I/O Management and Disk schedule
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I/O Management and Disk schedule. Device. Purpose. Partner. Data rate (KB/sec). I/O Devices. Keyboard. Input. Human. .01. Mouse. Input. Human. .02. Voice input. Input. Human. .02. Scanner. Input. Human. 200. Voice output. Output. Human. .06. Line printer. Output. Human.

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I/O Management and Disk schedule

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I o management and disk schedule

I/O Management and Disk schedule


I o management and disk schedule

Device

Purpose

Partner

Data rate (KB/sec)

I/O Devices

Keyboard

Input

Human

.01

Mouse

Input

Human

.02

Voice input

Input

Human

.02

Scanner

Input

Human

200

Voice output

Output

Human

.06

Line printer

Output

Human

1

Laser printer

Output

Human

100

Graphics display

Output

Human

30,000

CPU to frame buffer

Output

Human

200

Network terminal

Input or output

Machine

.05

Network-LAN

Input or output

Machine

200

Optical disk

Storage

Machine

500

Magnetic tape

Storage

Machine

2,000

Magnetic disk

Storage

Machine

2,000


I o management and disk schedule

No interrupts

Interrupts

I/O Techniques

I/O to memory transfer through processor

Programmed I/O

Interrupt-driven I/O

Direct I/O to memory transfer

Direct memory access (DMA)

  • DMA unit uses the bus to transfer data to/from memory

    • when the CPU is not using it, or

    • forces CPU to temporarily suspend (cycle stealing)

  • I/O channels

    • selector

    • multiplexor


I o management and disk schedule

Device

I/O

Device

I/O

Device

I/O

Scheduling

& Control

Scheduling

& Control

Scheduling

& Control

Hardware

Hardware

Hardware

User

processes

User

processes

User

processes

I/O organization

File

System

Logical

I/O

Communic.

architecture

Physical

organization

Local

File system

Remote


I o management and disk schedule

I/O Buffering

  • Why buffer?

    • I/O too slow

    • not possible to swap out a whole process

    • risk of single-process deadlock

    • page in memory would have to be locked

  • I/O devices are:

    • block-oriented: disk and other storage devices

    • stream-oriented: terminals, printers, com ports, mouse

  • Types of buffers (blocks, line or byte at-a-time)

    • single buffer

    • double

    • circular


I o management and disk schedule

Disk concepts: review

See the following page

http://home.ubalt.edu/abento/751/6iomgmt/os0606.html


I o management and disk schedule

  • Seek time

    • time to move disk arm to a track

    • S = m x n + s (m is a constant per disk, number of tracks transversed, s startup time)

  • Rotational delay

    • waiting time for a given sector align with the head

    • disks: =~ 3,600 rpm, average = 8.3 msec

    • diskettes= 300-600 rpm, average = 100 and 200 msec

  • Access time = seek + rotational delay

  • Transfer time

    • time to spin record by the head (to read or write)

    • T = b/ rN (b is number of bytes to be transferred, r rotation speed rpm/sec, N number of bytes on a track)

Disk performance parameters


I o management and disk schedule

OS maintains a queue of requests for each I/O device

Name

Description

Characteristic

Disk scheduling policies

Selection according to requestor:

RSS

random scheduling

for analysis only

FIFO

first-in, first-out

fairest of them all

PRI

priority by process

control not based on disk queue management

LIFO

last-in, first-out

maximize locality and resource utilization

Selection according to requested item:

SSTF

shortest service first

high use, small queues

SCAN

back and forth over disk

better service distribution

C-SCAN

one way with fast return

lower service variability

N-step-SCAN

SCAN of N records at a time

service guarantee

FSCAN

N-step-SCAN with N= queue size

load sensitive


I o management and disk schedule

  • a buffer in main memory for disk sectors

  • locality and locality of reference is what makes it to score hits

  • reading ahead and write-back delay

  • data delivery from the cache:

    • move from cache to user memory

    • shared memory and pointers: point do not move

  • Replacement strategy (similar to pages)

    • LRU (least recently used)

    • LFU (least frequently used)

    • FBR (frequency-based replacement)

  • Hits are not moved to top of queue as they might, pointers are used

  • Empirical: misses and hits are a function of the cache size

Disk Cache


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