1. Lecture 21: Introduction to Process Scheduling

2. Review: Processes & Threads
Resource ownership – Process or Task
Scheduling/execution – Thread or lightweight process
One process,
one thread (MS-DOS)
One process,
multiple threads (Java Runtime)
Multiple processes,
one thread per process (Unix)
Multiple processes,
multiple threads (W2K, Solaris, Linux)

3. Review: Single Threaded and Multithreaded Process Models

4. Review: Process/Thread States
Possible process states
running
blocked
ready

5. Process behavior Different Types of jobs have different demands
a CPU-bound process
an I/O bound process

6. Process Scheduling
When to run a different process?
Which one to run?

7. When to schedule process
New process is created
Process terminates
A process blocks on I/O, semaphore, etc.
I/O interrupt occurs
Hardware clock interrupts

8. Preemptive vs. Non-preemptive
Non-preemptive Scheduling: Processes run until they have finished or block for I/O
Preemptive Scheduling: Can forcibly suspend a process and switch to another
Hardware Clock Interrupt helps in implementing preemption

9. Categories of Process Scheduling
Batch Systems (Compute Servers)
Many processes, not interactive, throughput is important
Interactive Systems PC
Real Time Systems
Guaranteed response times, meet deadlines

10. Scheduling Algorithm Goals
All Systems
Fairness – giving each process a fair share of the CPU
Policy enforcement – seeing that stated policy is carried out
Balance – keeping all parts of the system busy

11. Scheduling Algorithm Goals
Batch Systems
Throughput – maximize jobs per hour
Turnaround time – minimize time between submission and termination
CPU utilization – keep the CPU busy all the time

12. Scheduling Algorithm Goals
Interactive Systems
Response time – respond to requests quickly
Proportionality – meet users’ expectations

13. Scheduling Algorithm Goals
Real-time Systems
Meeting deadlines – avoid losing data
Predictability – avoid quality degradation in multimedia systems

14. First Come First Serve (FCFS)
Maintain queue of processes
Schedule first in queue
Scheduled job executes until it finishes or blocks for I/O
Simple, easy to implement
Not good for mix of short and long jobs
Example: a short job arriving soon after a long one

15. FCFS Arrival time CPU burst P1: 4 36 8 112 1 13 I/O burst 1613 16 20
P1:
I/O burst
P2:
P3:
P4:

16. FCFS 13 16 20
P1:
P2:
P3:
P4:
Which process goes next?
P1 idle P2 P3 P4

17. FCFS
Time next ready 40 76 60
192 13 16 20
P1:
P2:
P3:
P4:
Which process goes next?
P1 idle P2 P3 P4