1. CPU scheduling 6. Schedulers, CPU Scheduling 6.1. Schedulers
Short term, Long term, Medium term schedulers.
6.2. CPU Scheduling
CPU- I/O burst cycle
CPU scheduler
Dispatcher
6.3. Scheduling Criteria
6.4. Scheduling Algorithms
First Come First Served. Shortest Job First. Priority Scheduling. Round Robin. Multilevel queues, Multilevel feedback queues.
Textbook Silberschatz 6th ed. Chapter 6

2. CPU scheduling Schedulers Long Term Scheduler
Is called not so often.
Controls the degree of multiprogramming (# of processes in memory).
Good mix of I/O bound and CPU bound proc. ?
Some systems do not have long term scheduler. ?
Short Term (CPU) Scheduler
Is called very often
Decides how to pick a process
Does Context Switching
Must be very fast
A process migrates between the various scheduling queues throughout its lifetime.

3. Medium Term Scheduling
CPU scheduling
Medium Term Scheduling

4. Medium Term Scheduling
CPU scheduling
Medium Term Scheduling
Some operating systems, such as time-sharing systems, may introduce an additional, intermediate level of scheduling.
This medium-term scheduler, removes processes from memory (and from active contention for the CPU), and thus reduces the degree of multiprogramming.

5. CPU scheduling CPU-I/O Burst Cycle
Scheduling is a fundamental operating-system function.
Almost all computer resources are scheduled before use. The CPU is, of course, one of the primary computer resources. Thus, its scheduling is central to operating-system design.
The success of CPU scheduling depends on the following observed property of processes:

6. Histogram of CPU-burst Times
CPU scheduling
CPU-I/O Burst Cycle
Histogram of CPU-burst Times
I/O Bound program
many very short CPU bursts
CPU Bound program
a few very long CPU bursts

7. Preemptive and Non-Preemptive
CPU scheduling
Preemptive and Non-Preemptive
CPU scheduler Selects from among the processes in ready queue, and allocates the CPU to one of them
Queue may be ordered in various ways
CPU scheduling decisions may take place when a process:
1. Switches from running to waiting state
2. Switches from running to ready state
3. Switches from waiting to ready
Terminates
Scheduling under only 1 and 4 is nonpreemptive
Otherwise scheduling is preemptive

8. Preemptive and Non-Preemptive
CPU scheduling
Preemptive and Non-Preemptive

9. CPU scheduling Dispatcher
Dispatcher – Does the remaining tasks after the process selection – Gives control to the process.
Context Switch
Switching between Kernel-User modes
Jumping to the proper location in the program to continue it.
Dispatch latency - stop one process and start another – Should be very short