second, with each winner getting 20 msec of CPU time as a prize.
More important processes can be given extra tickets, to increase their odds of winning. If there are 100 tickets outstanding, and one process holds 20 of them, it will have a 20% chance of winning each lottery. In the long run, it will get about 20% of the CPU. In contrast to a priority scheduler, where it is very hard to state what having a priority of 40 actually means, here the rule is clear: a process holding a fraction f of the tickets will get about a fraction f of the resource in question.
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Advanced Operating Systems

32. Advanced Operating Systems
END
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Advanced Operating Systems