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Shortest Job First (SJF)
What is SJF?
 Shortest-job-first is a non-preemptive discipline in which
waiting job(or process) with the smallest estimated runtime-to-completion is run next
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 The SJF scheduling especially appropriate for batch jobs
for which the run times are known in advance
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 The SJF algorithm favours short jobs(or processors)at the
expense of longer ones
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Shortest Job First
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Non-preemptive - once CPU given to the process it cannot be
preempted until completes its CPU burst
Ready queue is treated as a priority queue based on smallest CPU
time requirement
Priorities are assigned in inverse order of time needed for completion
of the entire job
If equal time of completion, then FCFS is used for assigning priority
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Minimizes average turnaround time
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It cannot be implemented at the level of short term CPU scheduling
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Store estimated value in PCB for the current burst, and compare with
actual value
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One approach is to try and estimate the length of the next CPU burst,
based on the processes previous activity
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Tn+1 = at n + (1 – a)Tn
 This formula allows us to weight both the history of the
burst times and the most recent burst time
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If a = 1 then the history has no
effect and the guess is equal to the most recent burst time
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5 for a is often used so that equal weight is
given to recent and past history
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Provably optimal w
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t
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Disadvantages:
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In general, cannot be implemented
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If the ready
list is saturated, then processes with large service times tend to be
left in the ready list while small processes receive service
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This total starvation of large
processes may be a serious liability of this algorithm
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