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 in
the 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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Previous: LCN scheduling
Index : Operating System
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