1. OverView of Scheduling
Student: Xiuxian Chen
ID:93036
Instructor:Mort Anvari
Term: Fall 2000

2. Basic Concepts
Scheduling is a fundamental operating-system function. Almost all computer resources are scheduling before use. The CPU is one the primary computer resources. Thus, its scheduling is central to operating-system design.

3. Types of Scheduling Long-term Scheduling
Determines which programs are admitted to the system for processing
Controls the degree of multiprogramming
If more processes are admitted
less likely that all processes will be blocked better CPU usage
each process has less fraction of the CPU
The long term scheduler may attempt to keep a mix of processor-bound and I/O-bound processes

4. Types of Scheduling (Continue)
Medium-term Scheduling
Medium-term scheduling is part of the swapping function. Typically, the swapping-in decision is based on the need to manage the degree of multiprogramming.

5. Types of Scheduling (Continue)
Short-term Scheduling
Determines which process is going to execute next (also called CPU scheduling)
The short term scheduler is known as the dispatcher
Is invoked when an event occurs
Clock interrupts
I/O interrupts
Operating system calls and traps
Signals

6. Types of Scheduling (Continue)
I/O Scheduling
Decision as to which process’s pending I/O request shall be handled by available I/O device

7. Scheduling and Process State Transition

8. Scheduling Criteria CPU utilization Throughput Turnaround time
Waiting Time
Response time

9. Scheduling algorithms
First-Come-First-Served (FCFS)
Each process joins the Ready queue
When the current process ceases to execute, the oldest process in the Ready queue is selected
A short process may have to wait a very long time before it can execute
Favors CPU-bound processes
I/O processes have to wait until CPU-bound process completes

10. Scheduling algorithms (continue)
Round-Robin
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

11. Scheduling algorithms (continue)
Priorities of Scheduling
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

12. Scheduling algorithms (continue)
Shortest Process Next
Non preemptive policy
Process with shortest expected processing time is selected next
Short process jumps ahead of longer processes
Predictability of longer processes is reduced
If estimated time for process not correct, the operating system may abort it
Possibility of starvation for longer processes

13. Scheduling algorithms (continue)
Feedback Scheduling
Process is demoted to the next lower-priority queue each time it returns to the ready queue
Longer processes drift downward
To avoid starvation, preemption time for lower-priority processes is longer

14. Scheduling algorithms (continue)
Multiprocessor Thread Scheduling
Load sharing
processes are not assigned to a particular processor
Gang scheduling
a set of related threads is scheduled to run on a set of processors at the same time
Determined processor assignment
threads are assigned to a specific processor
Dynamic Scheduling
number of threads can be altered during course of execution

15. Scheduling algorithms (continue)
Real-Time Scheduling
Static table-driven
determines at run time when a task begins execution
Static priority-driven preemptive
traditional priority-driven scheduler is used
Dynamic planning-based
Dynamic best effort

16. References
Operating systems: internals and Design Principles, third Edition by William Stallings