CPU scheduling fundamentals – Shortest Job First (SJF) In the SJF scheduling algorithm, which job is selected to execute next from the ready queue?

Introduction / Context:
SJF (Shortest Job First) is a classic CPU scheduling algorithm used in operating systems to minimize average waiting time by always running the process with the shortest next CPU burst. This concept appears in OS design, performance engineering, and interview questions on scheduling strategies.

Given Data / Assumptions:

• SJF chooses among ready processes based on predicted or known CPU burst length.
• All jobs are assumed to be in the ready queue and comparable by their expected CPU time.
• We are referring to non-preemptive SJF unless otherwise noted; preemptive SJF is often called Shortest Remaining Time First (SRTF).

Concept / Approach:

SJF minimizes the average waiting time by selecting the job that will finish the soonest, given its CPU burst length. If burst times are known exactly, SJF is provably optimal for average waiting time. In practice, burst times are estimated using exponential averaging or other heuristics derived from past behavior.

Step-by-Step Solution:

Verification / Alternative check:

Construct a small example with three jobs having CPU bursts 2, 4, and 8 units. Applying SJF (pick 2 first) yields lower average waiting time than FIFO (arrival order) or LIFO.

Why Other Options Are Wrong:

Last entered/first entered (options a and b) describe stack/queue discipline, not SJF. Longest waiting (option c) resembles aging or fair queueing, not SJF. “None of the above” is incorrect because one option matches exactly.

Common Pitfalls:

Confusing SJF with SRTF; assuming SJF uses arrival time; overlooking starvation risk for long jobs (often mitigated by aging).

Final Answer:

with the least processor needs.