input output systems

7/21/2019 Input output systems

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Silberschatz, Galvin a nd Gagne 200213.1

Chapter 13: I/O Systems

I/O HardwareApplication I/O InterfaceKernel I/O SubsystemTransforming I/O Requests to Hardware O perationsStreamsPerformance


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I/O Hardware

Incredible variety of I/O devicesCommon concepts

PortBus ( daisy chain or shared direct access)Controller (host adapter)

I/O instructions control devicesDevices h ave a ddresses, used by

Direct I/O instructionsMemory-mapped I/O


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A Typical PC Bus Structure


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Device I/O Port Locations on PCs (partial)


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Polling

Determines state of device command-ready busy

Error

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


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Interrupts

CPU Interrupt request line triggered by I/O device

Interrupt handler receives interrupts

Maskable to ignore or delay s ome interrupts

Interrupt vector to dispatch interrupt to correct handlerBased on prioritySome unmaskable

Interrupt mechanism also used for exceptions


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Interrupt-Driven I/O Cycle



Intel Pentium Processor Event-Vector Table



Direct Memory Access

Used to avoid programmed I/O for large data movement

Requires D MA controller

Bypasses C PU to transfer data directly b etween I/O

device and memory



Six Step Process to Perform DMA Transfer



Application I/O Interface

I/O system calls e ncapsulate device behaviors i n genericclassesDevice-driver layer hides d ifferences a mong I/Ocontrollers from kernelDevices vary in many d imensions

Character-stream or blockSequential or random-accessSharable or dedicatedSpeed of operationread-write, read only, or write only



A Kernel I/O Structure



Characteristics of I/O Devices



Block and Character Devices

Block devices i nclude disk drives Commands include read, write, seekRaw I/O or le-system accessMemory-mapped le a ccess possible

Character devices i nclude keyboards, mice, serial ports Commands include get, putLibraries layered on top allow line editing



Network Devices

Varying enough from block a nd character to have owninterface

Unix and W indows NT/9 i /2000 include socket interfaceSeparates n etwork p rotocol from network o peration

Includes select functionality

Approaches va ry widely (pipes, FIFOs, streams, queues,mailboxes)


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Clocks and Timers

Provide current time, elapsed time, timer

If programmable interval time used for timings, periodicinterrupts

ioctl (on UNIX) covers o dd aspects o f I/O such asclocks and timers


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Blocking and Nonblocking I/O

Blocking - process s uspended until I/O completed Easy t o use and understandInsufficient for some needs

Nonblocking - I/O call returns as much as available

User interface, data copy (buffered I/O)Implemented via multi-threadingReturns q uickly w ith count of bytes r ead or written

Asynchronous - process r uns w hile I/O executes

Difficult to useI/O subsystem signals p rocess w hen I/O completed


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Kernel I/O Subsystem

SchedulingSome I/O request ordering via per-device queueSome O Ss try fairness

Buffering - store data in memory w hile transferring

between devices To cope with device sp eed mismatchTo cope with device transfer size mismatchTo maintain copy s emantics


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Sun Enterprise 6000 Device-Transfer Rates


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Kernel I/O Subsystem

Caching - fast memory holding copy of dataAlways just a copyKey t o performance

Spooling - hold output for a device

If device can serve only o ne request at a timei.e., Printing

Device reservation - provides exclusive access to adevice

System calls for allocation and deallocationWatch out for deadlock


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Error Handling

OS can recover from disk read, device unavailable,transient write failures

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

System error logs h old problem reports


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Kernel Data Structures

Kernel keeps s tate info for I/O components, includingopen le tables, network c onnections, character devicestate

Many, many co mplex data structures t o track b uffers,memory allocation, dirty blocks

Some u se o bject-oriented methods a nd message passingto implement I/O


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UNIX I/O Kernel Structure


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I/O Requests to Hardware Operations

Consider reading a le from disk for a process:

Determine device holding leTranslate name to device representationPhysically r ead data from disk into buffer

Make data available to requesting processReturn control to process


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Life Cycle of An I/O Request


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STREAMS

STREAM a full-duplex communication channel betweena user-level process and a device

A STREAM consists of:- STREAM head interfaces with the user process

- driver end interfaces with the device- zero or more STREAM modules between them.

Each module co ntains a read queue and a write q ueue

Message passing is used to communicate betweenqueues


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The STREAMS Structure


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Performance

I/O a major factor in system performance:

Demands C PU to execute device driver, kernel I/O codeContext switches due to interruptsData copying

Network traffic e specially st ressful


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Intercomputer Communications


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Improving Performance

Reduce number of context switchesReduce data co pyingReduce interrupts b y u sing large transfers, smartcontrollers, pollingUse DMA

Balance C PU, memory, bus, and I/O performance forhighest throughput


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Device-Functionality Progression

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