Consider an operating system capable of loading and executing a single sequential user process at a time. The disk head scheduling algorithm used is First Come First Served (FCFS). If FCFS is replaced by Shortest Seek Time First (SSTF), claimed by the vendor to give better benchmark results, what is the expected improvement in the I/O performance of user programs?
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Answer: D
To find the expected improvement in I/O performance, we need to analyze how disk requests are generated and queued in this system:
- Single Sequential Process: The operating system can run only one user process at a time, and this process is sequential.
- Synchronous/Sequential I/O: A sequential process issues a disk I/O request and blocks (waits) until that request is completed before it can proceed to issue the next request.
- Queue Length: Because there is only one process running and it requests blocks sequentially, the disk controller's request queue will contain at most one outstanding request at any given time.
When the queue length is at most , any disk scheduling algorithm (whether FCFS, SSTF, SCAN, or LOOK) has no choice of requests to reorder. It must service the single pending request immediately.
Consequently, SSTF behaves identically to FCFS in this environment. The expected improvement in the I/O performance of the user programs is .
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