cpu scheduling by furqan

7/31/2019 CPU Scheduling by Furqan

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CPU Scheduling

Presented by:

Muhammad Ameen

Muhammad Furqan


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Why CPU Scheduling Is Required?

The objective of multiprogramming is tohave some process running at all timesto maximize CPU utilization.

Almost all computer resources arescheduled before used.


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CPU and I/O Burst Cycle

The execution of a process consists of acycle of CPU and I/O wait.

A process begins with a CPU burst , followed byan I/O burst , followed by another CPU burst andso on. The last CPU burst will end will a systemrequest to terminate the execution.

The CPU burst durations vary from process

to process and computer to computer.An I /O bound program has many very short CPUbursts.A CPU bound program might have a few very

long CPU bursts.


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CPU Scheduler

Whenever, the CPU becomes idle, the OSmust select one of the processes in theready-queue to be executed.The selection process is carried out the CPUscheduler.The CPU scheduler select a process from the

ready queue and allocates the CPU to thatprocess.


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Types of Scheduling

There are four types of scheduling that anOS has to perform:

Long Term scheduling Medium-Term Scheduling Short-Term Scheduling I/O Scheduling


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Long Term scheduling

The long term scheduling determineswhich programs are admitted to thesystem for processing.

It controls the level of multiprogramming.Long term scheduling is performed when anew process is created.The criteria used for long-term schedulingmay include first-come-first serve, priority,expected execution time, and I/Orequirements.


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Medium-Term Scheduling

The medium-term scheduling is a part ofswapping function.This is a decision to add a process to those

that are at least partially in main memory andtherefore available for execution.

The swapping- In decision is made on the needto manage the degree of multiprogramming andthe memory requirements of the swapped-outprocess.


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Short-Term Scheduling

A decision of which ready process to executenext is made in short-term scheduling.

I/O SchedulingThe decision as to which processs pendingI/O requests shall be handled by theavailable I/O device is made in I/O

scheduling.


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Scheduling Criteria

CPU utilization : keep the CPU as busy as possible (from 0% to100%)

Throughput : # of processes that complete their execution per

time unitTurnaround time :Amount of time to execute a particularProcess

Waiting time :Amount of time a process has been waiting in theready queue

Response time:Amount of time it takes from when a request wassubmitted until the first response is produced


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Optimization Criteria

Max CPU utilizationMax throughput

Min turnaround timeMin waiting timeMin Response time


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Scheduling Algorithms

CPU scheduling deals with the problem ofdeciding which of the processes in the

ready queue is to be allocated the CPU.There are many different CPU schedulingalgorithms, which we will discuss now.


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Scheduling Algorithms

First-Come First-Served (FCFS)SchedulingShortest-Job First Scheduling (SJF)Priority SchedulingRound-Robin Scheduling


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First-Come First-Served(FCFS) Scheduling

The process that requests the CPU first isallocated the CPU first. It is no preemptivealgorithm.Can easily be implemented with a FIFOqueue.When a process enters the ready queue, its

PCB is linked onto the tail of the queue.When CPU is free, it is allocated to theprocess at the head of the queue.


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Advantages

Very simpleDisadvantages

Long average and worst-case waitingtimesPoor dynamic behavior (convoy effect -short process behind long process)


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Example: Process Burst Time

P1 24

P2 3

P3 3

Suppose that the processes arrive in the orderP1, P2,P3.The Gantt Chart for the schedule is:

P 1 P 2 P 3 24 27 30

0

Waiting time for P1 = 0; P2 = 24; P3 = 27

Average waiting time: (0 + 24 + 27)/3 = 17


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Cont Suppose that the processes arrive in the orderP2 , P3 , P1 . The Gantt chart for the schedule is:

P 1 P 3 P 2

6 3 30 0

Waiting time for P1 = 6; P2 = 0 ; P3 = 3Average waiting time: (6 + 0 + 3)/3 = 3


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Shortest-Job First Scheduling (SJF)

This algorithm associates with each processthe length of its next CPU burst.

When the CPU is available, it is assignedthe process that has the smallest next CPUburst.It is a non-preemptive policy.


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Cont

If a new process arrives with CPU burstlength less than remaining time of currentexecuting process, preempt the currently

executing process and allocate the CPU tothe new process.


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Cont

Advantages:Minimizes average waiting times.

Problems:How to determine length of next CPUburst?Problem: starvation of jobs with longCPU bursts.


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Priority Scheduling

In priority scheduling, a priority (an integer)is associated with each process.Priorities can be assigned either externallyor internally.The CPU is allocated to the process withthe highest priority (smallest integer meanshighest priority).

Priority scheduling can be:PreemptiveNo preemptive


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ProblemStarvation or indefinite blocking lowpriority processes may never execute.

Solution Aging as time progresses increase thepriority of the process.


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Round-Robin Scheduling

Each process gets a small unit of CPU time(called time quantum ), usually 10-100milliseconds.

After this time has elapsed, the process ispreempted and added to the end of theready queue.

Ready queue is treated as a circular queue.

CPU scheduler goes around the readyqueue, allocating the CPU to each processfor a time interval of 1 time quantum.


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cpu scheduling by furqan

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