I o management in representative operating systems
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I/O Management in Representative Operating Systems. UNIX SVR4 I/O Management. Principles Each I/O device is associated with a special file Special files are managed by the file system To read or write to a device, a read or write request is made for a special file associated with the device

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I/O Management in Representative Operating Systems

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I o management in representative operating systems

I/O Managementin Representative Operating Systems


Unix svr4 i o management

UNIX SVR4 I/O Management

  • Principles

    • Each I/O device is associated with a special file

    • Special files are managed by the file system

    • To read or write to a device, a read or write request is made for a special file associated with the device

  • Types of I/O

    • Buffered

      • I/O passes through system buffers

      • Types of buffers

        • System buffer caches

        • Character queues

    • Unbuffered

      • Uses the DMA facility: transfer takes place directly between the I/O module and the process I/O area

CS-550 (M.Soneru): I/O Management in Representative Operating Systems: [Sta’01]


Unix svr4 i o management cont

UNIX SVR4 I/O Management (cont.)

CS-550 (M.Soneru): I/O Management in Representative Operating Systems: [Sta’01]


Unix svr4 i o management cont1

UNIX SVR4 I/O Management (cont.)

  • Buffer cache

    • All I/O operations with the disk (block oriented) are handled through the buffer cache

    • Data transfer between the buffer cache and the user process space uses DMA: memory-to-memory copy

    • Buffer cache management uses three lists

      • Free List: List of all buffers available for allocation

      • Device List: List of all buffers currently associated with each disk

      • Driver I/O Queue: List of buffers in the process of an I/O or waiting for an I/O on a particular device

    • Implementation

      • All buffers are either on the free list or on the driver I/O queue list

      • No physically separate lists: a list is maintained as pointers associated with each buffer

      • Each buffer is associated with a device and remains associated with that device even on the free list, until reused and associated with another device

CS-550 (M.Soneru): I/O Management in Representative Operating Systems: [Sta’01]


Unix svr4 i o management cont2

UNIX SVR4 I/O Management (cont.)

  • Buffer cache (cont.)

    • Operation

      • Uses the readers/writers model: may be read multiple times

      • On a reference to physical block X on device Y, the OS first checks if the block is in the buffer cache

      • It searches the Device List for block X on device Y

      • If entry exists, it contains a pointer to the first buffer in the chain

      • Block replacement uses a least-recently-used algorithm: the free list maintains the least-recently-used order

  • Character queue

    • Used for character-oriented devices (e.g., terminals, printers)

    • A character queue is either written by the process and read by the I/O device or written by device and read by the process

    • Producer-consumer model is used: a character, one read, is destroyed

CS-550 (M.Soneru): I/O Management in Representative Operating Systems: [Sta’01]


Unix svr4 i o management cont3

UNIX SVR4 I/O Management (cont.)

  • Unbuffered I/O

    • Fastest I/O, uses DMA between the process space and the device space

    • Operation

      • Process performing unbuffered I/O is locked in main memory and cannot be swapped out

      • I/O device performing unbuffered I/O is assigned to the process for the duration of the transfer

  • Types of devices and the buffering used for each

    • Disk drives: unbuffered I/O or buffer cache

    • Tape drives: unbuffered I/O or buffer cache

    • Terminals: character queue

    • Communication lines: character queue

    • Printers: unbuffered I/O or character queue

CS-550 (M.Soneru): I/O Management in Representative Operating Systems: [Sta’01]


Windows 2000 i o management

Windows 2000 I/O Management

  • I/O manager

    • Responsible for all I/O for the OS

    • Provides uniform interface that all types of drivers can call

CS-550 (M.Soneru): I/O Management in Representative Operating Systems: [Sta’01]


Windows 2000 i o management cont

Windows 2000 I/O Management (cont.)

  • Cache manager

    • Provides caching service in main memory for the entire I/O subsystem, serving file systems and network components

    • It manages the size of the cache for each activity

    • Provides two services to improve system performance

      • Lazy write

        • Writes done in the cache only and not on disk

        • When processor utilization is low, cache manager writes changes to disk

      • Lazy commit

        • Similar to lazy write for transaction processing

        • Committed information is written in cache only

        • Background process writes information to the file system log at a later time

CS-550 (M.Soneru): I/O Management in Representative Operating Systems: [Sta’01]


Windows 2000 i o management cont1

Windows 2000 I/O Management (cont.)

  • File system drivers

    • Treated like any other device driver

    • Messages for certain volumes routed to the appropriate software driver for that device adapter

  • Network drivers

    • Integrated networking capabilities

    • Support for distributed applications

  • Hardware device drivers

    • Access the hardware registers of peripheral devices through entry points in the Executive dynamic link libraries

    • A set of such routines exists for every platform supported by W2K: routine names are the same for all platforms

    • Device drivers portable across different platforms

CS-550 (M.Soneru): I/O Management in Representative Operating Systems: [Sta’01]


Windows 2000 i o management cont2

Windows 2000 I/O Management (cont.)

  • Asynchronous and synchronous I/O

    • Asynchronous mode

      • Optimum application performance

      • Application initiates I/O operation and the calling thread continues execution while the I/O operation is queued by the I/O manager and then performed

      • Application needs to be notified when operation is complete

    • Synchronous mode

      • Application is blocked until the I/O operation completes

    • Methods for signaling I/O completion for the asynchronous mode

      • Signaling a device kernel object

      • Signaling an event kernel object

      • Alertable I/O

      • I/O completion ports

CS-550 (M.Soneru): I/O Management in Representative Operating Systems: [Sta’01]


Windows 2000 i o management cont3

Windows 2000 I/O Management (cont.)

  • Methods for signaling I/O completion for the asynchronous mode

    • Signaling a device kernel object

      • Indicator associated with a device object is set when an operation on that object is complete

      • The thread that invoked the I/O operation continues execution and then checks the indicator when it needs the outcome of the I/O operation

      • Method is not appropriate for handling multiple requests on the same device (e.g., multiple actions on a single file)

    • Signaling an event kernel object

      • Thread creates event for each request

      • Thread can then wait on a single or multiple requests

      • Method is appropriate for handling multiple simultaneous I/O requests on a single device or file

CS-550 (M.Soneru): I/O Management in Representative Operating Systems: [Sta’01]


Windows 2000 i o management cont4

Windows 2000 I/O Management (cont.)

  • Methods for signaling I/O completion for the asynchronous mode (cont.)

    • Alertable I/O

      • Uses an Asynchronous Procedure Call (APC) queue associated with the thread

      • The thread I/O requests and the I/O manager places the results in the APC queue for that thread

    • I/O completion ports

      • Used on the W2K server to optimize the use of threads

      • A pool of threads is available for use: no need to create a new thread for each new request

CS-550 (M.Soneru): I/O Management in Representative Operating Systems: [Sta’01]


Windows 2000 i o management cont5

Windows 2000 I/O Management (cont.)

  • Redundant Array of Independent Disks (RAID) support

    • Hardware RAID

      • Separate physical disks combined into one or more logical disks by the disk controller

      • Controller interface responsible for the creation and regeneration of redundant information

    • Software RAID

      • Noncontiguous disk space combined into one or more logical partitions by the fault-tolerant software disk driver

      • Implemented by the OS and available on the server with any set of multiple disks

CS-550 (M.Soneru): I/O Management in Representative Operating Systems: [Sta’01]


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