Which one of the following CPU scheduling algorithms cannot be preemptive?

Question

A database has 25,000 fixed-length records of 100 bytes (primary key 15 bytes). The data file is block-aligned so each record is fully contained within a block. The file is indexed by a primary index file, also block-aligned and ordered. Block size is 1024 bytes and a block pointer is 5 bytes. Each index entry stores the block’s anchor key (15 bytes) and a pointer (5 bytes). Binary search on an index file of \(b\) blocks needs \(\lceil \log_2 b \rceil\) block accesses in the worst case. Given a key, the number of block accesses required to identify the data file block that may contain the record, in the worst case, is ____________.

Answer 1

The question is asking which scheduling policy, by its very definition, has no way to interrupt a running process once it has started. That rules out algorithms that only sometimes preempt and points at the one that structurally never can.

SRTF (Shortest Remaining Time First): constantly compares the remaining burst time of the running process against any newly arrived process, and switches the CPU over the moment a shorter job appears. This constant re-evaluation makes it preemptive by nature.
FCFS (First Come First Serve): processes are served strictly in arrival order, and whichever process is running keeps the CPU until it completes or blocks itself; nothing in the rule ever takes the CPU away early for a later arrival. There is no preemptive variant of FCFS, since interrupting arrival order would break the "first come first serve" rule itself.
Round Robin: hands out a fixed time quantum and force-switches to the next process the instant that quantum runs out, regardless of whether the current process is done. This forced time-based switch is a textbook case of preemption.
Priority Scheduling: commonly implemented with preemption, where a newly arrived higher priority process immediately grabs the CPU from a lower priority one that is running. So it supports a preemptive mode, even though a non-preemptive version also exists.

Three of the four algorithms, SRTF, Round Robin, and Priority, are able to run preemptively. FCFS is the odd one out, since its entire definition is built around never interrupting the currently running process.

Let's summarize:

Preemption means taking the CPU away from a running process before it finishes on its own.
FCFS never does this by definition, while SRTF, Round Robin, and Priority scheduling all can.

So the algorithm that cannot be preemptive is FCFS, option (B).

Answer 2

The question is asking which scheduling policy, by its very definition, has no way to interrupt a running process once it has started. That rules out algorithms that only sometimes preempt and points at the one that structurally never can.

SRTF (Shortest Remaining Time First): constantly compares the remaining burst time of the running process against any newly arrived process, and switches the CPU over the moment a shorter job appears. This constant re-evaluation makes it preemptive by nature.
FCFS (First Come First Serve): processes are served strictly in arrival order, and whichever process is running keeps the CPU until it completes or blocks itself; nothing in the rule ever takes the CPU away early for a later arrival. There is no preemptive variant of FCFS, since interrupting arrival order would break the "first come first serve" rule itself.
Round Robin: hands out a fixed time quantum and force-switches to the next process the instant that quantum runs out, regardless of whether the current process is done. This forced time-based switch is a textbook case of preemption.
Priority Scheduling: commonly implemented with preemption, where a newly arrived higher priority process immediately grabs the CPU from a lower priority one that is running. So it supports a preemptive mode, even though a non-preemptive version also exists.

Three of the four algorithms, SRTF, Round Robin, and Priority, are able to run preemptively. FCFS is the odd one out, since its entire definition is built around never interrupting the currently running process.

Let's summarize:

Preemption means taking the CPU away from a running process before it finishes on its own.
FCFS never does this by definition, while SRTF, Round Robin, and Priority scheduling all can.

So the algorithm that cannot be preemptive is FCFS, option (B).

Which one of the following CPU scheduling algorithms cannot be preemptive?

Show Hint

The Correct Option is B

Solution and Explanation

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