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Chapter 13: I/O Systems

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Chapter 13: I/O Systems

! I/O Hardware!Application I/O Interface!Kernel I/O Subsystem!Transforming I/O Requests to Hardware

Operations!Streams!Performance

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I/O Hardware! Incredible variety of I/O devices!Common concepts

uPortuBus (daisy chain

or shared direct access)uController (host

adapter)! I/O instructions control devices!Devices have addresses, used by

uDirect I/O instructionsuMemory-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 ucommand-readyubusyuError

!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 some interrupts! Interrupt vector to dispatch interrupt to correct

handleruBased on priorityuSome nonmaskable

! Interrupt mechanism also used for exceptions

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Interrupt-Driven I/O Cycle

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Intel Pentium Processor Event-Vector Table

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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

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Six Step Process to Perform DMA Transfer

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Application I/O Interface! I/O system calls encapsulate device

behaviors in generic classes!Device-driver layer hides differences among

I/O controllers from kernel!Devices vary in many dimensions

uCharacter-stream or blockuSequential or random-accessuSharable or dedicateduSpeed of operationuread-write, read only, or write only

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A Kernel I/O Structure

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Characteristics of I/O Devices

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Block and Character Devices

!Block devices include disk drivesuCommands include read, write, seek uRaw I/O or file-system accessuMemory-mapped file access possible

!Character devices include keyboards, mice, serial portsuCommands include get, putuLibraries layered on top allow line editing

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Network Devices!Varying enough from block and character to

have own interface

!Unix and Windows NT/9x/2000 include socket interfaceuSeparates network protocol from network

operationuIncludes select functionality

!Approaches vary widely (pipes, FIFOs, streams, queues, mailboxes)

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Clocks and Timers

!Provide current time, elapsed time, timer

!Programmable interval timer used for timings, periodic interrupts

!ioctl (on UNIX) covers odd aspects of I/O such as clocks and timers

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Blocking and Nonblocking I/O!Blocking - process suspended until I/O

completeduEasy to use and understanduInsufficient for some needs

!Nonblocking - I/O call returns as much as availableuUser interface, data copy (buffered I/O)uImplemented via multi-threadinguReturns quickly with count of bytes read or written

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Blocking and Nonblocking I/O

!Asynchronous - process runs while I/O executesuDifficult to useuI/O subsystem signals process when I/O completed

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Two I/O Methods

Synchronous Asynchronous

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Kernel I/O Subsystem

!SchedulinguSome I/O request ordering via per-device

queueuSome OSs try fairness

!Buffering - store data in memory while transferring between devicesuTo cope with device speed mismatchuTo cope with device transfer size mismatchuTo maintain “copy semantics”

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Device-status Table

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Kernel I/O Subsystem!Caching - fast memory holding copy of data

uAlways just a copyuKey to performance

!Spooling - hold output for a deviceuIf device can serve only one request at a time ui.e., Printing

!Device reservation - provides exclusive access to a deviceuSystem calls for allocation and deallocationuWatch 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 request fails

!System error logs hold problem reports

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I/O Protection

!User process may accidentally or purposefully attempt to disrupt normal operation via illegal I/O instructionsuAll I/O instructions defined to be privilegeduI/O must be performed via system calls

üMemory-mapped and I/O port memory locations must be protected too

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Use of a System Call to Perform I/O

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Kernel Data Structures

!Kernel keeps state info for I/O components, including open file tables, network connections, character device state

!Many, many complex data structures to track buffers, memory allocation, “dirty” blocks

!Some use object-oriented methods and message passing to 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 file from disk for a process:

uDetermine device holding file uTranslate name to device representationuPhysically read data from disk into bufferuMake data available to requesting processuReturn control to process

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Life Cycle of An I/O Request

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Performance

! I/O a major factor in system performance:

uDemands CPU to execute device driver, kernel I/O code

uContext switches due to interruptsuData copyinguNetwork traffic especially stressful

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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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Device-Functionality Progression