Operating System: Chap13 I/O Systems

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Operating System:

Chap13 I/O Systems

National Tsing-Hua University

2016, Fall Semester

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Outline



Overview



I/O Hardware



I/O Methods



Kernel I/O Subsystem



Performance



Application Interface

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Chapter13 I/O Systems

Overview



The two main jobs of a computer



I/O

^and

Computation



I/O devices : tape, HD, mouse, joystick, network card, screen, flash disks, etc



I/O subsystem : the methods to control all I/O devices



Two conflicting trends

 Standardization of HW/SW interfaces

 Board variety of I/O devices

Operating System Concepts – NTHU LSA Lab 3

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Overview



Device drivers : a uniform device-access interface to the I/O subsystem

 Similar to system calls between apps and OS



Device categories

 Storage devices: disks, tapes

 Transmission devices: network cards, modems

 Human-interface devices: keyboard, screen, mouse

 Specialized devices: joystick, touchpad

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I/O Hardware



Port : A connection point between I/O devices and the host

 E.g.: USB ports



Bus : A set of wires and a well-defined protocol that specifies messages sent over the wires

 E.g.: PCI bus



Controller : A collection of electronics that can operate a port, a bus, or a device

 A controller could have its own processor, memory, etc. (E.g.: SCSI controller)

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Typical PC Bus Structure

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Basic I/O Method (Port-mapped I/O)

 Each I/O port (device) is identified by a unique port address

 Each I/O port consists of four registers (1~4Bytes)

 Data-in register: read by the host to get input

 Data-out register: written by the host to send output

 Status register: read by the host to check I/O status

 Control register: written by the host to control the device



Program interact with an I/O port through

special I/O instructions

(different from mem. access)

 X86: IN, OUT

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Device I/O Port Locations on PCs (partial)

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I/O Methods Categorization



Depending on how to address a device:

 Port-mapped I/O

Use different address space from memory

Access by special I/O instruction (e.g. IN, OUT)

 Memory-mapped I/O

Reserve specific memory space for device

Access by standard data-transfer instruction (e.g. MOV)

 More efficient for large memory I/O (e.g. graphic card)

 Vulnerable to accidental modification, error

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I/O Methods Categorization



Depending on how to interact with a device:

 Poll (busy-waiting): processor periodically check status register of a device

 Interrupt: device notify processor of its completion



Depending on who to control the transfer:

 Programmed I/O: transfer controlled by CPU

 Direct memory access (DMA) I/O: controlled by DMA controller (a special purpose controller)

Design for large data transfer

Commonly used with memory-mapped I/O and interrupt I/O method

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Six-Step Process to Perform DMA (Direct Memory Access)

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Review Slides ( I )



Definition of I/O port? Bus? Controller?



I/O device and CPU communication?

 Port-mapped vs. Memory-mapped

 Poll vs. Interrupt

 Programmed I/O vs. DMA



Steps to handle an interrupt I/O and DMA

request?

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Chapter13 I/O Systems Operating System Concepts - NTHU EOS Lab 13

Kernel I/O Subsystem

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I/O Subsystem



I/O Scheduling – improve system performance by ordering the jobs in I/O queue

 e.g. disk I/O order scheduling



Buffering – store data in memory while transferring between I/O devices

 Speed mismatch between devices

 Devices with different data-transfer sizes

 Support copy semantics

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I/O Subsystem



Caching – fast memory that holds copies of data

 Always just a copy

 Key to performance



Spooling – holds output for a device

 e.g. printing (cannot accept interleaved files)



Error handling – when I/O error happens

 e.g. SCSI devices returns error information



I/O protection

 Privileged instructions

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UNIX I/O Kernel Data Structure



Linux treats all I/O devices like a file

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Device-status Table

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Blocking and Nonblocking I/O



Blocking - process suspended until I/O completed

 Easy to use and understand

 Insufficient for some needs

 Use for synchronous communication & I/O



Nonblocking

 Implemented via multi-threading

 Returns quickly with count of bytes read or written

 Use for asynchronous communication & I/O

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Life Cycle of An I/O Request

In buffer Physical I/O

Data transfer Check buffer

cache

Move process from run queue to wait queue

Allocate kernel buffer Schedule I/O

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Performance



I/O is a major factor in system performance

 It places heavy demands on the CPU to execute device driver code

 The resulting context switches stress the CPU and its hardware caches

 I/O loads down the memory bus during data copy between controllers and physical memory, …

 Interrupt handling is a relatively expensive task

Busy-waiting could be more efficient than interrupt- driven if I/O time is small

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Improving Performance



Reduce number of context switches



Reduce data copying



Reduce interrupts by using large transfers, smart controllers, polling



Use DMA



Balance CPU, memory, bus, and I/O performance for highest throughput

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Review Slides ( II )



What are the key I/O services

 Scheduling

 Cache

 Buffering

 Spooling

 Error handling

 I/ protection



How to improve system performance?

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Chapter13 I/O Systems Operating System Concepts - NTHU EOS Lab 23

Application I/O Interface

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A Kernel I/O Structure

 Device drivers: a uniform device-access interface to the I/O subsystem; hide the differences among device

controllers from the I/O sub-system of OS

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Characteristics of I/O Devices

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I/O Device Class



Device class is fairly standard across different OS

 Block I/O

 Char-stream I/O

 Memory-mapped file access

 Network sockets

 Clock & timer interfaces



Back-door interfaces (e.g. ioctl() )

 Enable an application to access any functionality

implemented by a device driver without the need to invent a new system call

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Block & Char Devices



Block devices: disk drives

 system calls: read( ), write( ), seek( )

 Memory-mapped file can be layered on top



Char-stream devices: mouse, keyboard, serial ports

 system calls: get( ), put( )

 Libraries layered on top allow line editing

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Network Devices



Varying enough from block and character to have own interface

 System call: send(), recv(), select()

 select() returns which socket is waiting to send or receive, eliminates the need of busy waiting



Many other approaches

 pipes, FIFOS, STREAMS, message queues

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Reading Material & HW



13.1 – 13.6



Problem Set

 13.2

 13.5

 13.6

 13.8

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Interrupt Vector Table



Intel Pentium Processor:

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CPU and device Interrupt handshake

1. Device asserts interrupt request (IRQ)

2. CPU checks the interrupt request line at the beginning of each instruction cycle

3. Save the status and address of interrupted process

4. CPU acknowledges the interrupt and search the

interrupt vector table for interrupt handler routines

5. CPU fetches the next instruction from the interrupt handler routine

6. Restore interrupted process after executing interrupt handler routine

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Interrupt Prioritization



Maskable interrupt: interrupt with

priority lower than current priority is not recognized until pending interrupt is

complete



Non-maskable interrupt (NMI): highest- priority, never masked



Often used for power-down, memory error

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Chapter13 I/O Systems Operating System Concepts – NTHU LSA Lab 33

Interrupt-Driven I/O

CPU executing checks for interrupts between instructions

CPU Controller

initiates I/O 2

input ready, output complete, or error Generates interrupt signal

3 CPU receiving interrupt,

transfers control to Interrupt handler

4

interrupt handler processes data, returns from interrupt

5

CPU resumes processing of interrupted task

6 device driver

initiates I/O 1

7

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Summary of Services in I/O Subsystem

 The management of the name space for files and devices

 Access control to files and devices

 Operation control

 File system space allocation

 Disk allocation

 Buffering, caching, and spooling

 I/O scheduling

 Device status monitoring, error handling, and failure recovery

 Device driver configuration and initialization

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I/O Requests to Hardware Operations



Consider reading a file from disk for a process



Determine device holding file



Translate name to device representation



Physically read data from disk into buffer



Make data available to requesting process



Return control to process

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Layered File System revisited

Logical File System Application Programs

File-Organization Module

Basic File System

I/O Control

Devices

Given a symbolic file name, use directory to provide values needed by FOM

Transform logical address to physical block address Numeric disk address

Driver: hardware instructions write(data_file, item)

file name

information of data_file

2144th block

driver 1, cylinder 73, surface 2, sector 10

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Intercomputer Communications

 Network traffic could cause high context switch rate

 Interrupt generated during keyboard & network I/O

 Context switch occurs between prog. & kernel (drivers)

keyboard

e.g. telnet

e.g. telnet session

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STREAMS



A full-duplex communication channel

between a user-level process and a device



STREAM provides a framework for a modular

and incremental approach to writing device

drivers and network protocols

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The STREAM Structure

 A STREAM consists of

 STREAM head interfaces with user process

 Driver end interfaces with the device

 zero or more STREAM modules between them

 Each module contains a read and a write queue

 Message passing is used to communicate between

queues