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ROUND ROBINCPU

SCHEDULING

ALGORITHM


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WHAT IS ROUND ROBIN SCHEDULING?

The round-robin (RR) scheduling algorithm is designed especially for timesharingsystems. It is similar to FCFS scheduling, but preemption is added to

switch between processes. A small unit of time, called a time quantum or time

slice, is defined. A time quantum is generally from 10 to 100 milliseconds. The

ready queue is treated as a circular queue. The CPU scheduler goes around the

ready queue, allocating the CPU to each process for a time interval of up to 1

time quantum.

To implement RR scheduling, we keep the ready queue as a FIFO queue of

processes. New processes are added to the tail of the ready queue. The CPU

scheduler picks the first process from the ready queue, sets a timer to interrupt

after 1 time quantum, and dispatches the process.

One of two things will then happen. The process may have a CPU burst of

less than 1 time quantum. In this case, the process itself will release the CPUvoluntarily. The scheduler will then proceed to the next process in the ready

queue. Otherwise, if the CPU burst of the currently running process is longer

than 1 time quantum, the timer will go off and will cause an interrupt to the

operating system. A context switch will be executed, and the process will be

put at the tail of the ready queue. The CPU scheduler will then select the nextprocess in the ready queue.

PROS OF ROUND ROBIN Starvation can never occur, since no priority is given. Order of time unit allocation

is based upon process arrival time, similar to FCFS. Balanced throughput between FCFS and SJF, shorter jobs are completed faster

than in FCFS and longer processes are completed faster than in SJF.

CONS OF ROUND ROBIN

Poor average response time, waiting time is dependent on number of processes,and not average process length.

Because of high waiting times, deadlines are rarely met in a pure RR system.


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Table depicting various

scheduling algorithms and

their corresponding

parameters.

Scheduling algorithm CPUOverhead Throughput Turnaround time Response time

First In First Out Low Low High Low

Shortest Job First Medium High Medium Medium

Priority based scheduling Medium Low High High

Round-robin scheduling High Medium Medium High

Multilevel Queue scheduling High High Medium Medium
http://en.wikipedia.org/wiki/Scheduling_algorithmhttp://en.wikipedia.org/wiki/Scheduling_algorithmhttp://en.wikipedia.org/wiki/CPUhttp://en.wikipedia.org/wiki/CPUhttp://en.wikipedia.org/wiki/Throughputhttp://en.wikipedia.org/wiki/Response_timehttp://en.wikipedia.org/wiki/Response_timehttp://en.wikipedia.org/wiki/First_In_First_Outhttp://en.wikipedia.org/wiki/Shortest_Job_Firsthttp://en.wikipedia.org/wiki/Fixed_priority_pre-emptive_schedulinghttp://en.wikipedia.org/wiki/Fixed_priority_pre-emptive_schedulinghttp://en.wikipedia.org/wiki/Round-robin_schedulinghttp://en.wikipedia.org/wiki/Multilevel_feedback_queuehttp://en.wikipedia.org/wiki/Multilevel_feedback_queuehttp://en.wikipedia.org/wiki/Multilevel_feedback_queuehttp://en.wikipedia.org/wiki/Round-robin_schedulinghttp://en.wikipedia.org/wiki/Fixed_priority_pre-emptive_schedulinghttp://en.wikipedia.org/wiki/Shortest_Job_Firsthttp://en.wikipedia.org/wiki/First_In_First_Outhttp://en.wikipedia.org/wiki/Response_timehttp://en.wikipedia.org/wiki/Throughputhttp://en.wikipedia.org/wiki/CPUhttp://en.wikipedia.org/wiki/Scheduling_algorithm


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IMPROved ROUND ROBIN(Context switch

method)1.1 In this improvement we try to decrease the turnaround time by decreasing the number of the switching

operations between one process and the other. This operation is done by checking the remain time of the

execution process, if the remain time is less than the time quantum then the process will stay in the processor

(CPU) until it complete its execution, so there will be no switching operation and this will eliminate the switch that

needed by the context switch.

Consider the following example.

Process no Burst time

1 350

2 125

3 4754 250

5 75

Taking time quantum of 60 units.

Context switch shown with simple ROUND ROBIN algorithm


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Context switches shown with improved round robin

Conclusion

We observe that using this algorithm turnaround time andwaiting time decreases.


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Lets consider a suitable example to evaluate this algorithm.

Example:Consider 5 processes with burst times 30,62,74,88,34(take time

quanta=25).

According to algorithm we need to arrange these bursttimes in ascending order.


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Gantt chart for simple round robin scheduling.

By improvising the algorithm we get.

N=5Median=Y(n+2)/2=62

New time quanta= 62+88/2=75

Using above time quanta Improved Gantt chart is:-

Result

Table showing comparison between the variousparameters for the following data.

Qt=quantum time

Cs=context switch

Awt=average waiting time

Att=average turn around time

We can use this for processes with or without arrival

times.


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Conclusion:We observe that the proposed algorithm

gives increased time quanta but reduces others parameters

significantly ie its more efficient than existing RR


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TERMINOLOGIES USED IN

CPU scheduling

CPU Utilization. We want to keep the CPU asbusy as possible.

Throughput. If the CPU is busy executing

processes, then work is being done. One

measure of work is the number of processes

that are completed per time unit, called

throughput. For long processes, this rate maybe one process per hour; for short transactions,

it may be 10 processes per second.

Turnaround time. From the point of view of a

particular process, the important criterion is

how long it takes to execute that process. The

interval from the time of submission of a

process to the time of completion is theturnaround time. Turnaround time is the sum

of the periods spent waiting to get into

memory, waiting in the ready queue, executing

on the CPU, and doing I/O.

Waiting time. The CPU scheduling algorithm

does not affect the amount of the time during

which a process executes or does I/O; it affectsonly the amount of time that a process spends

waiting in the ready queue. Waiting time is the

sum of periods spend waiting in the ready

queue.


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Response time. In an interactive system,

turnaround time may not be the best criterion.

Often, a process can produce some output

fairly early and can continue computing newresults while previous results are being output.

PROCESS STATE DIAGRAM

Various Scheduling Algorithms Used are:-

FCFS(First Come First Serve)

SJF(shortest job first)

Round Robin

Priority scheduling

Multilevel Queue Scheduling

Multilevel Feedback Queue,Multilevel Queue Scheduling among Queues


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Typical example of Round robin


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Improved round robin(combining sjf andround robin)In our Proposed RR scheduling algorithm we

have

combined the working of SJF (shortest job

first)

scheduling algorithm along with

contemporary RR

scheduling algorithmProposed algorithm1. Allocate all processes to the CPU only one

time as like present Round Robin scheduling

algorithm.

2. After first time we select shortest job from the

waiting queue and it shortest job assign to the

CPU.3. After that we select next shortest job and do

step 2

4. Till the complete execution of all processes we

repeat steps 2 and 3 that means while all the

processes has not been finished(executed).

Example


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