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Special Topics on Operating System

Special Topics on Operating System. R. C. Chang. Tentative Topics. Linux Internal Microkernels Window CE Internal Inferno Other Embedded Operating Systems. Trends of Operating System. Server : UNIX/Linux, Windows/NT Desktop : Windows 98/NT, UNIX/Linux

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Special Topics on Operating System

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  1. Special Topics on Operating System R. C. Chang

  2. Tentative Topics • Linux Internal • Microkernels • Window CE Internal • Inferno • Other Embedded Operating Systems

  3. Trends of Operating System • Server : UNIX/Linux, Windows/NT • Desktop : Windows 98/NT, UNIX/Linux • Embedded : WIN/CE, Inferno,VxWork, JINI,…

  4. Operating System Structure • Monolithic Kernel • UNIX, Windows/98, Linux • First Generation Microkernel • Mach, Chorus,… • Second Generation Microkernel • L4, Exokernel,...

  5. 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

  6. Hardware Basics • A typical PC Motherboard

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

  8. Software Basics • What is an operating system? • Memory management • Processes • $ ps • PID TTY STAT TIME COMMAND • 158 pRe 1 0:00 -bash • 174 pRe 1 0:00 sh /usr/X11R6/bin/startx • 175 pRe 1 0:00 xinit /usr/X11R6/lib/X11/xinit/xinitrc -- • 178 pRe 1 N 0:00 bowman • 182 pRe 1 N 0:01 rxvt -geometry 120x35 -fg white -bg black • 184 pRe 1 < 0:00 xclock -bg grey -geometry -1500-1500 -padding 0 • 185 pRe 1 < 0:00 xload -bg grey -geometry -0-0 -label xload • 187 pp6 1 9:26 /bin/bash • 202 pRe 1 N 0:00 rxvt -geometry 120x35 -fg white -bg black • 203 ppc 2 0:00 /bin/bash • 1796 pRe 1 N 0:00 rxvt -geometry 120x35 -fg white -bg black • 1797 v06 1 0:00 /bin/bash • 3056 pp6 3 < 0:02 emacs intro/introduction.tex • 3270 pp6 3 0:00 ps • $

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

  10. Kernel Data Structures • Link Lists • Hash Tables • Caches • Abstract Interfaces • /proc/filessystems

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

  12. An Abstract Model of VM

  13. 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

  14. 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 --> Disacrd • Dirty page --> swapping • Linux uses a Least Recently Used page aging technique to choose pages

  15. 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)

  16. 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

  17. 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

  18. 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

  19. 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

  20. Free_area data structure Allocated groups of pages

  21. 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

  22. Memory Mapping • Virtual memory is represented by an mm_structure data structure • vm_area_structure

  23. Demand Paging • Page fault • Search for vm_area_struct (AVL Tree) • If no (access illegal virtual address)--> 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)

  24. The Linux Page Cache

  25. 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 --> single page read ahead

  26. 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 > free_page_high do nothing • otherwise kswapd try to reduce the number of physical pages being used by the system:

  27. 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

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

  29. Swapping Pages In • Demand paging • swapin operation

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