ITFN 2601Introduction to Operating

Systems

Lecture 4

Scheduling

Agenda

SchedulingBatchInteractiveReal-TimeThreads

Scheduling: When

New Process is CreatedParent ProcessChild Process

Process ExitsWhen a process blocksI/O Interrupt occurs

Clock InterruptsNon-preemptivePreemptive

Objectives of a Good Scheduling Policy

FairnessEfficiencyLow response time (important for

interactive jobs)Low turnaround time (important for

batch jobs)High throughput

Scheduling

ThroughputThe amount of useful work accomplished per unit time. This depends, of course, on what constitutes “useful work.” One common measure of throughput is jobs/minute (or second, or hour, depending on the kind of job)

UtilizationFor each device, the utilization of a device is the fraction of time the device is busy. A good scheduling algorithm keeps all the devices (CPUs, disk drives, etc.) busy most of the time.

Scheduling

TurnaroundThe length of time between when the job arrives in the system and when it finally finishes

Response TimeThe length of time between when a job arrives in the system and when it starts to produce output. For interactive jobs, response time might be more important than turnaround.

Wait TimeThe amount of time the job is ready (runnable but not running). This is a better measure of scheduling quality than turnaround since the scheduler has no control of the mount of time the process spends computing or blocked waiting for I/O.

Preemption

Needs a clock interrupt (or equivalent)Needed to guarantee fairnessFound in all modern general purpose

operating systemsWithout preemption, the system

implements “run to completion (or yield)”

Scheduling Algorithms(Batch)

FIFO (First In First Out)FairestLow throughputHigh Turnaround

Shortest FirstHigh ThroughputLow TurnaroundUnfair for Large Jobs

Scheduling Algorithms(Batch, cont)

Shortest RemainingHigh Turnaround on Long JobsUnfair for Large Jobs

Multi-Scheduling (CPU or Memory Limited)HIGH Turnaround (disk swaps)Throughput highly variable, probably lowFairness highly variable

First-Come First-Served

The simplest possible scheduling discipline is called First-come, first-served (FCFS). The ready list is a simple queue (first-in/first-out). The scheduler simply runs the first job on the queue until it blocks, then it runs the new first job, and so on. When a job becomes ready it is simply added to the end of the queue.

FCFS

Main advantage of FCFS is that it is easy to write and understand

No starvationIf one process gets into an infinite loop, it

will run forever and shut out all the othersFCFS tends to excessively favor long

bursts. CPU-bound processes

Shortest Job First (SJF)

Whenever the CPU has to choose a burst to run, it chooses the shortest one

Non-preemptive policyPreemptive version of the SJN, called

shortest remaining time firstStarvation is possible

Scheduling Algorithms(Interactive)

Round RobinFairest overallResponse time variable but finite

Priority SchedulingFair

“More Fair” for users with higher prioritiesResponse time inverse to priority

Round-Robin

Round-robin (RR). RR keeps all the processes in a queue and runs the first one, like FCFS. After a length of time q (called quantum), if the current burst hasn’t completed, it is moved to the tail of the queue and the next process is started

Round Robin

An important preemptive policyEssentially the preemptive version of FCFSThe key parameter is the quantum size qWhen a process is put into the running state, a

timer is set to qIf the timer goes off and the process is still

running, the OS preempts the process.The process is moved to the ready stateThe next job in the queue is selected to run

Round Robin

Quantum can’t be too largeQuantum can’t be too smallWhat quantum should we choose

TradeoffSmall q makes system more responsiveLarge q makes system more efficient since

there’s less switching

Round Robin, Example

Priority Scheduling

Always run the highest priority processPreemptive or non-preemptivePriorities can be assigned externally to

processes based on their importanceAssigned (and changed) dynamically

Other Interactive Scheduling

Multiple QueuesShortest Process NextGuaranteed SchedulingLottery SchedulingFair-Share Scheduling

Scheduling Algorithms(Interactive, cont)

Multi-QuantizedResponse time proportionate to quantaMore bookkeeping

Shortest Process NextEstimation of process lengthUnfair for large jobsFast response for small jobs

Scheduling Algorithms(Interactive, cont)

Guaranteed SchedulingLottery Scheduling

Alotted time proportional to Job Size/Importance

SharingFair by user, not necessarily fair by jobResponses become disproportionate

Scheduling Algorithms(Real-Time)

Small JobsHigh PriorityPeriodic/AperiodicSchedulable?

Iff the sum of the ratios CPU Time to Period time is less than one

Sum(CPU/Period) <= 1Static/Dynamic?

Scheduler Mechanism

Some processes are intrinsically more important at some times than othersTime-dependent responseHigh-priority request

How can a process raise it’s scheduling priority?

Thread Scheduling

User-level threadsProcess has a Thread SchedulerSame concept as Process SchedulerConflicts with Process Scheduling

Kernel Level ThreadsKernel has Thread and Process SchedulerTwo Quanta’s

Thread QuantaProcess Quanta

Summary

Scheduler responsible for many goals

Scheduling algorithms complex

Know your math!