1. Uniprocessor Scheduling
Scheduling of Processes

3. Types of Scheduling Long-term (least frequent decision) Medium-term
When an new process is created, will it be admitted to the ready queue?
Controls degree of multiprogramming (balances response/turnaround time and processor utilization)
Can consider priority, expected execution time, I/O requirements, etc.
Responds to requests for interactive sessions
Medium-term
Which processes should have frames allocated?
Also affects degree of multiprogramming
Already discussed in context of memory management
Short-term (most frequent decision)
Which process should execute next?
Focus of this lesson
I/O – next lesson

4. Short-Term Scheduling
Performed by part of kernel referred to as dispatcher
Invoked in response to
Timer interrupts
I/O interrupts
System calls
Signals (e.g. from semaphores)
Some long-term and medium-term scheduling events

5. Short-Term Scheduling Criteria
Performance Related
Other
User Oriented
Turnaround time
Response time
Deadlines
Predictability
System Oriented
Throughput
Processor utilization
Fairness
Enforcing priorities
Balancing resources

6. Short Term Scheduling Policies
Can be described in terms of
Selection function
Decision mode

7. Selection Function Determines which of the ready processes to execute
Example inputs:
s: total service time required by the process
e: time spent so far in execution
w: time spent so far in system (executing and waiting)
May also consider resource management or other execution characteristics of processes

8. Decision Mode Specifies times at which selection function is invoked
Nonpreemptive
Selection function used only when running process
Blocks for I/O
Requests OS service
Terminates
Preemptive
Selection function is used at other times
Timer interrupts
Process admission
Selection function is used when a process
blocks
terminates
enters the system
time slice expires.

9. Process Scheduling Example

10. FCFS/FIFO Policy Nonpreemptive
Always select the process that has been in the ready queue the longest 5 10 15 20 A B C D E

11. Round Robin Policy
Preemptive -- processes run for a time-slice or quantum
Always select the process that has been in the ready queue the longest 5 10 15 20 A B C D E

12. Virtual Round Robin Policy
I/O-bound processes typically do not use a complete quantum
Put processes with pending I/O in auxiliary queue, remember how much time remains in their time slice
Give preference to processes in auxiliary queue
Allow to run for remainder of time slice

14. Shortest Process Next Policy
Nonpreemptive
Always select process with shortest estimated processing time 5 10 15 20 A B C D E

15. Shortest Remaining Time Policy
Preemptive
When a new process enters the system, or when the running process leaves the running state
Select process with shortest estimated remaining processing time 5 10 15 20 A B C D E

16. HRRN Policy HRRN = Highest Response Ratio Next Nonpreemptive
Always select the process that has the highest value of A B C D E 5 10 15 20

17. Multi-Level Feedback Policy
Preemptive based on time quantum
Maintain a ready queue for each priority
When a process is preempted, move it to a lower priority queue
Note: If there are no processes waiting, don’t preempt current process
With quantum = 1, long processes will have very large turnaround time
Give processes from queue RQi quantum = 2i

19. Quantum = 1 5 10 15 20 A B C D E

20. Quantum = 2i
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