Detail Explanation of Weighted Round Robin Scheduling | Real Time System

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weighted round robin schedulingWeighted Round Robin Scheduling,
Real Time System Notes | Sixth Semester,
BSc.CSIT | Tribhuvan University (TU)

Weighted Round Robin Scheduling
Weighted Round Robin (WRR) scheduling is used to facilitate controlled sharing of the network bandwidth. It is commonly used for scheduling real time traffic in high speed switched networks. Different jobs are given different weights, which refers to the fraction of processor time allocated to the job. A job with weight ‘wt’gets ‘wt’ time slices every round and length of a round is equal to the sum of the weights of all the ready jobs.

By adjusting the weights of jobs, we can speed up or retard the progress of each job towards it’s completion. By giving each job a fraction of the processor, a Round Robin scheduler delays the completion of every job. Weighted Round Robin is not suited for scheduling precedence constrained jobs as response time of a chain of jobs can be unduly large. This scheduling is reasonable with producer and consumer jobs as they execute concurrently in a pipelined fashion.

figure (a)

Consider two sets of job J1={J1,1 , J1,2} and J2={J2,1 , J2,2}
Release time of all jobs are 0 and execution time 1.
J1,1 and J2,1 execute on processor P1 and J1,2 , J2,2 on P2.
Suppose J1,1 is predecessor of J1,2 and J2,1 is predecessor of J2,2.

figure (b)

both job complete at the time T4 if jobs are scheduled in Weighted Round Robin.
figure (c)

If jobs are executed one after another, one of the chain can complete in time 2 while other in 3.
It is better to schedule jobs on RR because both sets can complete a few time slices after time 2.

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