OS Spring’04

Scheduling

Operating SystemsSpring 2004

OS Spring’04

Multiprogramming Multiprogramming: having multiple

jobs (processes) in the systemInterleaved (time sliced) on a single CPUConcurrently executed on multiple CPUsBoth of the above

Why multiprogramming?Responsiveness, utilization, concurrency

Why not?Overhead, complexity

OS Spring’04

ResponsivenessJob 1arrives

Job 1terminates

Job1 Job2 Job3

Job 2terminates

Job 3terminates

Job 2arrives

Job 3arrives

Job1Job3

Job2

Job 1 terminates Job 3 terminates

Job 2 terminates

OS Spring’04

Utilization

idle

idle

idle

idle

idle

idle

1st I/Ooperation

I/Oends

2nd I/Ooperation

I/Oends

3rd I/Ooperation

CPU

Disk

CPU

Disk idle idle

idle

idleJob1 Job1

Job1 Job1Job2

Job2

Job2

OS Spring’04

Workload matters!? Does it really matter?Yes, of course:

If all jobs are CPU bound (I/O bound), multiprogramming does not help to improve utilization

A suitable job mix is created by a long-term scheduling

Jobs are classified on-line to be CPU (I/O) bound according to the job’s history

OS Spring’04

Concurrency Concurrent programming

Several process interact to work on the same problem ls –l | more

Simultaneous execution of related applications Word + Excel + PowerPoint

Background execution Polling/receiving Email while working on

smth else

OS Spring’04

The cost of multiprogramming Switching overhead

Saving/restoring context wastes CPU cycles

Degrades performanceResource contentionCache misses

ComplexitySynchronization, concurrency control, deadlock avoidance/prevention

OS Spring’04

Short-Term Scheduling

running

ready blocked

created

schedule

preempt

event done

wait for event

terminated

OS Spring’04

Short-Term scheduling Process execution pattern consists

of alternating CPU cycle and I/O wait CPU burst – I/O burst – CPU burst – I/O

burst...

Processes ready for execution are hold in a ready (run) queue

STS schedules process from the ready queue once CPU becomes idle

OS Spring’04

Preemptive/non-preemptive Four events in which the scheduler may decide

to switch the CPU to a new processRunning process terminatesRunning process blocks (e.g., for I/O)Non-ready process becomes ready (or starts) Clock interrupt

The latter two cause preemption Advantages:

Better time sharing, responsivenessGuard against endless loops

Disadvantages:Higher programming complexity

Example: Unix disables interrupts within kernel modeDisrupts real-time

OS Spring’04

Metrics: Response time

Job arrives/becomes ready to run Starts running

Job terminates/blocks waiting for I/O

TwaitTrun

Tresp

Tresp= Twait + Trun

Response time (turnaround time) is the average over the jobs’Tresp

OS Spring’04

Metrics

Wait time: average of Twait

This parameter is under the system control

ThroughputNumber of complete process terminations within a time unit

Utilization Fraction of time CPU is utilized

OS Spring’04

Off-line vs. On-line scheduling Off-line algorithms

Get all the information about all the jobs to schedule as their inputOutputs the scheduling sequencePreemption is never needed

On-line algorithmsJobs arrive at unpredictable timesVery little info is available in advancePreemption compensates for lack of knowledge

OS Spring’04

First-Come-First-Serve (FCFS) Schedules the jobs in the order in

which they arriveOff-line FCFS schedules in the order the jobs appear in the input

Runs each job to completion Both on-line and off-line Simple, a base case for analysis Poor response time

OS Spring’04

Shortest Job First (SJF) Best response time

Short Long job

Long job Short

Inherently off-lineAll the jobs and their run-times must be available in advance

OS Spring’04

Preemption Preemption is the action of stopping

a running job and scheduling another in its place

Context switch: Switching from one job to another

OS Spring’04

Using preemption On-line short-term scheduling algorithms

Adapting to changing conditions e.g., new jobs arrive

Compensating for lack of knowledge e.g., job run-time

Periodic preemption keeps system in control

Improves fairnessGives I/O bound processes chance to run

OS Spring’04

Shortest Remaining Time first (SRT)

Job run-times are known Job arrival times are not known When a new job arrives:

if its run-time is shorter than the remaining time of the currently executing job:preempt the currently executing job and schedule the newly arrived job

else, continue the current job and insert the new job into a sorted queue

When a job terminates, select the job at the queue head for execution

OS Spring’04

Round Robin (RR) Both job arrival times and job run-

times are not known Run each job cyclically for a short

time quantumApproximates CPU sharing

Job 1arrives

Job1 3

Job 2arrives

Job 3arrives

2 1 2 31 2 31 2 1 Job2

Job 3terminates

Job 1terminates

Job 2terminates

OS Spring’04

ResponsivenessJob 1arrives

Job 1terminates

Job1 Job2 Job3

Job 2terminates

Job 3terminates

Job 2arrives

Job 3arrives

Job1Job3

Job2

Job 1 terminates Job 3 terminates

Job 2 terminates

OS Spring’04

Priority Scheduling Example: prioritize processes according

to their past CPU usageEquivalent to SJF with predicted next CPU burst time

16

1

8

1

4

1

2

1:

2

1

10 ,)1(

3211

1

nnnnn

nnn

TTTTE

ETE

Tn is the duration of the n-th CPU burst En+1 is the estimate of the next CPU burst

OS Spring’04

Multilevel feedback queues

quantum=10

quantum=20

quantum=40

FCFS

new jobs terminated

OS Spring’04

Multilevel feedback queues Priorities are implicit in this scheme Very flexible Starvation is possibleShort jobs keep arriving => long jobs

get starved Solutions:

Let it beAging

OS Spring’04

Real-life examples Solaris 2 Window 2000 Linux