HRN policy corrects some of the weakness in LCN and SRTN policies, particularly the excessive discrimination towards longer jobs and excessive favouritism towards short new jobs. HRN is a non-preemptive scheduling discipline I n which the priority of each job is a function not only of the job’s service time but also of the amount of time the job has been waiting for service. Once a job gets the CPU, it runs to completion. Dynamic priorities in HRN are calculated as follows;
Response Ratio = (time waiting + execution time received so far)
/ (execution time received so far)
HRN policy always selects the process with highest response ratio for execution. Since the execution time received appears in the denominator, shorter jobs will get preference. But because time waiting appears inthe numerator, longer jobs that have been waiting will also be given favourable treatment. HRN policy can perform well due to two reasons: no requests are starved of attention at the beginning of their life, as newly arriving requests would have a very high response ratio would increase steadily while they wait for service. In this manner, HRN incorporates the desired aspects of LCN and SRTN policies. Longer requests would not face difficulties in either starting or finishing their execution, while the weighted turnaround times of shorter jobs are not adversely affected by the presence of long jobs in the system.
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