Multiprogramming performance effect: Under multiprogramming, how does average turnaround time typically change for short jobs versus long jobs?

Introduction / Context:
Multiprogramming allows multiple jobs to share the CPU by interleaving execution. This impacts turnaround time, which is the total time from job submission to completion. Understanding the tradeoffs for short versus long jobs is core to operating system scheduling concepts.

Given Data / Assumptions:

• The system supports multiple jobs in memory at once.
• CPU and I O overlap is possible; when one job waits, another can run.
• Turnaround time includes waiting, running, and I O times.

Concept / Approach:

Short jobs benefit because they can grab small CPU quanta and complete sooner instead of waiting for a single long job to finish. Long jobs, however, face preemption and time slicing that can slightly extend their turnaround time compared to running alone because of context switching and competition for CPU.

Step-by-Step Solution:

Verification / Alternative check:

Empirical studies of round-robin and multi level feedback queue policies show improved responsiveness for short and interactive tasks, often at a small cost to long CPU bound tasks, aligning with the selected option.

Why Other Options Are Wrong:

• Lengthened; shortened or lengthened; lengthened contradict typical multiprogramming outcomes.
• Shortened; shortened assumes no overhead or competition effects on long jobs.
• None of the above is incorrect because one option matches standard behavior.

Common Pitfalls:

Equating throughput improvements with universal turnaround improvements; ignoring context switch overhead and queueing effects on large jobs.

Final Answer:

shortened; lengthened