1 Device I/O Port Locations on PCs (partial) Polling Computer - - PDF document

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CMSC 421 Section 0202 I/O Systems Chapter 13: I/O Systems

Silberschatz, Galvin and Gagne 2002 13.2 Operating System Concepts

Outline

I/O Hardware Application I/O Interface Kernel I/O Subsystem Transforming I/O Requests to Hardware Operations Streams Performance

Silberschatz, Galvin and Gagne 2002 13.3 Operating System Concepts

I/O Hardware

• There is an incredible variety in I/O devices
• There are shared common concepts

Device communicates with system using a connection point Port Bus (daisy chain or shared direct access) Controller (host adapter) Handshake between CPU and device controller done with

polling/interrupts

Offloading to DMA for large I/O

• Device-registers (control, state, data) accessible via the

controller of device allow a computer to control a device

• I/O instructions control devices
• Devices have addresses, used by

Direct I/O instructions Memory-mapped I/O

Silberschatz, Galvin and Gagne 2002 13.4 Operating System Concepts

A Typical PC Bus Structure

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Silberschatz, Galvin and Gagne 2002 13.5 Operating System Concepts

Device I/O Port Locations on PCs (partial)

Silberschatz, Galvin and Gagne 2002 13.6 Operating System Concepts

Polling

Computer system determines state of device using

Polling Interrupts

Polling determines state of device using certain

commands

command-ready bit Busy bit Error bit Command register Data in/out registers

Busy-wait cycle to wait for I/O from device

Silberschatz, Galvin and Gagne 2002 13.7 Operating System Concepts

Interrupts

CPU’s Interrupt Request Line is triggered by I/O device,

and CPU checks after each instruction, and device sets interrupt address

Device raises interrupt CPU catches interrupt and dispatches it Handler clears the interrupt

Interrupt handler receives interrupts Maskable to ignore or delay some interrupts Interrupt vector to dispatch interrupt to correct handler

Based on priority Some unmaskable Chaining of handlers

Interrupt mechanism also used for exceptions

Silberschatz, Galvin and Gagne 2002 13.8 Operating System Concepts

Interrupt-Driven I/O Cycle

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Silberschatz, Galvin and Gagne 2002 13.9 Operating System Concepts

Intel Pentium Processor Event-Vector Table

Silberschatz, Galvin and Gagne 2002 13.10 Operating System Concepts

Direct Memory Access

Used to avoid programmed I/O for large data movement Requires DMA controller Bypasses CPU to transfer data directly between I/O

device and memory

CPU writes DMA control block DMA controller interacts with device using

DMA-request raised by device to signal the DMA controller

that device has data

DMA-acknowledge raised by DMA to tell device to put data

• n data bus

Silberschatz, Galvin and Gagne 2002 13.11 Operating System Concepts

Six Step Process to Perform DMA Transfer

Silberschatz, Galvin and Gagne 2002 13.12 Operating System Concepts

Application I/O Interface

I/O system calls encapsulate device behaviors in generic

classes

Device-driver layer hides differences among I/O device

controllers from kernel

Devices vary in many dimensions

Character-stream or block Sequential or random-access Sharable or dedicated Speed of operation read-write, read only, or write only

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Silberschatz, Galvin and Gagne 2002 13.13 Operating System Concepts

A Kernel I/O Structure

Silberschatz, Galvin and Gagne 2002 13.14 Operating System Concepts

Characteristics of I/O Devices

Silberschatz, Galvin and Gagne 2002 13.15 Operating System Concepts

Block and Character Devices

Block devices include disk drives

Commands include read, write, seek Raw I/O (device is viewed as a linear array of logical blocks)

• r file-system access

Memory-mapped file access possible Executing a program can be done with memory-

mapping of the executable to VM Character devices include keyboards, mice, serial ports

Commands include get, put Libraries layered on top allow line editing

Silberschatz, Galvin and Gagne 2002 13.16 Operating System Concepts

Network Devices

Varying enough from block and character to have own

interface

Unix and Windows NT/9x/2000 include socket interface

Similar to electric sockets Local and remote sockets and connecting them with

addresses

Separates network protocol from network operation Includes select functionality that manage a set of sockets

and informs the CPU which socket have data or are ready to accept more data Approaches vary widely

pipes, FIFOs, streams, queues, mailboxes

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Silberschatz, Galvin and Gagne 2002 13.17 Operating System Concepts

Clocks and Timers

Provide current time, elapsed time, timer Programmable Interval Timer can be used for

implementing timers (via periodic interrupts)

OS can implement many timers using a single

Programmable timer ioctl (on UNIX) covers odd aspects of I/O such as

clocks and timers

Silberschatz, Galvin and Gagne 2002 13.18 Operating System Concepts

Blocking and Nonblocking I/O

Blocking - process suspended until I/O completed

Easy to use and understand Insufficient for some needs

Nonblocking - I/O call returns as much data as available

User interface, data copy (buffered I/O) Implemented via multi-threading Returns quickly with count of bytes read or written

Asynchronous - process runs while I/O executes

Difficult to use I/O subsystem signals process when I/O completed

Silberschatz, Galvin and Gagne 2002 13.19 Operating System Concepts

Kernel I/O Subsystem

Scheduling to improve performance

Some I/O request ordering via per-device queue Some OSs try fairness

Buffering - store data in memory while transferring

between devices

To cope with device speed mismatch To cope with device transfer size mismatch To maintain “copy semantics” To allow programmer to overwrite user I/O buffer as

soon as I/O command returns, without having to worry about when data have been output to the device

Can also be implemented with VM and copy-on-write

Silberschatz, Galvin and Gagne 2002 13.20 Operating System Concepts

Sun Enterprise 6000 Device-Transfer Rates

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Silberschatz, Galvin and Gagne 2002 13.21 Operating System Concepts

Kernel I/O Subsystem

Caching - fast memory holding copy of data

Always just a copy Key to performance Sometimes can be used together with buffering

Spooling - hold output for a device

If device can serve only one request at a time i.e., printers, tape drives, etc

Device reservation - provides exclusive access to a

device

System calls for allocation and deallocation of devices Watch out for deadlocks

Silberschatz, Galvin and Gagne 2002 13.22 Operating System Concepts

Error Handling

OS can recover from disk read, device unavailable,

transient write failures

Most return an error number or code when I/O request

fails

System error logs hold problem reports

Silberschatz, Galvin and Gagne 2002 13.23 Operating System Concepts

Kernel Data Structures

Kernel keeps state info for I/O components, including

• pen file tables, network connections, character device

state

Many, many complex data structures to track buffers,

memory allocation, “dirty” blocks, etc

Some use object-oriented methods and message passing

to implement I/O

Silberschatz, Galvin and Gagne 2002 13.24 Operating System Concepts

UNIX I/O Kernel Structure

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Silberschatz, Galvin and Gagne 2002 13.25 Operating System Concepts

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

Silberschatz, Galvin and Gagne 2002 13.26 Operating System Concepts

Life Cycle of An I/O Request

Silberschatz, Galvin and Gagne 2002 13.27 Operating System Concepts

STREAMS

• STREAM – a full-duplex communication channel between a

user-level process and a device

• A STREAM consists of:

STREAM head that interfaces with the user process using

read/write or get/put message in a synchronous manner

driver end that interfaces with the device and zero or more STREAM modules between them, that

communicate via message passing in asynchronous manner

• Each module contains a read queue and a write queue
• Message passing is used to communicate between queues, and

may implement flow control

• STREAMs provide a modular and incremental approach to

developing device drivers and protocols

Silberschatz, Galvin and Gagne 2002 13.28 Operating System Concepts

The STREAMS Structure

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Silberschatz, Galvin and Gagne 2002 13.29 Operating System Concepts

Performance

I/O is a major factor in system performance:

Demands CPU to execute device driver, kernel I/O code Context switches due to interrupts Data copying Network traffic especially stressful

Silberschatz, Galvin and Gagne 2002 13.30 Operating System Concepts

Intercomputer Communications

Silberschatz, Galvin and Gagne 2002 13.31 Operating System Concepts

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

• But where should I/O functionality be implemented?

User process? Kernel? Device driver? Device controller?

Silberschatz, Galvin and Gagne 2002 13.32 Operating System Concepts

Device-Functionality Progression