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Deciding which process/thread should use a Resource

(CPU, disc, etc.)

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A priority number (integer) is associated with each process

The CPU is allocated to the process with the highest priority (smallest

integer highest priority)

A priority scheduling algorithm can be preemptive or non preemptive.

NonPreemptive

Pre-emptive

A Non preemptive

priority schedulingalgorithm will simply

put the new process

at the tail of the

Ready Queue.

A Preemptive priority

scheduling algorithm will

preempt the CPU if thepriority of the newly arrived

process is higher than the

priority of the currently

running process..

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SJF is a priority scheduling where priority is the predicted next CPU burst

time.

Problem Starvation

low priority processes may never execute

Solution Aging

as time progresses increase the priority of the process

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Round-robin scheduling

Pre emptive

Based on FIFO

time slice or quantum

Processes run only for a limited amount of time . After this time has

elapsed, the process is preempted and added to the end of the readyqueue

If there are n processes in the ready queue and the time quantum is

q, then each process gets 1/n of the CPU time in chunks of at most q

time units at once. No process waits more than (n-1)q time units.

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Process Burst Time

P1 53

P2

17

P3 68

P4 24

The Gantt chart is:

Typically, higher average turnaround than SJF, but better response

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162

Time Quantum = 20

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19 APRIL 2011 COPYRIGHT VIRTUAL UNIVERSITY OF PAKISTAN

Process Burst TimeP1

53 33 13P2

17

P3 68

48

28

8P4

24 4

The Gantt chart with quantum 20 is:

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162

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Process Turnaround Time Waiting Time

P1 134 134

53 = 81P2

37 37 17 = 20

P3

162 162 68 = 94

P4

121 121 24 = 97

Average waiting time = 73

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Very large quantum size

Processes run for long periods

Degenerates to FIFO

Very small quantum size

System spends more timecontext switching than runningprocesses

Quantum size

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Quantum size

Best quantum time size

Depends on system

Interactive: response time key factor

Batch: turnaround time key factor

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Ready queue is partitioned into separate queues:

foreground (interactive)

background (batch)

Each queue has its own scheduling algorithm

foreground RR

background FCFS

Scheduling must be done between the queues

Fixed priority scheduling; (i.e., serve all from foreground then from background).Possibility of starvation.

Time slice each queue gets a certain amount of CPU time which it can scheduleamongst its processes; i.e., 80% to foreground in RR 20% to background in FCFS

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Different processes have different needs

Short I/O-bound interactive processes should generallyrun before processor-bound batch processes

Multilevel feedback queues

Arriving processes enter the highest-level queue andexecute with higher priority than processes in lowerqueues

Long processes repeatedly descend into lower levels

Gives short processes and I/O-bound processes higherpriority

Long processes will run when short and I/O-boundprocesses terminate

Processes in each queue are serviced using round-robin

Process entering a higher-level queue preempt running

processes

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Similarly Aging can be done to prevent starvation i.e. the processes that

waits too long in the lower priority queue are moved to a higher priority

queue

Multilevel-feedback-queue scheduler defined by the following parameters:

number of queues

scheduling algorithms for each queue

method used to determine when to upgrade a process

method used to determine when to demote a process

method used to determine which queue a process will enter when that

process needs service

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Three queues:

Q0 RR with time quantum 8 milliseconds

Q1

RR time quantum 16 milliseconds Q2 FCFS

Scheduling

A new job enters queue Q0 which is served FCFS. When it gains CPU, job

receives 8 milliseconds. If it does not finish in 8 milliseconds, job ismoved to queue Q1.

At Q1job is again served FCFS and receives 16 additional milliseconds. Ifit still does not complete, it is preempted and moved to queue Q2.

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