Q.Which of the following is false about SJF?
S1: It causes minimum average waiting time
S2: It can cause starvation
(A) Only S1
(B) Only S2
(C) Both S1 and S2
(D) Neither S1 nor S2

Shortest Job First Scheduling

Imagine you're at a railway ticket counter. There are five people ahead of you. One wants a complex refund that will take 15 minutes. Another just needs a quick printout — 30 seconds. A third is buying a single ticket — 2 minutes. If the clerk serves them in the order they arrived, you might wait 20 minutes before your turn. But if the clerk instead handles the quickest tasks first, everyone who just needs a minute or two gets served almost instantly, and the long-winded customer waits a bit longer. The average waiting time for the whole group drops dramatically.

That's the core intuition behind Shortest Job First (SJF) scheduling.

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The Precise Statement

SJF is a CPU scheduling algorithm where the process with the smallest total execution time (burst time) is selected to run next. If two processes have the same burst time, tie-breaking is typically done by arrival order (FCFS).

There are two flavours:

• Non-preemptive SJF: Once a process gets the CPU, it runs to completion. The scheduler picks the shortest job among those that have arrived at the moment of decision.
• Preemptive SJF (also called Shortest Remaining Time First, SRTF): If a new process arrives with a burst time shorter than the remaining time of the currently running process, the CPU is taken away and given to the new one.

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How It Works (Non-preemptive Example)

Consider three processes arriving at time 0:

FCFS order (P1 → P2 → P3):

• P1 finishes at 6, P2 at 14, P3 at 17.
• Average waiting time = (0 + 6 + 14) / 3 = 6.67

SJF order (P3 → P1 → P2):

• P3 finishes at 3, P1 at 9, P2 at 17.
• Average waiting time = (0 + 3 + 9) / 3 = 4.0

The improvement is clear.

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The Catch: Starvation

SJF has a serious flaw. If short jobs keep arriving, a long job may never get the CPU — it gets pushed back indefinitely. This is called starvation. In practice, this is unacceptable for interactive systems where every process must eventually make progress.

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Key Takeaways for Exams

• SJF minimizes average waiting time.
• It requires knowledge of burst times — which is unrealistic in general-purpose systems.
• Non-preemptive SJF is simpler; preemptive SJF (SRTF) gives even lower average waiting time but more context switches.
• Starvation is the main practical drawback.

Shortest Job First: pick the smallest job next, get the lowest average wait — but only if you know the future.