Itfn 2601 introduction to operating systems
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ITFN 2601 Introduction to Operating Systems. Lecture 4 Scheduling. Agenda. Scheduling Batch Interactive Real-Time Threads. Scheduling: When. New Process is Created Parent Process Child Process Process Exits When a process blocks I/O Interrupt occurs Clock Interrupts Non-preemptive

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ITFN 2601 Introduction to Operating Systems

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Itfn 2601 introduction to operating systems

ITFN 2601Introduction to Operating Systems

Lecture 4




  • Scheduling

    • Batch

    • Interactive

    • Real-Time

    • Threads

Scheduling when

Scheduling: When

  • New Process is Created

    • Parent Process

    • Child Process

  • Process Exits

  • When a process blocks

  • I/O Interrupt occurs

    • Clock Interrupts

    • Non-preemptive

    • Preemptive

Objectives of a good scheduling policy

Objectives of a Good Scheduling Policy

  • Fairness

  • Efficiency

  • Low response time (important for interactive jobs)

  • Low turnaround time (important for batch jobs)

  • High throughput



  • Throughput

    The 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)

  • Utilization

    For 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.



  • Turnaround

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

  • Response Time

    The 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 Time

    The 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.



  • Needs a clock interrupt (or equivalent)

  • Needed to guarantee fairness

  • Found in all modern general purpose operating systems

  • Without preemption, the system implements “run to completion (or yield)”

Scheduling algorithms batch

Scheduling Algorithms(Batch)

  • FIFO (First In First Out)

    • Fairest

    • Low throughput

    • High Turnaround

  • Shortest First

    • High Throughput

    • Low Turnaround

    • Unfair for Large Jobs

Scheduling algorithms batch cont

Scheduling Algorithms(Batch, cont)

  • Shortest Remaining

    • High Turnaround on Long Jobs

    • Unfair for Large Jobs

  • Multi-Scheduling (CPU or Memory Limited)

    • HIGH Turnaround (disk swaps)

    • Throughput highly variable, probably low

    • Fairness highly variable

First come first served

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.

Itfn 2601 introduction to operating systems


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

  • No starvation

  • If one process gets into an infinite loop, it will run forever and shut out all the others

  • FCFS tends to excessively favor long bursts. CPU-bound processes

Shortest job first sjf

Shortest Job First (SJF)

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

  • Non-preemptive policy

  • Preemptive version of the SJN, called shortest remaining time first

  • Starvation is possible

Scheduling algorithms interactive

Scheduling Algorithms(Interactive)

  • Round Robin

    • Fairest overall

    • Response time variable but finite

  • Priority Scheduling

    • Fair

      • “More Fair” for users with higher priorities

    • Response 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 robin1

Round Robin

  • An important preemptive policy

  • Essentially the preemptive version of FCFS

  • The key parameter is the quantum size q

  • When a process is put into the running state, a timer is set to q

  • If the timer goes off and the process is still running, the OS preempts the process.

  • The process is moved to the ready state

  • The next job in the queue is selected to run

Round robin2

Round Robin

  • Quantum can’t be too large

  • Quantum can’t be too small

  • What quantum should we choose

    • Tradeoff

      • Small q makes system more responsive

      • Large q makes system more efficient since there’s less switching

Round robin example

Round Robin, Example

Priority scheduling

Priority Scheduling

  • Always run the highest priority process

  • Preemptive or non-preemptive

  • Priorities can be assigned externally to processes based on their importance

  • Assigned (and changed) dynamically

Other interactive scheduling

Other Interactive Scheduling

  • Multiple Queues

  • Shortest Process Next

  • Guaranteed Scheduling

  • Lottery Scheduling

  • Fair-Share Scheduling

Scheduling algorithms interactive cont

Scheduling Algorithms(Interactive, cont)

  • Multi-Quantized

    • Response time proportionate to quanta

    • More bookkeeping

  • Shortest Process Next

    • Estimation of process length

    • Unfair for large jobs

    • Fast response for small jobs

Scheduling algorithms interactive cont1

Scheduling Algorithms(Interactive, cont)

  • Guaranteed Scheduling

  • Lottery Scheduling

    • Alotted time proportional to Job Size/Importance

  • Sharing

    • Fair by user, not necessarily fair by job

    • Responses become disproportionate

Scheduling algorithms real time

Scheduling Algorithms(Real-Time)

  • Small Jobs

  • High Priority

  • Periodic/Aperiodic

  • Schedulable?

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

    • Sum(CPU/Period) <= 1

  • Static/Dynamic?

Scheduler mechanism

Scheduler Mechanism

  • Some processes are intrinsically more important at some times than others

    • Time-dependent response

    • High-priority request

  • How can a process raise it’s scheduling priority?

Thread scheduling

Thread Scheduling

  • User-level threads

    • Process has a Thread Scheduler

    • Same concept as Process Scheduler

    • Conflicts with Process Scheduling

  • Kernel Level Threads

    • Kernel has Thread and Process Scheduler

    • Two Quanta’s

      • Thread Quanta

      • Process Quanta



  • Scheduler responsible for many goals

  • Scheduling algorithms complex

  • Know your math!

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