ENVS 1800 Lecture Notes - Lecture 11: Logical Address, Auxiliary Memory, Hard Disk Drive Performance Characteristics
14 May 2018
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ENVS 1800 Tutorial 11 Notes – First-Come, First-Served Scheduling and Secondary storage
scheduling
Introduction
Secondary storage scheduling
• With virtual storage, every memory access request points to a logical address, not a
physical one.
• Since the logical address is within the space of the process itself, the translation process
assures that it is not possible to point to a physical address belonging to another process
• Unless the page tables have been set up intentionally to share frames between the
processes
• Thus, virtual storage provides simple, effective separation protection between
processes.
• On a busy system, it is common to have a number of disk requests pending at any given
time.
• The operating system software will attempt to process these requests in a way that
enhances the performance of the system.
• As you might expect by now, there are several different disk scheduling algorithms in
use.
First-Come, First-Served Scheduling
• First-come, first-served (FCFS) scheduling is the simplest algorithm.
• As requests arrive, they are placed in a queue and are satisfied in order.
• Although this may seem like a fair algorithm, its inefficiency may result in poorer service
to every request in the queue.
• The problem is that seek time on a disk is long and somewhat proportional to the
distance that the head has to move.
• With FCFS, one can expect the head to move all over the disk to satisfy requests.
• It would be preferable to use an algorithm that minimizes seek distances.
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