1. Chapter 2.2 : Process Scheduling
Process concept 
Process scheduling
Interprocess communication
Deadlocks
Threads

2. Scheduling Select process(es) to run on processor(s)
Process state is changed from “ready” to “running”
The component of the OS which does the scheduling is called the scheduler

3. Types of Scheduling Scheduling is divided into various levels.
These levels are defined by the location of the processes
A process can be
available to be executed by the processor
partially or fully in main memory
in secondary memory
is not started yet

4. Types of Scheduling Long Term Scheduling. Medium Term Scheduling.
The decision to add to the pool of processes to be executed.
Medium Term Scheduling.
The decision to add to the process in main memory.
Short Term Scheduling.
The decision as to which process will gain the processor.
I/O Scheduling.
The decision as to which process's I/O request shall be handled by a device.

5. Scheduling Criteria
Fairness : each process should get a fair share of the CPU
Efficiency: keep CPU 100% utilized
Response time : should be minimized for interactive users
Turnaround : minimize batch turnaround times
Throughput : maximize number of jobs processed per hour

6. User-Oriented, Performance Criteria
Criteria Aim
Response Time low response time,
maximum number of
interactive users
Turnaround Time time between submission
and completion
Deadlines maximise deadlines met

7. System-oriented, Performance Criteria
Criteria Aim
Throughput allow maximum number of jobs to complete
Processor maximise percentage of time processor is busy
utilisation
Overhead minimise time processor busy executing OS

8. System oriented, other criteria
Criteria Aim
Fairness treat processes the same avoid starvation
Enforcing Priorities give preference to higher priority processes
Balancing Resources keep the system resources busy

9. Important Factors I/O boundedness of a process
CPU boundedness of a process
Is the process interactive or batch?
Process priority
Page fault frequency
Preemption frequency
Execution time received
Execution time required to complete

10. Types of Scheduling
A scheduling algorithm is non-preemptive (run to completion) if the CPU cannot be taken away by the OS.
A scheduling algorithm is preemptive if the CPU can be taken away by the OS.
Cooperative Scheduling is the scheduling algorithm used by some operating systems (such as Windows versions up to 95 in which processes relinquish the control of CPU for some reason (I/O, waiting for some event etc.)

11. The Interrupting Clock
The OS sets the interrupting clock to generate an interrupt at some specified future time.
This interrupt time is the process quantum.
Provides reasonable response times and prevents the system being held up by processes in infinite loops.

12. Scheduling Algorithms
FCFS
Round Robin
Virtual Round Robin
Priority
Priority Classes
Shortest Job First
Shortest Remaining Time
Highest Response Ratio Next
Feedback Queues

13. FCFS (First Come First Serve)
Implementation:
As each process becomes ready, it joins the ready queue.
When the current process finishes the oldest process is selected next.
Characteristics:
Simple to implement
Nonpremptive
Penalises short and I/O-bound processes

14. Round Robin (RR) Implementation: Characteristics:
Processes are dispatched FIFO. But are given a fixed time on the CPU (quantum - time slice).
Characteristics:
Preemptive
Effective in time sharing environments
Penalises I/O bound processes

15. Quantum Size Some Options: Large or small quantum
Fixed or variable quantum
Same for everyone or different
If quantum is to large RR degenerates into FCFS
If quantum is to small context switching becomes the primary job being executed
A good guide is quantum should be slightly larger than the time required for a typical interaction

16. Virtual Round Robin (VRR)
A modification to the RR algorithm to remove the bias towards CPU bound processes.
Implementation:
Two “ready” queues, one called an AUX queue for storing “completed” IO processes
AUX queue has priority over READY queue
IO processes only runs for remaining time
Characteristics:
Performance studies indicate fairer than RR

17. Priority Implementation: Characteristics:
Each process is assigned a priority and the scheduler always selects the highest priority process first
Characteristics:
High priority processes may run indefinitely, so decrease the priority of these processes at regular intervals
Assign high priority to system processes with known characteristics such as being I/O bound

18. Priority Classes Highest Lowest Priority Class 4 Priority Class 3

19. Implementation: Characteristics:
Processes are grouped into priority classes
Round Robin is used within a class
When selecting process start with the highest class. If the class is empty, use a lower class
Characteristics:
If priorities are not adjusted from time to time, lower classes may starve to death

20. Shortest-Job-First (SJF)
Sometimes known as Shortest Process Next (SPN)
Implementation:
The process with the shortest expected execution time is given priority on the processor

21. Characteristics: Nonpremptive Reduces average waiting time over FIFO
Always produces the minimum average turnaround time
Must know how long a process will run
Possible user abuse
Suitable for batch environments. Not useful in a timesharing environment

22. Shortest Remaining Time (SRT)
Preemptive counterpart of SPN
Implementation:
Process with the smallest estimated run-time to completion is run next
A running process may be preempted by a new process with a shorter estimate run-time

23. Characteristics:
Still requires estimates of the future
Higher overhead than SJF
No additional interrupts are generated as in RR
Elapsed service times must be recorded

24. Highest Response Ratio Next (HRRN)
How do you get around the problem of Indefinite postponement?
Implementation:
Once a job gets the CPU it runs it to completion
The priority of a job is a function of the job's service time and the time it has been waiting for service
priority = (time waiting + service time) / service time

25. Characteristics:
Nonpremptive
Shorter jobs still get preference over longer jobs
However aging ensures long jobs will eventually gain the processor
Estimation still involved

26. Feedback Queues There is a network of ready queues
Sometimes called multi-level feedback queues
Implementation:
There is a network of ready queues
A new process enters at the top queue
Moves through the queue FIFO

27. I/O processes:
If the job requires I/O before quantum expiration it leaves the network and comes back at the same level queue
CPU bound processes:
If the quantum expires first, the process is placed on the next lower queue
This continues until it reaches the bottom queue

28. Dispatching:
A process is only placed on the CPU if all higher level queues are empty
A running process is preempted by a process arriving in a higher queue
Processes from lower level queues receive a larger quantum
Modifications:
In some systems processes can proceed back up the network by becoming I/O bound

29. A Scheduling Mechanism Should:
Favour short jobs
Favour I/O bound jobs to get good I/O device utilisation
Determine the nature of a job and schedule accordingly