Chapter 9 Uniprocessor Scheduling Operating Systems: Internals and Design Principles, 6/E William Stallings Patricia Roy Manatee Community College, Venice,

Chapter 9Uniprocessor Scheduling

Operating Systems:Internals and Design Principles, 6/E

William Stallings

Patricia RoyManatee Community College, Venice, FL

©2008, Prentice Hall

Aim of Scheduling

• Assign processes to be executed by the processor(s)

• Response time• Throughput• Processor efficiency

Processor Scheduling

• Known as the dispatcher• Invoked when an event occurs

– Clock interrupts– I/O interrupts– Operating system calls– Signals

Processor Scheduling Criteria

• User-oriented– Response Time

• Elapsed time between the submission of a request until there is an output

– Turnaround Time• The time between the submission and the

completion of a process

• System-oriented– Effective and efficient utilization of the

processor

Decision Mode

• Nonpreemptive– Once a process is in the running state, it will

continue until it terminates or blocks itself for I/O

Decision Mode

• 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

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

Priority Queuing

Priorities

• Lower-priority may suffer starvation– Allow a process to change its priority based

on its age or execution history

First-Come-First-Served

• Each process joins the Ready queue

• When the current process ceases to execute, the process that waits the longest in the Ready queue is selected

Process Scheduling Example

First-Come-First-Served

• 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– Low utilization for I/O devices– Possibly low utilization for processor

Round Robin

• Clock interrupt is generated at periodic intervals

• When an interrupt occurs, the currently running process is placed in the ready queue– Next ready job is selected

• Known as time slicing

Round Robin• Uses preemption based on a clock

Round Robin

• Time quantum is– The time interval between two clock interrupts– The maximum time allocated to a process’s

execution in each turn

Round Robin

• Which type of processes are favored by round robin scheduling?– CPU or I/O bound processes?

Round Robin

Example: A, C, D, E: CPU bound processes;

B: I/O bound process.

What should we do to treat

them fairly in terms of CPU allocation?

A

B

C

D

E

Round Robin

• Which type of processes are favored by round robin scheduling?– CPU or I/O bound processes?

• Favors CPU-bound processes– CPU-bound process gets more CPU time– I/O process has to wait for CPU-bound

processes to consume their quanta before its turn of executing on the CPU

Queuing Diagram

Virtual round robin

Use an auxiliary queue to hold processes that have I/O requests serviced

Process in auxiliary queue gets preference over those in the main ready queue

Process from the auxiliary queue runs for remaining time from last time-slice

Shortest Process Next

• Process with shortest expected processing time is selected next

• Short process jumps ahead of longer processes

• Nonpreemptive policy

Shortest Process Next

Shortest Process Next

• Possibility of starvation for longer processes• If estimated time for process not correct, the

operating system may abort it• How to estimate a process execution time?• Average of history?

• Simple average: Sn+1 = 1/n Ti

• Exponential average: Sn+1 = Tn + (1-)Sn

– Greater weights to more recent instances

Shortest Remaining Time• Preemptive version of shortest process next

policy

Highest Response Ratio Next

• Choose next process with the greatest ratio

– shorter processes are favored– aging without service increases the ratio so that

a longer process will eventually be served

Highest Response Ratio Next

• Choose next process with the greatest ratio

Feedback Scheduling• Don’t know remaining time process needs

to execute• Penalize jobs that have been running

longer: putting them into lower priority ready queues

Feedback Scheduling

Feedback Scheduling

Fair-Share Scheduling

• User’s application runs as a collection of processes (threads)

• User is concerned about the performance of the application

• Need to make scheduling decisions based on process sets

Fair-Share Scheduling

• CPUj(i) = CPUj(i-1)/2

• GCPUk(i) = GCPUk(i-1)/2

• Pj(i) = Basej + CPUj(i)/2 + GCPUk(i)/(4Wk)

Fair-Share Scheduler

In-Class Exercise

• Prob. 9.2 Consider the following set of processes:

Perform FCFS, RR (q=4), SPN, SRT, HRRN, Feedback (q=2i) on them and get the Finish Time and Turnaround Time for each process.

Process Name Arrival Time Processing Time

ABCDE

013912

35255

Appendix

Exponential Smoothing Coefficients

Use Of Exponential Averaging

Use Of Exponential Averaging

Effect of Size of Preemption Time Quantum

Effect of Size of Preemption Time Quantum