Special Topics on Operating System

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Special Topics on Operating System

• R. C. Chang

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Tentative Topics

• Linux Internal
• Microkernels
• Window CE Internal
• Inferno
• Other Embedded Operating Systems

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Trends of Operating System

• Server UNIX/Linux, Windows/NT
• Desktop Windows 98/NT, UNIX/Linux
• Embedded WIN/CE, Inferno,VxWork, JINI,

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Operating System Structure

• Monolithic Kernel
• UNIX, Windows/98, Linux
• First Generation Microkernel
• Mach, Chorus,
• Second Generation Microkernel
• L4, Exokernel,...

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Linux Internal

• Text Book The Linux Kernel by David A. Rusling,
  (Please Download the book from Internet)
• Reference Books Linux Kernel Internals (Second
  Edition) by M. Beck et al., Addison-Wesley, 1998

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Hardware Basics

• A typical PC Motherboard

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PC Hardware

• PC, SP, PS
• Memory
• D-Cache, I-Cache
• Cache Coherence
• Bus
• ISA, PCI
• Controllers and Peripherals
• IDE, SCSI, NIC,
• Address Space

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Software Basics

• What is an operating system?
• Memory management
• Processes
• ps
• PID TTY STAT TIME COMMAND
• 158 pRe 1 000 -bash
• 174 pRe 1 000 sh /usr/X11R6/bin/startx
• 175 pRe 1 000 xinit /usr/X11R6/lib/X11/xini
  t/xinitrc --
• 178 pRe 1 N 000 bowman
• 182 pRe 1 N 001 rxvt -geometry 120x35 -fg
  white -bg black
• 184 pRe 1 lt 000 xclock -bg grey -geometry
  -1500-1500 -padding 0
• 185 pRe 1 lt 000 xload -bg grey -geometry
  -0-0 -label xload
• 187 pp6 1 926 /bin/bash
• 202 pRe 1 N 000 rxvt -geometry 120x35 -fg
  white -bg black
• 203 ppc 2 000 /bin/bash
• 1796 pRe 1 N 000 rxvt -geometry 120x35 -fg
  white -bg black
• 1797 v06 1 000 /bin/bash
• 3056 pp6 3 lt 002 emacs intro/introduction.tex
• 3270 pp6 3 000 ps

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• Device Drivers
• Controller Chips
• NCR810 SCSI
• The Filesystems
• EXT2,CDROM,...

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Kernel Data Structures

• Link Lists
• Hash Tables
• Caches
• Abstract Interfaces
• /proc/filessystems

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Memory Management

• Virtual Memory
• large address space
• protection
• memory mapping
• fair physical memory allocation
• shared virtual memory

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An Abstract Model of VM
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Demand Paging

• Load virtual pages that are currently bein used
  by the executing program
• Page fault handling
• Linux uses demand paging to load executable
  images into a processes virtual memory

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Swapping

• If a process needs to bring a virtual page into
  physical memory and there are no free physical
  pages available
• Clean or read-only pages --gt Disacrd
• Dirty page --gt swapping
• Linux uses a Least Recently Used page aging
  technique to choose pages

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Physical and Virtual Addressing Modes

• OS usually runs in physical address model
• Alpha AXP processor does not have a special
  physical addressing model.
• It divides up the memory space into several areas
  and designates two of them as physical address
  area
• KSEG (0xfffffc000310000)

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Access Control
ASM Address Space Match KRE kernel mode can
read URE user mode can read PFN Page Frame
Number Page_Dirty, Page_ACCESSED
V Valid FOE Fault on Execute FOW
Write FOR Read
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Caches

• Buffer Cache block device drivers(hard disks)
• Page Caches Speed up access to images and data
  on disk.
• Swap Cache only modified(dirty) pages are saved
  in the swap file.
• Hardware Caches Data/Instruction, TLB

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Linux Page Tables
Each Platform that Linux runs on must provide
translation macro that allow the kernel to
traverse the page tables for a particular
processor
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Page Allocation and Deallocation

• Physical page mem_map structure
• mem_map_t for each physical page(initialized at
  the boot time)
• Important fields
• count, age, man_nr (physical frame number)
• free-area vector is used to find free pages

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Free_area data structure
Allocated groups of pages
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Page Allocation/Deallocation

• Buddy algorithm
• Pages are allocated in blocks which are powers of
  2 in size
• The page deallocation code recombine pages into
  large blocks of free pages whenever it can

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Memory Mapping

• Virtual memory is represented by an mm_structure
  data structure
• vm_area_structure

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Demand Paging

• Page fault
• Search for vm_area_struct (AVL Tree)
• If no (access illegal virtual address)--gt SIGSEGV
• Check for type of access
• Decide the page in swap file or somewhere in disk
• if the page page table entry is invalid but not
  empty then it is in swap file
• nopage operation(by using page cache)

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The Linux Page Cache
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The Linux Page Cache

• Memory mapped files are read a page at a time and
  these pages are stored in the page cache
• Search through page_hash_table
• Linux --gt single page read ahead

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Swapping Out and Discarding Pages

• Kernel Swap Daemon(kswapd)
• kernel thread
• keep enough free pages in the system
• started by init process and wait for kernel swap
  timer to periodcally expire
• if of free pages gt free_page_high do nothing
• otherwise kswapd try to reduce the number of
  physical pages being used by the system

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Swap out pages

• Reduce the size of the buffer and page caches
• clock algorithm
• Swapping out System V shared memory pages
• update multiple page table entries
• Swapping out and discarding pages
• clock algorithm

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The Swap Cache

• When swapping pages out, Linux avoids writing
  pages if it does not have to
• Swap cache --gt a PTE for a swapped out page
• If a swap cache is non-zero, a page in swap file
  has not been modified --gt no need to write the
  page to the swap file

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Swapping Pages In

• Demand paging
• swapin operation