1. Uniprocessor Scheduling
Chapter 9

2. Aim of Scheduling To improve:
Response time: time it takes a system to react to a given input
Turnaround Time (TAT) = Total time spent in the system
= waiting time + service time
Throughput: jobs per minute (inverse of TAT)

3. Types of Scheduling

4. Medium-Term Scheduling
Long-Term Scheduling
Determines which programs are admitted to the system for processing
Controls the degree of multiprogramming
More processes, smaller percentage of time each process is executed
Which job to admit? FCFS, Priority, expected exec time, I/O req.s
Medium-Term Scheduling
Part of the swapping function
Swapping-in decision is based on the need to manage the degree of multiprogramming
Short-Term Scheduling
Known as the dispatcher
Executes most frequently
Invoked when an event occurs: Clock interrupts, I/O, OS calls, Signals, etc.

5. Evaluation Criteria for Short-Term Scheduling Policies:
User-oriented, performance related
Turnaround time: time between submission and completion
Response Time: time between submission and the first output.
User-oriented, other
Predictability: For users, it is important that a job runs the same way (Time & cost) regardless of the system load
System-oriented, Performance related
Maximize Throughput: rate at which processes are completed
Processor utilization: percentage of time that the processor is busy
System-oriented, other
Fairness
Enforcing priorities?
Balancing resources? – Is it favoring processes that do not request stressed resources?

6. The Use of Priorities Decision Mode
Scheduler chooses a process of higher priority over one of lower priority
Use multiple ready queues to represent each level of priority
Lower-priority may suffer starvation
allow a process to change its priority based on its age or execution history
Decision Mode
Nonpreemptive
Once a process is in the running state, it will continue until it terminates or blocks itself
Preemptive
Currently running process may be interrupted and moved to the Ready state due to an external event.
No single process can monopolize the processor for very long  Better service

7. Process Scheduling Example

8. First-Come-First-Served (FCFS)5 10 15 20 A B C D E
Each process joins the Ready queue
Nonpreemptive
The oldest process in the ready queue is selected to run next
Disadvantage:
A short process may have to wait a very long time before it can execute
Advantage: Favors CPU-bound processes
I/O processes have to wait until CPU-bound process completes

9. Round-Robin (RR) Preemption based on a clock (time slicing)5 10 15 20 A B C D E
Preemption based on a clock (time slicing)
Clock interrupt is generated at periodic intervals (time slice is determined a priori)
When an interrupt occurs, the currently running process is placed in the ready queue and next ready job is selected to run

10. Shortest Process Next (SPN)5 10 15 20 A B C D E
Nonpreemptive
Process with shortest expected processing time is selected next.
For batch jobs, user is required to estimate the running time. If estimated time for process not correct, the OS may abort it.
For interactive jobs, OS tries to predict it.
Short process jumps ahead of longer processes
Possibility of starvation for longer processes

11. Process Scheduling Example

12. Shortest Remaining Time (SRT)5 10 15 20 A B C D E
Preemptive version of shortest process next policy
Achieves better turnaround time as compared to SPN, because a short job is given immediate preference to a running longer job
Risk of starvation for long jobs
Must estimate the remaining processing time  Not easy!

13. Highest Response Ratio Next (HRRN)5 10 15 20 A B C D E
Nonpreemptive
An important performance criteria to minimize is:
Normalized TurnAround Time = TAT / ServiceTime
Choose next process with the highest response ratio R (so that it does not get any higher!):
No starvation possible
with this formula, shorter jobs are favored against longer ones

14. Feedback (FB) 5 10 15 20 A B C D E
Hard to predict the remaining time a process needs to execute.
Instead, we can penalize jobs that have been running longer
Each subsequent time that a job is preempted, it is moved to a lower-priority ready queue. This way, a longer process will gradually drift downward in the hierarchy of queues.
starvation may occur if new jobs enter the system frequently
To remedy this, increase priority if a job spends too much time in a lower priority queue and give more time quantum to lower priority queues.

15. A Comparison of Scheduling Policies

16. Characteristics of Various Scheduling Policies