1. Uniprocessor Scheduling I
Chapter 9

2. Goals of Scheduling Quick response time Fast throughput
Processor efficiency

3. Type of Scheduling (1) Long-term Medium-term
performed when new process is created
The decision to add to the pool of processes to be executed (chapter 3, process control)
Medium-term
swapping
The decision to add to the number of processes that are partially or fully in main memory (chapter 7 and 8, control structure)

4. Types of Scheduling Short-term I/O which ready process to execute next
decision as to which ready process will be executed by the processor (chatter 9 and 10)
I/O
decision as to which process’s pending I/O request shall be handled by available I/O device (chapter 11)

5. Scheduling and Process State Transition
New
Long-term
scheduling
Long-term
scheduling
Ready,
suspend
Running
Exit
Ready
Short-term
scheduling
Medium-term
scheduling
Blocked,
suspend
Blocked
Medium-term
scheduling

6. A Seven-State Model - States
Ready: The process is in main memory and available for execution.
Blocked: The process is in main memory and awaiting an event.
Blocked, suspend: The process is in secondary memory and awaiting an event.
Ready, suspend: The process is in secondary memory but is available for execution as soon as it is loaded into main memory.

7. Queuing Diagram for Scheduling
Long-term
scheduling
Time-out
Batch
jobs
Short-term
scheduling
Ready Queue
Processor
Release
Medium-term
scheduling
Interactive
users
Ready, Suspend Queue
Medium-term
scheduling
Blocked, Suspend Queue
Blocked Queue
Event Wait
Event
Occurs

8. 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

9. Medium-Term Scheduling
Swapping
Based on the need to manage multiprogramming

10. Short-Term Scheduling
Known as the dispatcher
Invoked when an event occurs
clock interrupts
I/O interrupts
operating system calls
signals

11. Short-Tem Scheduling Criteria
User-oriented
Related to the behavior of the system as perceived by the individual user or process.
Response Time: Elapsed time between the submission of a request until there is output.
System-oriented
The focus if on effective and efficient utilization of the processor

12. Short-Term Scheduling Criteria
Performance-related
Performance-related criteria are quantitative and generally can be readily measured.
measurable such as response time and throughput
Not performance related
predictability: a given job should run in about the same amount of time and at about the same cost regardless of the load on the system

13. Priorities
Scheduler will always choose a process of higher priority over one of lower priority
Have 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

14. Priority Queuing A set of queues to instead of a single ready queue
RQ0
Processor
Release
Dispatch
A set of queues to instead of a single ready queue
RQ1 . .
Admit .
RQn
Priority
[RQi] >[RQj]
for i<j
Preemption
Event Wait
Event
occurs
Blocked Queue

15. Decision Mode Nonpreemptive Preemptive
Once a process is in the running state, it will continue until it terminates or blocks itself for I/O
Preemptive
Currently running process may be interrupted and moved to the Ready state by the operating system
Allows for better service since any one process cannot monopolize the processor for very long

16. An Example for Scheduling Policies
Process
Service Time
Arrival Time 1 3 2 2 6 3 4 4 6 4 5 5 8 2

17. First-Come-First-Served (FCFS)5 10 15 20
Each process joins the Ready queue
When the current process ceases to execute, the oldest process in the Ready queue is selected 1 2 3 4 5

18. Turnaround Time and Service Time
Turnaround Time Tq - the total time that the item spends in the system (waiting time plus service time, or finish time mines arrival time).
Service Time Ts - Finish Time minus Start Time.
Normalized Turnaround Time - the ratio of turnaround time to service time
Tq / Ts

19. FCFS -Normalized Turnaround Time
Process Arrival Serve Start Finish Turnaround Tq/Ts
Time Time Ts Time Time Time Tq A B C D
Mean

20. Difficulties with First-Come-First-Served
A short process may have to wait a very long time before it can execute
Favors CPU-bound processes
CPU-bound processes: the processes mostly use processor.
I/O processes have to wait until CPU-bound process completes

21. Round-Robin q=1 Uses preemption based on a clock5 10 15 20
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
Next ready job is selected. 2
Ready Time 1 2 3 5 7 10 2 14 4 6 9 3 13 6 8 4 16 12 8 5 11

22. Round-Robin q=4 7 Ready Time 2 4 Ready Time 6 15 85 10 15 20 1 7
Ready Time 2 2 4 3
Ready Time 6 15 4 8 5
P2 rest part ready time is 7
P4 rest part ready time is 15
P5 ready time is 8
P4 rest part ready time is 15
P5 ready time is 8