08 Operating System Support by Sk6813Vm


									        Chapter 8

Operating System Support

    Focus on Architecture
   Functions of Operating System

• Managing Resources
    Access to System Utilities
    Access to files
    Access to I/O devices
    Managing Interrupts & Bus Control (DMA)
    Error detection and response

• Scheduling Processes (or tasks)
    Program creation
    Program execution
    I/O Processing

• Managing Memory Utilization
    Virtual memory,
       Types of Operating Systems

What do we want to Optimize in each?

• Interactive

• Batch

• Uni-tasking

• Multi-tasking

• Real-Time
Batch Operating System Resident Monitor

                   • Jobs are batched in queue

                   • Monitor handles scheduling

                   • Monitor controls sequence of
                     events to process batch

                   •When one job is finished,
                    control returns to Monitor
                    which loads next job
       Desirable Hardware Support for OS

• Memory protection
   — To protect the Monitor & Utilities

• Timer
   — To prevent a job monopolizing the system

• Privileged instructions
   — Only executed by Monitor
   — e.g. I/O, files

• Interrupts
   — Allows for relinquishing and regaining control

   — Allows for optimizing bus usage
The Case for Multi-programmed Batch Systems

• I/O devices are very slow
   Waiting is inefficient use of computer

• When one program is waiting for I/O,
  another can use the CPU
Multi-Programming with Three Programs
Utilization:   Uni-programmed vs Multi-programmed
Multiprogramming Resource Utilization

• What are the challenges of scheduling ?
  —When do we start a new program ?

  —When a program is blocked, what program is
   executed next?

  —When several programs are blocked, which gets
   first dibs at the resource needed?

  —How long can one process monopolize a resource?
How can we model Processing?

A Five State   Process   Model
Types of Scheduling Needed
  Keeping Track of Process Information

If Processes can be interrupted and restarted,
  what is necessary to be able to stop and
  restart programs?
Process Control Block Layout
Scheduling Time Sequence Example:
Some Key Elements of O/S
                Process Scheduling:

What happened to the Medium-term Queue ?
            Memory Management

What are the Memory Management Issues?

• How can we keep the optimum “portions” of programs
  in memory to optimize the use of the resources ?

• How can we protect one program from corrupting
  other programs ?

• What do we do when all programs in memory and
  stalled and memory is full ?
               Memory Management
• Uni-programming
   — Memory split into two
   — One for Operating System (monitor)
   — One for currently executing program

• Multi-programming
   — “User” part is sub-divided and shared among active

• Note: Memory size implications
  - 16 bits  64K memory addresses
  - 24 bits  16M memory addresses
  - 32 bits  4G memory addresses
  - 64 bits    ? memory addresses

• Problem: I/O is so slow compared with CPU
  that even in multi-programming system, CPU
  can be idle most of the time

• Solutions:
  — Increase amount of main memory
     – Expensive

  — Swapping
           How Does Swapping Work?

• Long term queue of “processes stored on disk”
• Processes moved in as space becomes available
• As a process completes it is moved out of main
  memory to make room for other process(es)
• If none of the processes in memory are ready (i.e. all
  I/O blocked)
   — Swap out a blocked process (intermediate queue)
   — Swap in a ready process or a new process

    But swapping is an I/O process…
       Isn’t I/O slow?
       So why does swapping make sense ?
of Swapping
Other Structures to help with Memory Management

     • Partitioning

     • Paging
      Splitting memory into sections to allocate to processes
       (including Operating System!)

Fixed Size Partitions:
• Equal-sized partitions
   —Potentially a lot of wasted memory

• Variable-sized partitions
   —Process is stored into smallest reasonable “hole”

Dynamic partitions
   — memory leak
   — need periodic compaction
Fixed Partitioning
Effect of Dynamic Partitioning – memory leaks
    What about Relocation Challenges?

• Can’t expect that process will load into the same
  place in memory as last time, but
• Instructions contain addresses
   — For Locations of data
   — For Addresses for instructions (branching)

• How about having logical address and physical
  addresses ?
   — Logical address - relative to beginning of program
   — Physical address - actual location in memory (this time)

• Mechanisms:
   — Use Base Address
   — Automatic (hardware) Conversion
• Split memory into equal sized, “small” chunks
   - Page frames

• Operating System maintains list of free

• Then:
  — Allocate the required number page frames to a process
     – A process does not require contiguous page frames

  — Each process has its own page table
Allocation of Free Frames
Paging Addresses- Logical and Physical
         Paging Implementation Issues

• Demand paging
  —Do not require all pages of a process in memory
  —Bring in pages as required

• Page fault
  —Required page is not in memory
  —Operating System must swap in required page
  —May need to swap out a page to make space
  —Perhaps select page to throw out based on recent
  Paging has a Potential for “Thrashing”

• Too many processes in too little memory

• Operating System spends all its time

• Little or no real work is done

• Solutions
  — Good page replacement algorithms
  — Reduce number of processes running
  — Add more memory
                  Virtual Memory
• We do not need all pages of a process in memory for
  it to run - We can swap in pages only as required

• So - we can now run processes that are bigger than
  total memory available!

Differentiation “memory models”:
• Main memory is called real memory
   — physical memory
• User/programmer can see much bigger memory space
   — virtual memory

• Page Tables can become huge and can’t fit into
   — need multiple level tables, or
   — inverted tables (Why inverted tables ?)
Alternate Inverted Page Table Structure
    Problem with Inverted Page Table

• Every virtual memory reference causes two
  physical memory accesses
  —Fetch page table entry
  —Fetch data

• Use Translation Lookaside Buffer
  — special cache for page table(s)
TLB and Cache Operation
  (special Cache for tables)

• Segmentation is “partitioning” memory that is
  visible to the programmer
   - Note: Paging is not visible to the programmer

• Usually different segments are allocated to
  program and data

• There may be a number of program and data
  segments per process (program)
  — e.g. to support protection levels, priority levels,
   organization, flexibility, etc.
        Advantages of Segmentation

• Simplifies handling of growing data structures

• Allows programs to be altered and recompiled
  independently, without re-linking and re-loading

• Lends itself to sharing among processes

• Lends itself to protection

Can paging and segmentation be combined?
Pentium II Address Translation Mechanism

 “Segment” uses 2 bits to provide 4 levels of protection, typically:
 0: OS kernel,   1: OS,   2: apps needing special security,   3: general apps
 Pentium II              (Uses hardware for segmentation & paging)

• Unsegmented, unpaged
   — virtual address = physical address
   — Used in Low complexity, High performance systems

• Unsegmented, paged
   — Memory viewed as paged linear address space
   — Protection and management via paging (Ex: Berkeley UNIX)

• Segmented, unpaged
   — Collection of local address spaces
   — Protection to single byte level, Translation table needed is on chip when
     segment is in memory, provide predictable access times

• Segmented, paged
   — Segmentation used to define logical memory partitions subject to access
   — Paging manages allocation of memory within partitions (Ex: Unix System V)
OS Review


        long-term scheduler (queue of all jobs potentially schedulable)
        short-term scheduler (queue of processes that are ready to execute)
        medium-term scheduling (queue of jobs that can reside in memory)
        blocked monitoring (queue of processes blocked for resources)

        new – ready – running – blocked – exit   state machine

  Memory management:
      paging – frames, pages, page fault, page table, logical/physical addr
      virtual memory – inverted page table, Translation Lookaside Buffer

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