In-Depth Notes on Device Management in Computer Systems

Device Management

  • Memory Management in Early Systems

    • Single-user systems
    • Fixed partitions
    • Dynamic partitions
    • Relocatable partitions

  • Memory Management in Recent Systems

    • Swapping (moving processes in and out of the memory)
    • Paging
    • Paged Memory Allocation (JT, PMT, MMT)
    • Demand Paging Memory Allocation (JT, PMT - Modified, MMT)
    • Segmentation
      • Segmented Memory Allocation (JT, SMT, MMT)
      • Segmented/Demand Paged Allocation (JT, PMT, SMT, MMT)
    • Page Replacement Algorithms
    • FIFO (First-In-First-Out)
    • LRU (Least Recently Used)
    • OPT (Optimal)
    • Important Terms
    • Trashing
    • Page Fault
    • Working Set
    • Reference Bit
    • Modified Bit
    • Virtual Memory

Introduction to Device Management

  • System Device
  • Device Handler Seek Strategies
  • Groups of Storage Media
  • I/O System Component
  • Communication Among Devices
  • Management of I/O Requests

Device Management Functions

  • Tracking the Status of Each Device
    • Examples include disk drives, printers, modems, etc.
  • Using Preset Policies
    • Determines which process gets a device and duration
  • Allocating Devices
  • Reallocating Devices
    • At the process level
    • At the job level

Peripheral Devices Categories

  1. Dedicated Devices

    • Assigned only to one job at a time (e.g., tape drives, printers)
    • Disadvantage: Inefficient if not used 100% of the time

  2. Shared Devices

    • Assigned to several processes (e.g., disk pack)
    • Interleaving requests must be controlled by Device Manager
    • Conflicts resolved based on policies

  3. Virtual Devices

    • Dedicated devices transformed into shared (e.g., using spooling for printers)
    • Spooling speeds up slow dedicated I/O devices

I/O Scheduling Algorithms

  • Goals of Scheduling Algorithms:

    • Minimize arm movement
    • Minimize mean response time
    • Minimize variance in response time

  • Seek Strategies:

    • FCFS (First-Come-First-Served)
    • Average performance, high arm movement
    • SSTF (Shortest Seek Time First)
    • Minimizes overall seek time; prefers nearest request
    • SCAN
    • Moves arm from outer to inner tracks, servicing requests
    • LOOK
    • Similar to SCAN, but doesn't go to track edges unless needed
    • N-Step SCAN
    • Holds requests until arm returns
    • C-SCAN (Circular SCAN)
    • Serves requests on its path and then jumps back to the start
    • C-LOOK
    • Stops at the last requested track and jumps back

Disk Scheduling Example

  • Scenario: Disk drive with cylinder requests in FIFO order
  • Tasks: Calculate distances moved by different scheduling methods:
    • FCFS
    • SSTF
    • LOOK

Storage Media Types

  • Sequential Access Media
    • Records stored sequentially
  • Direct Access Storage Devices (DASD)
    • Records can be accessed directly
    • Categories include:
    • Magnetic Disks
    • Optical Discs
    • Flash Memory
    • Magneto-Optical Disks

Optical and Flash Memory

  • Optical Discs
    • High-intensity laser creates pits and lands to store data
    • Speed classification (e.g., 32x) indicates spin rate
  • CD-R Technology
    • Write-once, read-many
    • Uses a dye layer that is burned to mark data
  • CD-RW Technology
    • Data can be written, changed, and erased using phase change technology
  • DVD-ROMs
    • Store more data and have tighter spirals compared to CDs
  • Flash Memory
    • Emulates RAM but retains data without power
    • Frequent forms include Compact Flash and USB drives

I/O Subsystem Efficiency

  • Direct Memory Access (DMA)
    • Control unit accesses memory without CPU intervention
  • Buffers
    • Temporarily store data to synchronize I/O actions
  • Device Manager Responsibilities
    • Manages device activity and optimizes performance through task division

Key Takeaways

  • Different Seek Strategies perform differently under various loads.
  • The I/O subsystem's design significantly impacts data retrieval times and system efficiency.
  • Flash memory and various storage media types play crucial roles in modern computing systems.