CPU Scheduling CPU Scheduling

Process Scheduling is process of Process Scheduling is process of deciding CPU is assign to which process deciding CPU is assign to which process

There are two types of CPU There are two types of CPU Scheduling AlgorithmsScheduling Algorithms

1. Preemptive Scheduling1. Preemptive Scheduling 2. Non preemptive Scheduling 2. Non preemptive Scheduling

Non Preemptive Non Preemptive Scheduling Scheduling

In this method once the CPU has been In this method once the CPU has been allocated to a process ,the process allocated to a process ,the process keeps the CPU until it release the keeps the CPU until it release the CPU either by terminating or switching CPU either by terminating or switching to the waiting state to the waiting state

Preemptive Scheduling Preemptive Scheduling

In this method take the CPU forcibly In this method take the CPU forcibly from one process and assign to from one process and assign to another process another process

CPU scheduling decision depending on CPU scheduling decision depending on the following four circumstances the following four circumstances

1. when a process switches from the 1. when a process switches from the running state to the waiting staterunning state to the waiting state

2. when the process switches from running 2. when the process switches from running state to the ready statestate to the ready state

3. when the process switches from waiting 3. when the process switches from waiting state to the ready state state to the ready state

4. when a process terminates 4. when a process terminates

Scheduling Criteria Scheduling Criteria

CPU scheduling depending the CPU scheduling depending the following criteria's following criteria's

1.1. CPU Utilization CPU Utilization 2.2. Throughput Throughput 3.3. Turnaround TimeTurnaround Time4.4. Waiting TimeWaiting Time

Non Preemptive Non Preemptive Scheduling Scheduling

1.1. FCFS (First Come First Served)FCFS (First Come First Served)2.2. SJF ( Shortest Job First)SJF ( Shortest Job First)3.3. Priority Scheduling Priority Scheduling

FCFS FCFS

In this Schema the process that In this Schema the process that requests the CPU first is allocated the requests the CPU first is allocated the CPU first CPU first

The implementation of FCFS based on The implementation of FCFS based on FIFO queueFIFO queue

Consider the following set of processes Consider the following set of processes that arrive at time 0, with the length of that arrive at time 0, with the length of the CPU burst time given in the CPU burst time given in milliseconds milliseconds

Process Process Burst Time Burst Time p1 24p1 24 p2 3p2 3 p3 3p3 3

p1 p2 p3

0 24 27 30

Consider the following set of processes Consider the following set of processes that arrive at time 0, with the length of that arrive at time 0, with the length of the CPU burst time given in the CPU burst time given in milliseconds milliseconds

Process Process Burst Time Burst Time p1 6p1 6 p2 8p2 8 p3 7p3 7 p4 3p4 3

Priority Algorithm Priority Algorithm

In this technique a priority is In this technique a priority is associated with each process and the associated with each process and the CPU is allocated to the process with CPU is allocated to the process with highest priority highest priority

If two processes are having equal If two processes are having equal priority processes are scheduled priority processes are scheduled based on FCFS based on FCFS

Consider the following set of processes Consider the following set of processes that arrive at time 0, with the length of that arrive at time 0, with the length of the CPU burst time and priorities are the CPU burst time and priorities are given in below given in below

Process Process Burst Time Priority Burst Time Priority p1 10 3p1 10 3 p2 1 1p2 1 1 p3 2 3p3 2 3 p4 1 4p4 1 4 p5 5 2p5 5 2

p2 p5 p1 p3 p4

0 1 6 16 18 19

TAT Wait Time

P1

P2

P3

P4

p5

16-0=16

1-0=1

18-0=18

19-0=19

6-0=6

16-10=6

1-1=0

18-2=16

19-1=18

6-5=1

Preemptive algorithmsPreemptive algorithms

Preemptive SJFPreemptive SJF Preemptive PriorityPreemptive Priority Round Robin Round Robin

Preemptive SJFPreemptive SJFThis technique depending on two This technique depending on two

conditioncondition1. When new job enter into ready queue1. When new job enter into ready queue check the CPU burst time of new job check the CPU burst time of new job

with the remaining CPU burst time of with the remaining CPU burst time of the Current jobthe Current job

case 1: if the CPU burst time of new job case 1: if the CPU burst time of new job less than remaining CPU burst time of less than remaining CPU burst time of the current job CPU suspend the the current job CPU suspend the current job temporarily and take new current job temporarily and take new job for executionjob for execution

Case 2: if the remaining CPU burst time of Case 2: if the remaining CPU burst time of current job is less than or equal to new job current job is less than or equal to new job

then the CPU continues execution with then the CPU continues execution with the current job the current job

2. When job terminate2. When job terminate

Consider four jobs along with CPU burst Consider four jobs along with CPU burst time and arrival time time and arrival time

Process Process Burst Time Arrival Time Burst Time Arrival Time j1 5 0j1 5 0 j2 2 2 j2 2 2 j3 1 3j3 1 3 j4 3 5j4 3 5

j1 j2 j2 j3 j1 j4

0 2 3 4 5 8 11

TAT Wait Time

j1

j2

j3

j4

8-0=8

4-2=2

5-3=2

11-5=6

8-5=3

2-2=0

2-2=0

6-3=3

Round Robin Algorithm Round Robin Algorithm

Round Robin designed for time Round Robin designed for time sharing systemssharing systems

In this technique each process assign In this technique each process assign certain time quantum or time slice at certain time quantum or time slice at all timeall time

The time quantum ranges from 10 to The time quantum ranges from 10 to 100 mille seconds 100 mille seconds

Example of RR with Time Quantum = 20Example of RR with Time Quantum = 20

ProcessProcess Burst TimeBurst TimePP11 5353 PP22 1717 PP33 6868 PP44 2424

P1 P2 P3 P4 P1 P3 P4 P1 P3 P3

0 20 37 57 77 97 117 121 134 154 162

Multi level QueueMulti level Queue Ready queue is partitioned into separate queues:Ready queue is partitioned into separate queues:

foreground (interactive)foreground (interactive)background (batch)background (batch)

Each queue has its own scheduling algorithmEach queue has its own scheduling algorithm– foreground – RRforeground – RR– background – FCFSbackground – FCFS

Scheduling must be done between the queuesScheduling must be done between the queues– Fixed priority scheduling; (i.e., serve all from Fixed priority scheduling; (i.e., serve all from

foreground then from background). Possibility of foreground then from background). Possibility of starvation.starvation.

– Time slice – each queue gets a certain amount Time slice – each queue gets a certain amount of CPU time of CPU time

Multi level Queue Multi level Queue SchedulingScheduling

Multi level Feedback QueueMulti level Feedback Queue

A process can move between the various queues A process can move between the various queues Multilevel-feedback-queue scheduler defined by the Multilevel-feedback-queue scheduler defined by the

following parameters:following parameters:– number of queuesnumber of queues– scheduling algorithms for each queuescheduling algorithms for each queue– method used to determine when to upgrade a method used to determine when to upgrade a

processprocess– method used to determine when to demote a method used to determine when to demote a

processprocess– method used to determine which queue a process method used to determine which queue a process

will enter when that process needs servicewill enter when that process needs service

Example of Mult i level Feedback QueueExample of Mult i level Feedback Queue

Three queues: Three queues: – QQ00 – RR with time quantum 8 milliseconds – RR with time quantum 8 milliseconds

– QQ11 – RR time quantum 16 milliseconds – RR time quantum 16 milliseconds

– QQ22 – FCFS – FCFS SchedulingScheduling

– A new job enters queue A new job enters queue QQ00 which is servedwhich is served FCFS. When it gains CPU, job receives 8 FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue milliseconds, job is moved to queue QQ11..

– At At QQ11 job is again served FCFS and receives 16 job is again served FCFS and receives 16 additional milliseconds. If it still does not additional milliseconds. If it still does not complete, it is preempted and moved to queue complete, it is preempted and moved to queue QQ22..

Multi level Feedback Multi level Feedback QueuesQueues

Multiple-Processor SchedulingMultiple-Processor Scheduling

CPU scheduling more complex when CPU scheduling more complex when multiple CPUs are availablemultiple CPUs are available

Homogeneous processorsHomogeneous processors within a within a multiprocessormultiprocessor

Load sharingLoad sharing Asymmetric multiprocessingAsymmetric multiprocessing – only one – only one

processor accesses the system data processor accesses the system data structures, alleviating need for data sharingstructures, alleviating need for data sharing

Real-Time SchedulingReal-Time Scheduling

Hard real-timeHard real-time systems – required to systems – required to complete a critical task within a complete a critical task within a guaranteed amount of timeguaranteed amount of time

Soft real-timeSoft real-time computing – requires that computing – requires that critical processes receive priority over critical processes receive priority over less fortunate onesless fortunate ones