Rensselaer polytechnic institute csc 432 operating systems david goldschmidt ph d
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Rensselaer Polytechnic Institute CSC 432 – Operating Systems David Goldschmidt, Ph.D. Operating Systems { week 06.b }. Short-term (CPU ) scheduling. the dispatcher operates here. CPU scheduling algorithms (i).

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Rensselaer polytechnic institute csc 432 operating systems david goldschmidt ph d

Rensselaer Polytechnic Institute

CSC 432 – Operating Systems

David Goldschmidt, Ph.D.

Operating Systems{week 06.b}


Short term cpu scheduling

Short-term (CPU) scheduling

the dispatcher operates here


Cpu scheduling algorithms i

CPU scheduling algorithms (i)

  • CPU scheduling requires an algorithm todetermine which process to dispatch next

  • Scheduling algorithms include:

    • First-Come, First-Served (FCFS)

    • Shortest-Job-First (SJF)

    • Round-Robin (RR)

    • Priority

    • Multilevel Queue (MQ)


Cpu scheduling algorithms ii

process

CPU scheduling algorithms (ii)

  • Preemptive scheduling preempts a running process before itstime slice expires

    • Or it preempts a processbecause its time slice has expired

  • Non-preemptive scheduling gives a process exclusive uninterrupted access to the CPU for the entirety of its execution

process

process

process


Cpu scheduling algorithms iii

CPU scheduling algorithms (iii)

  • Compare scheduling algorithms by measuring

    • CPU utilization – keep CPU as busy as possible

    • Throughput – maximize the number of processes that complete their execution per unit time

    • Turnaround time – minimize the elapsed time to fully execute a particular process

    • Waiting time – minimize the elapsed time a process waits in the ready queue


Round robin rr

P3

P1

P2

P3

P1

P3

P1

0

2

4

6

8

10

12

13

Round-Robin (RR)

  • RR is a preemptive algorithmthat gives all ready processesa fair time slice of CPU time

    • Using a time slice of 2 ms....

      time


Priority scheduling i

Priority Scheduling (i)

  • Associate a priority number with each process

    • The dispatcher selects the processwith the highest priority

    • For multiple ready processes withidentical priority numbers, use FCFS (or ...)

    • Key problem is starvation

      • Overcome starvation by aging, increasingthe priority of a process as it ages


Priority scheduling ii

process

Priority Scheduling (ii)

  • Is priority scheduling preemptiveor non-preemptive?

    • Non-preemptive priority scheduling places higher-priority processes at the head of the queue

    • Preemptive priority scheduling requires a running process to be interrupted and preempted upon the arrival of a higher-priority process

(use this one for Project #1)


Multiclass systems

Multiclass systems

  • Operating systems that support priority schemes are often called multiclass systems

use a separate schedulingalgorithm for each queue


Multilevel queue mq

Multilevel Queue (MQ)

  • Assign processes to multiple queues,each with its own scheduling algorithm


Multilevel feedback queue mfq

Multilevel Feedback Queue (MFQ)

  • Dynamically assign processes to multiple queues based on actual CPU burst times

    • i.e. feedback

quantum is synonymouswith time slice


Practice

Practice!

  • Apply the FCFS, SJF, RR, and Preemptive Priority scheduling algorithms to this table:

    • For RR, use a time slice of 10 ms

    • Calculate the wait and turnaround times ofeach process, as well as overall averages

lower number indicates

a higher priority

recalculate using

context switch

time tcs = 20 μs


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