Special Topics on Operating
System
R. C. Chang
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Tentative Topics
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Linux Internal
Microkernels
Window CE Internal
Inferno
Other Embedded Operating Systems
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Trends of Operating System
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Server : UNIX/Linux, Windows/NT
Desktop : Windows 98/NT, UNIX/Linux
Embedded : WIN/CE, Inferno,VxWork,
JINI,…
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Operating System Structure
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Monolithic Kernel
– UNIX, Windows/98, Linux
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First Generation Microkernel
– Mach, Chorus,…
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Second Generation Microkernel
– L4, Exokernel,...
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Linux Internal
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Text Book: The Linux Kernel by David A.
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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
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A typical PC Motherboard
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PC Hardware
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PC, SP, PS
Memory
– D-Cache, I-Cache
– Cache Coherence
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Bus
– ISA, PCI
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Controllers and Peripherals
– IDE, SCSI, NIC,…
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Address Space
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Software Basics
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What is an operating system?
– Memory management
– Processes
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$ 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
$
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Device Drivers
– Controller Chips
• NCR810 SCSI
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The Filesystems
– EXT2,CDROM,...
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Kernel Data Structures
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Link Lists
Hash Tables
– Caches
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Abstract Interfaces
– /proc/filessystems
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Memory Management
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Virtual Memory
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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
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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
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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
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Linux uses a Least Recently Used page
aging technique to choose pages
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Physical and Virtual Addressing
Modes
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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
V : Valid
FOE: Fault on Execute
FOW:
Write
FOR:
Read
ASM: Address Space Match
KRE: kernel mode can read
URE: user mode can read
PFN: Page Frame Number
Page_Dirty, Page_ACCESSED
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Caches
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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
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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)
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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
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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
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Virtual memory is represented by an
mm_structure data structure
– vm_area_structure
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Demand Paging
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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)
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The Linux Page Cache
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The Linux Page Cache
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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
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Swapping Out and Discarding
Pages
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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:
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Swap out pages
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Reduce the size of the buffer and page
caches
– clock algorithm
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Swapping out System V shared memory
pages
– update multiple page table entries
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Swapping out and discarding pages
– clock algorithm
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The Swap Cache
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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
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Swapping Pages In
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Demand paging
swapin operation
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