I/O System

Objectives of I/O system:

• Take an application I/O request and send it to the physical device.
• Take whatever response comes back from the device and send it to the application.
• Optimize the performance of the various I/O requests.

I/O requests: In general, there may be many I/O requests for a device at the same time. These requests may come from multiple processes or the same process.

I/O Issues: The operating system is able to improve overall system performance if it can keep the various devices as busy as possible. It is important for the operating system to handle device interrupts as quickly as possible.

DISK SCHEDULING

Purpose of Disk Scheduling: Select a disk request from the queue of I/O requests and decide when to process this I/O request.

Issues in Disk Scheduling: Throughput (the number of disk requests that are completed in some period), Fairness (some disk requests may have to wait a long time before being served).

Goal of disk scheduling:

• High throughput
• Fairness and
• In any disk system with a moving read/write head, the seek time between cylinders takes a significant amount of time. This travelling head time should be minimized.

DISK SCHEDULING ALGORITHMS

• FCFS scheduling: first-come-first-served
• SSTF scheduling: shortest-seek-time-first
• SCAN scheduling
• C-SCAN scheduling: circular SCAN
• LOOK scheduling
• C-LOOK scheduling

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